Lighting Control - KNX

Lighting Control 

KNX Association

KNX ADVANCED COURSE 

Table of Contents 

1 General ..................................................................................................................... 4 

2 Conventional Brightness Control: Sun shines – Light switches off ............................. 4 

3 Principle .................................................................................................................... 5 

4 Constant Lighting Control .......................................................................................... 5 

4.1 Constant Lighting Control: Areas of Application, Objective ................................ 6 

4.2 Types of Closed-loop Control ............................................................................ 6 

4.3 Usable Bus Devices .......................................................................................... 7 

4.4 Characteristics of Sensors and Actuators .......................................................... 7 

4.4.1 Sensors ......................................................................................................... 7 

4.4.2 Closed-loop Controller types.......................................................................... 8 

4.4.3 Actuators ....................................................................................................... 9 

4.5 Parameterisation Notes, Flags, Possible Errors, Bus Load etc. ........................10 

4.6 Parameterisation Example................................................................................12 

4.6.1 Functions ......................................................................................................12 

4.6.2 Example .......................................................................................................13 

4.6.3 Parameters ...................................................................................................13 

4.6.4 Group Addresses ..........................................................................................15 

4.6.5 Linking of Sensor and Actuator Objects, Operation ......................................15 

4.6.6 Additional Notes ...........................................................................................16 

4.7 Installation Notes ..............................................................................................16 

4.7.1 Alignment of the Measuring Sensor ..............................................................16 

4.7.2 Several Light Strips with a Varying Proportion of External Light: ...................16 

4.7.3 Fundamental Mismatch ................................................................................17 

5 Brightness Control ....................................................................................................18 

5.1 Areas of Application, Objective .........................................................................18 

5.2 Types of Open-loop Lighting Control ................................................................18 

5.2.1 Continuous Control .......................................................................................18 

5.2.2 Two-step Control ..........................................................................................19 

5.3 Usable Bus Devices .........................................................................................20 

5.3.1 General ........................................................................................................20 

5.3.2 Sensors ........................................................................................................20 

5.3.3 Actuators ......................................................................................................20 

5.3.4 Controllers ....................................................................................................20 

5.4 Parameterisation Notes, Flags, Bus Load etc. ..................................................21 


5.6 Installation Notes ..............................................................................................25 

6 Brightness Control, combined with Master/Slave Control .........................................25 

6.1 Objective ..........................................................................................................25 

6.2 Principle ...........................................................................................................25 

Home and Building Management Systems 

KNX Association 

Lighting Control Lighting Control_E0310a 2/34


6.3 Available Devices .............................................................................................26 


6.5 Installation and Solution Notes .........................................................................27 

7 Appendix Tasks .......................................................................................................29 

7.1 Task 1: Lighting – Control dependent on External Light ....................................29 

7.2 Task 2: Lighting – Closed Loop Control Master-Slave with separate Actuator ..33 

7.3 General hints for the calibration of lighting closed loop control .........................34 





1 General 

The lighting in modern buildings is generally no longer switched manually. The users in 

principle demand the implementation of intelligent open- and closed-loop lighting control 

systems. This should result in the efficient operation of the lighting. Efficiency has three 

meanings in this context: firstly, there should be an obvious saving of energy; secondly, 

intelligent lighting systems should largely prevent lights from being switched on 

unnecessarily which should lead finally to an automatic protection of resources! 

2 Conventional Brightness Control: Sun shines – Light switches 

off 

If dimmed fluorescent lamps are used for example instead of switched lamps, the service 

life of the fluorescent material is lengthened considerably even when the maximum power 

is reduced by only 10%. This again results in cost savings and protection of the 

environment. A well-designed lighting control system also means a more pleasant working 

environment for the user e.g. from office workstations. It is also worth mentioning in this 

context that closed-loop lighting control should – where possible – use existing (free) 

external light which results in a two-fold energy saving, particularly during the warmer 

seasons of the year. A reduced level of artificial light means less electrical power, less 

waste heat and thus also a lower cooling capacity if the rooms are air-conditioned. 

Moreover, scientific studies have proven, that an integrated blinds and illumination 

automation will reduce the sun induced heat energy into a building up to 80%, 

Thus even a 4-fold saving can be made, perhaps in certain buildings avoid to install air 

condition at all. 





Light strip 1 Light strip 2 Light strip 3 

~ 26 % ~ 48 % ~ 70 % 

Receiver Receiver Receiver 

500 lux required light intensity 

Necessary 

artificial light 

Existing daylight 

Goal: Brightness control optimised for all areas of the room 

3 Principle 

Closed-loop and open-loop lighting control systems are based on the modulation of the 

lighting level inside the room by the measurement of either the level of external light 

(independent variable) or the measurement and feedback of the level of internal light 

(dependent variable) which contains a variable proportion of external light. In both 

variants, the primary goal is to maintain a required level of internal light as constant as 

possible. In general, it is a feature of a distributed system such as KNX that the individual 

tasks in the closed-loop or open-loop control systems are distributed among different 

devices: sensors, actuators and controller modules. This has both benefits and 

disadvantages as outlined below in detail. 

4 Constant Lighting Control 

A closed-loop control circuit is used for constant lighting control. The required level of 

brightness in the room or the level of lighting at the desk is measured as a controlled 

variable together with the interference from the external light and then fed back to the 

actuators in the appropriate manner. 

Controlling system 

Controlled system 

Z 

W Controller Y Actuator Contr. variable X 

X+Z 

- + 

Measuring device 

Brightness 

sensor 

W ... Reference variable (e.g. brightness, setpoint) 

Y ... Control value (dimming value 1-100%) 

Z ... Interference (level of external light) 

X ... Actual value (lux value at workstation) 

Figure 1: Constant lighting control 





4.1 Constant Lighting Control: Areas of Application, Objective 

This type of control system is mainly used in commercial installations where it is important 

to adhere to certain regulations such as those governing the workplace but not to make 

more light available than necessary. The aim is to create optimum working conditions 

while at the same time saving costs. The user should also be offered an optimum level of 

convenience and the internal room controller should operate independently of other 

parameters which cannot be detected here. 

4.2 Types of Closed-loop Control 

A distinction is made between Closed-loop Control and the so-called Integral Reset. In the 

first control system, the control value is directly influenced by the setpoint/actual value 

differential. For example, an absolute dimming value is sent to the actuators via a 

feedback function (dependent on the measured lighting level but with a variable proportion 

of external light). The negative feedback of the control value on the comparison point of 

the closed-loop control circuit as well as the system deviation with forward gain are 

governed by one function. The greater the influence on the control value, the higher the 

failure rate of the system deviation. 

Integral Reset on the other hand operates according to the two-step principle. The control 

value for the brightness level of the actuators is modified indirectly or gradually, namely 

via relative dimming telegrams. After this type of dimming process, the lighting sensor 

measures the surface again that is to be constantly illuminated, compares it with the 

setpoint and then decides again in which direction further dimming should take place. The 

respective level of change carried out to the control value remains constant at each new 

activation. It is not dependent on the degree of system deviation 

Two-step control is actually not a proper closed-loop control system as it rarely achieves 

the required setpoint. This type of control only has an opening/closing point instead of 

continuous dimming processes which must be provided with a sufficiently large hysteresis 

to prevent the triggering of continuous opening/closing operations. This type of lighting 

control is described in more detail under the section “Brightness control”. 





4.3 Usable Bus Devices 

Since we are specifically discussing the maintenance of a constant level of illuminance, 

only actuators that allow variable illumination are permitted. They must therefore be 

dimming actuators. A required characteristic of KNX dimming actuators is that it must be 

possible to control them in three different ways: switching, relative dimming via 4 bit 

information and absolute dimming via 8 bit information. Sensors for this control system 

must first only be able to measure the actual brightness value with sufficient accuracy and 

frequency without supplying upper limit values in the required band width. As regards 

accuracy, the sensor that offers a logarithmic resolution is the most beneficial. This means 

that is must be able to carry out more precise measurements in the lower range than in 

the upper range as the human eye reacts less to absolutely identical changes with an 

increasing level of brightness. When judging whether a closed-loop control system is 

better or worse than another type of system, only a relative estimation of accuracy is 

necessary. If an adjustment to 1500 lux is required for example and the maximum system 

deviation is +/- 150 lux i.e. 10 %, this is equivalent to an adjustment to 500 lux at an 

absolute value of +/- 50 lux. 

The third component of this control system is a controller which takes over the actual 

control task. It receives the measured brightness value as an input and compares it with 

the setpoint which is available e.g. as a parameter (fixed value) or as an object value (can 

be modified at any time via the bus). From these two values, it can determine the output 

(control value) according to the implemented algorithm (control function) and send it to the 

actuator. 

In practice, particularly due to cost and space restrictions, the manufacturers of these 

types of components decide to integrate the controllers into the sensors or even to build 

the entire controller and sensor component into the actuator. 

4.4 Characteristics of Sensors and Actuators 

4.4.1 Sensors 

As stated above, brightness sensors that are used for lighting control should have a 

measured-value resolution available that is adapted to the setpoint. A system deviation 

below approx. +/- 15% is still not noticed by the user. The accuracy or tolerance of the 

measuring sensor as well as the losses caused by A/D conversion in the KNX sensor 

must be added together i.e. lie below this figure. Example: The setpoint is 600 lux; the 

tolerance is therefore +/- 90 lux. The individual possible measured values must thus be 

separated by less than 90 lux. The fact that indirect measurements are always carried out 

causes a problem. It is not the luminous flux of the lamp that is measured, but the 

reflected light output from the reflected surface. It is easy to imagine that a dark surface is 

less reflective than e.g. a white desk. The structure of the surface can also influence the 

recording of the measured value. In principle, it can be said that the light is scattered to a 

greater or lesser degree and the measured value of the sensor is lower than for direct 

measurement. Due to the varying reflectance factors, sensor heads must have a variable 

gain available which can be adapted to the respective requirements. This gain must be 

carried out in front of the A/D converter as otherwise the resolution is reduced by the gain 

factor and the control precision decreases. The measured value can thus be adapted via 

this gain factor under the named requirements in the course of a calibration procedure so 





that it can be used simultaneously as a lux value that can be displayed. The correct 

measured brightness value is generally sent cyclically by the sensor. The value differential 

and time interval that are used for sending can be set in a wide range in most cases for 

the available devices. In addition to the cyclical sending variant, it is also possible to react 

quicker with event control if the measured brightness value changes significantly. 

4.4.2 Closed-loop Controller types 

The example outlined above now requires a closed-loop controller, which uses the 

setpoint/actual value comparison to determine how the control value must be modified in 

order to return to the desired equilibrium of setpoint = actual value. The details of this 

control technology will not be discussed here, only the feasible alternatives. 

This so-called closed-loop procedure can contain proportional, integral and differential 

feedback components. This means: 

proportional: a direct control output ‘y’ is calculated for the dimming actuator from the 

setpoint/actual value differential ‘x’ via simple, linear conversion function according to 

the function y = a . x 

integral: the control output is integrated with a specific rate i.e. it is zero at the 

beginning and reaches the calculated value y = a . x . t only after a certain period 

differential: the control output is determined from the rate of change of the system 

deviation y = x/t 

Only the P controller can be used directly by these basic functions of control technology. 

The use of a P controller however leads to a systematic deviation which cannot be 

reduced due to the requirement to avoid oscillations. Only an integrating controller would 

function better in our case as it finds a stable state after a certain period in which it 

remains while there are no changes to the external light intensity. The D controller reacts 

very quickly if there is a substantial change in the parameters. The response to this 

change dies down again after a certain period. 

Apart from the continuous control of the setpoint, it is now important for an optimum 

lighting control system that the change in the brightness level runs almost imperceptibly 

for the user. Extreme external variations in the lighting should also not influence the 

control output at such a rate that it causes an imbalance. P and D controllers are therefore 

rarely used for lighting control systems. Only integral action control is in practice sufficient 

to fulfil the requirements of the user. 

In most cases, the integral action control has an indirect rather than direct influence on the 

control output. We are then using the “integral reset” procedure. 

In practice, this means that the closed-loop controller modifies the control output stepwise 

and indeed always by the same amount per temporal unit, which is a modification of a true 

integral controller, since the steps are always of the same size. In the KNX system, 4 bit 

dimming telegrams (DPT 3.007) are predestined for this as they transfer these constant 

changes in value that do not contain any different values (apart from the sign) as is the 

case with an absolute 8 bit control output. Any considerable variations in the parameters 

always cause the same rate of change. Since the change in brightness should take place 

gradually, only the step widths 1/64 (1.6%) or 1/32 (3.2%) are recommended for DPT 

3.007. 4 bit dimming telegrams are sent continually at a certain rate, provided that the 

actual value does not meet the setpoint within the hysteresis. If the hysteresis range is 

reached, the controller stops. 





Figure 2: Example of a closed-loop controller with an integrated sensor 

In the objects of figure 1, it is possible to detect that further possibilities can be 

implemented apart from the actual dimming function (= control value). These options 

enable a very convenient use of the control system: 

Setpoint adjustment via absolute (direct) value control 

Master /slave function for additional lighting circuits 

Toggling between manual and automatic mode via all operator functions which the 

dimming actuator is aware of (i.e. switching, dimming and value setting) 

The previous statements about the procedure used by DPT 3.007 2 do not however mean 

that an integral reset can only be carried out with DPT 3.007 only. This procedure can 

also be implemented with DPT 5.001 telegrams. The controller thus starts with an initial 

value, mainly 0 or 255, and sends further telegrams as with DPT 3..07, which now contain 

8 bit values that are reduced or increased by a constant value compared to the previous 

value. Some devices also manage to retrieve the present dimming value of the actuator to 

be controlled and start from this one instead 0 or 100%. The benefit of this procedure 

compared to DPT 3.007 appears to be that the resolution increases until it reaches steps 

of 0.4%. This means it is possible to make even finer adjustments in the dimming process. 

The bus load should however be taken into account since it rises when smaller changes in 

the value are carried out. 

The device shown in figure 2 is an example for 8-bit closed loop control. 

4.4.3 Actuators 

No particular requirements are placed on the dimming actuators initially as only the 4 bit 

or 8 bit object is required for integral reset. It is however important to be able to select 

whether this type of actuator should switch off or not when dimming down continually. It is 

likewise the case with the opposite process. If the closed-loop control is switched on, it 

should be possible to switch on using 4 bit dimming commands. The timing/dimming curve 

must be active for both 8 bit value dimming and the 4 bit dimming process as the actuator 

otherwise dims brighter or darker step by step. The KNX dimmers and dimming actuators 

that are currently available on the market fulfil almost all these conditions which is not 

always the case with older devices. To optimise the dimming process, the dimming speed 





of the actuator and dimming step width and send interval of the controller should match 

precisely: 

If e.g. 1/64 dimming telegrams are sent every 2 seconds, the dimmer must be set so that 

it makes a full cycle of 0 – 100% in 128 sec and no faster. 

If it should also be possible to operate the dimmer manually, a shorter dimming period is 

of course required. Differences can therefore be found in this regard between the 

available devices as not all of them offer this possibility (see the note in the next section). 

4.5 Parameterisation Notes, Flags, Possible Errors, Bus Load etc. 

The previously named condition (dimming period of 128 sec) is optimised for the control 

system but not for manual operation. A compromise must often be made: the actuator 

dims at a slightly slower rate e.g. in 8 sec and the controller transmits at a slightly faster 

rate to avoid stepwise brightness processes. This increases the bus load however. 

Alternatively 1/32 is used as a step width instead of 1/64 to reduce the bus load again. 

This produces a further problem in the case of very bright luminaires (over-dimensioned or 

still new). The changes in brightness can already be greater than the set hysteresis after a 

single step of 1/32. This results in continuous fluctuations as the luminaire continually 

exceeds or falls below the target value. 

500 lx 

Hysteresis 

Interior 

lighting 

External brightness 

Figure 3: Mismatch of dimming step width and hysteresis 

The extension of the hysteresis or reduction of the dimming step width can provide some 

help: 

500 lx 

Hysteresis 

Interior 

lighting 

External brightness 

Figure 4: Correct ratio of dimming step width and hysteresis 





A better option is the use of dimming actuators with two time bases. This option has 

become more common since dimming actuators based on BCU2 have come on to the 

market. It is then really possible to implement a manual and automatic function: manual 

operation = short dimming time base, automatic = long dimming time base. 

Figure 5: Objects of a switch/dim actuator with a dual time base 





4.6 Parameterisation Example 

4.6.1 Functions 

We use 3 KNX components in this layout for a complete control system: a sensor with an 

integrated controller (type = integral reset by 8 bit), a dimming actuator with a dual 

dimming time base and a 3-fold push button with text display that enables the following 

functions. 

Function 

Effect 

Switch light manually (basic) 

Switches actuator on/off, interrupts the CLL*) 

control 

Dim light manually (basic) 

Dims the actuator, interrupts the CLL control 

Switch automatic CLL control on/off (basic) 

Starts the CLL control with the parameterised set 

point or stops it 

Set value manually for lighting 

(continuously) (optional) 

Sets the brightness value at the actuator between 

0 and 100%, interrupts the CLL control 

Presence (optional) 

CLL control triggered by a movement / presence 

detector; or simpler, just by switch 

Setpoint shift (optional) 

Variable LUX value setpoint control 

Calibration 

Only for commissioning (or for advanced users to 

recalibrate later) 

*) CLL: closed loop lighting 

The usual range in which setpoint values can fluctuate is between 250 lux (lighting level 

for less demanding activities) and 1500 lux (level for light-intensive laboratory 

workstations where optical equipment is used such as lenses, microscopes etc.). The 

sensor must maintain a system deviation of max. +/-15% throughout the range. 

Two further functions/group addresses are required to calibrate the arrangement after the 

installation: 

measured lux value (to compare with the external lux meter) 

calibration trigger 

The brightness sensor/controller already mentioned above also enables calibration to be 

carried out. The address ‘measured lux value’ is not really always necessary but strongly 

recommended to be used in order to have an indication about how far the real measured 

value and that one of the KNX sensor differ before the calibration is started. The 

calibration process will calculate just a (for the user not accessible) gain factor between 





the physically measured light level and the either parameterized or via object adjusted 

setpoint. 

4.6.2 Example 

The sensor would – after the initial full application download – send a value of 200. But a 

Luxmeter would see 500 Lux. The setpoint of the sensor is also 500 Lux. (It has to be – 

otherwise the calibration would be faulty). So after the calibration trigger telegram has 

been sent, the new measured value would be returned with “500”. 

Only a connection between the controller and the actuator is now missing: 

Automatic value control (Master value) 

4.6.3 Parameters 

Figure 6: Control parameters of the closed loop controller 

Explanations: 

Operating mode: “Constant light level control” is used for continuous dimming 

Number of slaves: Only 1 master channel required, so no slaves = 0 

Send meas. brightn.: (optional) the sensor value can be displayed and monitored 

Min. variation…: “15 Lux” means, with deviations of equal or more than 15 Lux a new 

value will be sent. 

Setpoint value: “Parameter” or “Comm. Object”; the latter one allows flexible setting 

of the setpoint 

Max. var. from setp.: Half of the double sided hysteresis (varies from 15 to 60 Lux) 

Max. step size …: varies from 0.5 up to 3%; = difference between 2 master value 

telegrams; 

Transmit next…: 2 sec; i.e., together with “max. step” 0-100% = 120 sec. 





Start and finish…: 

The controller will firstly read the value status, then calculate the 

next telegram to be sent, and also stop with a 0% value telegram 

Figure 7: Parameter (extract) of the switch/dim actuator 

This setting enables a manually operated dimming speed of 5 sec per 100%. 

The factor for the “dimming time 2” (plus its base timer “seconds”) rules an automatic 

dimming time of 120 sec. 

This time equals exactly the setting of the master channel of the brightness controller. 





4.6.4 Group Addresses 

To summarise, all the group addresses are listed here again: 

Figure 8: Group address configuration in a convenient constant lighting control system 

4.6.5 Linking of Sensor and Actuator Objects, Operation 

In our example a push button with text display (LCD) is used to provide proper labelling of 

the control functions and the (optional) display of values. 

Very often, however, customers don’t want to spend so much money, so they only use a 

local pushbutton for simple switching and dimming control as manual override, and the 

automatic activation will be done e.g. by a central scheduler, or by the twilight sensor on a 

weather central. 

Figure 9: Group address links between the 3 KNX devices 





4.6.6 Additional Notes 

In general, the actuator is also addressed via central functions (see above) which switch 

the lights on/off directly or also activate/deactivate the control for example via a time 

switch. This must also be taken into account in the controller. A direct central switching 

operation must also switch off the controller with positive drive. These addresses must 

therefore also be linked to the disable objects of the controller; otherwise the controller will 

always act against this external input and try to dim up the lights again. 

4.7 Installation Notes 

Many factors are decisive for the optimum installation of a brightness control system. 

Some are listed here: 

4.7.1 Alignment of the Measuring Sensor 

The surface that is to be measured should be as undisturbed as possible i.e. should never 

indicate different surface characteristics. External light should not directly penetrate the 

receiving lens if possible, likewise artificial light. 

Building ceiling 

Correct Incorrect 

Lamp 

Suspended 

ceiling 

Receiver 

Mounting height 2.5 - 6 m 

Light cone 

Correct 

Incorrect 

0 

4 

w 

A 

n 

A 

_ 

5 

9 

1 

Calibration required 

Measuring sensor +/-15% 

Daylight 

Figure 10: Optimum position of the measuring sensor (graphics do not resemble the sensor 

exactly) 

4.7.2 Several Light Strips with a Varying Proportion of External Light: 

In this case, which rarely occurs, a single brightness sensor is only sufficient if it is 

possible to generate different control values for the individual actuator channels. If you do 

not have a controller that can do this, a separate sensor must be installed for each light 

strip. As mutual influences (mostly unwanted) can arise, a partitioning of the sensors must 

take place and the overlapping of the lighting surfaces underneath the individual strips 

must be minimised. It can, however, invariably lead to oscillation processes or at least an 

unexpected distribution of brightness (although the measured values of the sensors are 

correct). The technically better solution nowadays is to only control one strip (whereby it is 

not possible to simply select any strip but the strip which is installed at the window side, 

which reacts the most sensitively to daylight changes). The other lighting strips are simply 





interfaced via a control function e.g. offset adjustment is a so-called master/slave circuit 

(see the explanations in section 5). 

Light strip 1 Light strip 2 Light strip 3 

~ 26 % ~ 48 % ~ 70 % 

Receiver 

Slave 2 Slave 1 

Master 

500 lx required illuminance 

Necessary 


Existing 

daylight 

Figure 11: Brightness control of a light strip, combined with offset control for the other light 

strips 

4.7.3 Fundamental Mismatch 

A mismatch always occurs in this system. This is based on the one hand on the different 

lighting spectrum of natural light and artificial light and on the other hand, on the different 

angle of illumination. The more external light comes in, the greater the measuring error, 

because the natural light also is picked up by the sensor rod directly instead of only from 

the controlled surface by reflection. A curve of the controlled variable is produced 

dependent on the external brightness which reaches its minimum in a ratio of 50:50 

between the proportion of external and internal light. The system must always be 

balanced under these lighting conditions. 

Ф internal 

[lx] 

Optimum adjustment value 

Actual 

value 

500 

Setpoint 

250 

Actual 

internal value 

Proportion of 


Proportion of 

external light 

Ф external 

[lx] 

Figure 12: Behaviour of the actual value dependent on the level of external light 





5 Brightness Control 

5.1 Areas of Application, Objective 

In contrast to lighting control in which you wish to achieve individual and optimum levels of 

light intensity as mentioned above, brightness control is mostly considered with regard to 

minimising installation costs without fully losing sight of the goal of reducing energy costs. 

It is however agreed between the planner and user that significant deviations from the 

setpoint/actual value can occur in the control system. This is particularly the case when 

only switchable lamps are integrated in the control system. 

Figure 13: Principle of brightness control 

5.2 Types of Open-loop Lighting Control 

For brightness control, a distinction is made between continuous control and two-step 

control. A sensor measures an external brightness value which is independent of the 

internal illuminance that is to be set. Starting from this measured value, the internal control 

value is determined at first via any calculation function which the room illuminance uses to 

reach the required setpoint. Technically this is an open control loop. The feedback loop is 

missing. This has both advantages and disadvantages. It is beneficial that this type of 

lighting control can never oscillate since the feedback loop has been omitted as 

mentioned. Starting with the measured value of a single sensor, many different control 

curves can be differentiated. The disadvantage is that a very extensive adjustment must 

be carried out for continuous control which takes at least one day. 

5.2.1 Continuous Control 

Continuous control – like the closed-loop control above – requires dimmers or dimming 

actuators which can be infinitely adjusted. As an automatic adjustment of the control value 

cannot take place (due to the missing feedback loop), this must be carried out by selecting 

a control function. In the simplest case, this can be a straight line determined by two pairs 

of values: a) maximum external brightness above which there should be 100% interior 





lighting and b) minimum brightness level above which the light should be switched off. 

Moreover, the control module must be in a position to carry out a hysteresis in order to 

ease control in the extreme range between switched on and switched off. 

Figure 14: Characteristic curve for brightness control – a hysteresis is advisable 

5.2.2 Two-step Control 

The characteristic curve described in the previous section is further simplified if only two 

states are possible for the controlled lamps: ON or OFF. 

Switching value 

1 

Switch Off point 

Hysteresis 

0 

0 

Switch On point 

2000 4000 6000 [ lux ] 

Measured value of brightness sensor 

Figure 15: Principle of two step control 





5.3 Usable Bus Devices 

5.3.1 General 

A separation between the sensor and actuator technology is advisable. The control 

system takes over the evaluation of the sensors (including push buttons) and triggers the 

actuators. 

5.3.2 Sensors 

Almost all the devices that can evaluate external inputs can be considered as sensors for 

brightness control. This includes the simple two-step control via a binary input that is 

coupled with a floating contact of a light sensor. The disadvantage in this case is that the 

switching points and hysteresis cannot be influenced via the KNX by telegram or at least 

by downloadable device parameters. 

Analogue inputs with KNX capability which supply a measured value in DPT 9.004 format 

in lux units are better in this case. The sensors that offer an almost logarithmic resolution 

of the measured signal are beneficial and of course completely cover the required area. If 

this is not the case, the measured-value resolution – as mentioned above – should be 

sufficiently precise for light control values approx. >50% so that visible fluctuations in the 

brightness level do not occur in the area in which the artificial light is predominant. As we 

saw above with closed-loop control, the maximum step width may not be higher than a 

3.2% dimming value differential. This characteristic must of course also apply here. This 

means that we should have at least 50/3.2 = 16 steps in the range 50%-100% for the 

dimming setpoint. You can now easily specify whether a sensor is sufficient in practice or 

is not precise enough. 

Example: A sensor records 1000 lux of external brightness, the dimming level lies at 50%. 

The resolution of the sensor dimming level lies at 50%. The resolution of the sensor must 

be at least 1000 / 16 = 6.25 lux. A small query regarding the theory – if this sensor with a 

resolution of 62.5 lux already had a dimming level of 50% at an external brightness of 500 

lux – is it suitable or not? 

5.3.3 Actuators 

As stated above under 3.4.3, there is no limit applicable here. All the actuators designed 

for normal dimming can be used. All the binary outputs can be used again for two-step 

control. 

5.3.4 Controllers 

The brightness control system covers several different control curves. It is therefore a 

good idea to implement the functions ‘measure external light level’ and ‘trigger actuators’ 

in a special module which doesn’t have to be directly linked with the light sensor via cable. 

In this case, it is possible to concentrate on the optimum position for placing the 

measuring sensor, as the measuring signal is already formatted as a DPT 9.004 telegram 

in the KNX-coupled sensor and then routed through the KNX network as appropriate. A 

controller module is then located in a distribution board which derives a range of different 

control curves from the incoming measured value. It is of course always possible to 

interface another system via a gateway in which DDC functions are also available. This is 





not a KNX lighting controller but the conversion is carried out externally in the DDC which 

then sends new dimming control values in response via the gateway to a new measured 

value of the lighting sensor. (E.g. gateways to Profibus). 

Figure 16: Conversion of the measured value of the light sensor in different control curves 

5.4 Parameterisation Notes, Flags, Bus Load etc. 

Having already discussed all the important characteristics of the actuators and sensors 

involved in the ‘Lighting control’ section, we can now briefly summarise here: statements 

regarding flags (readability of status objects) and inclusion of manual operation in the 

automatic system (time-limited interruption of control) also apply here of course. If the 

lighting should also be switched/dimmed manually via local push buttons or set directly to 

values, the control system must also be aware of the addresses, otherwise manual 

operation leads inevitably to an immediate correction by the automatic system. 

As we no longer have a control loop, it is also possible under certain conditions to 

implement minimum and maximum limit values for the dimming control value within which 

the control system operates. Otherwise it is switched off (manual operation). To prevent 

frequent switching in the two limit ranges, minimum ON and/or maximum OFF times can 

be requested as well as an ON/OFF hysteresis. The module that can fulfil these additional 

requirements is of course preferable to those with pure control curve functions. 

The bus load also plays a role here: it is not merely a question of value dimming but also 

whether and how the cyclical and/or event–controlled sending of the lighting measured 

value should be set best. It should first be considered that each new brightness value from 

the sensor results in the same number of value dimming telegrams as active control 

curves. 






Figure 17: Links between switch sensors, actuators and the control unit, consisting of the 

brightness sensor and control module 

In the above diagram, it can be seen that it is not only the external brightness and 

dimming control values (‘set value’) from the controller to the dimming actuators that are 

important, but also the ON/OFF lighting command, the optional 4 bit dimming and the 8 bit 

value setting of the manual push button. The application shown is – as in the closed-loop 

control described in paragraph 3 – able to convert increases and decreases in the 

dimming value into an adjustment of the control curves. The logic operation of the status 

value is required however as this determines the parallel displacement of the curve. What 

is still missing in this diagram would be a true automatic ON/OFF function which is 

controlled via a time switch. This KNX clock would also use the group addresses 

‘Enable/disable control’ and ‘Control ON/OFF’. 

Figure 18: Parameter of 2 different brightness sensors: linear vs. logarithmic value 

calculation 

The setting for “Sending on change” should not be too precise. The 1 st sensor allows 5% 

changes, always related to the last sent value. Such a value calculation goes along with 





the logarithmic impression of brightness by the human eye and is much better than a 

linear output (equidistant values) of sensor 2. Still an option of limiting the amount of bus 

telegrams per time unit should be available (not visible in this example here). 

Regarding sensor 2 (Figure 18 lower image): If a measured value interval of 64 lux is still 

sufficient in the lower range (see 2 nd example above), then the factor should be 8 instead 

of 4 as shown. During cyclical repetition of the measured value – if parameterised –values 

like those shown above are completely unsuitable: assuming that 10 control curves are 

implemented, up to 11 telegrams in total can occur every 650 msec. The bus load would 

then already be over 30%. The aim should however be to remain below 2%. Assuming 

that a maximum of 45 telegrams can be sent on the line on average, then 2% would 

correspond to 0.9 telegrams / sec. In the case of the aforementioned 11 telegrams per 

measured value, this means that a measured value may be sent approx. every 12 

seconds. This timing resolution is probably quick enough, primarily, because the controller 

calculates the new control values in proportion and does not always send constant 

dimming steps as in integral-action closed-loop control. The correct parameter 

combination for sending the cyclical measured values is then: 

Base = 130 ms; Factor = 12 sec / 130 ms = 92 

Dimming 

value 

250 0 

Measured 

value 

250 

250 500 

200 1000 

150 2000 

125 3000 

75 4500 

25 8000 

25 

1000 

8000 

Figure 19: Value table for lighting control: a monotone falling curve is important 

The table shown in the diagram figure 19 can be specified arbitrarily at first. An 

approximation of a comparable constant lighting control system can mainly be achieved 

over 3 points. The two values of complete darkness or the level of daylight that is 

sufficient to light up a room or part of it without artificial light can be quickly determined 

using a lux meter. The value pair that starts the proportional range should be verified next: 

in our example this is 500 lux / control value 250 (98%). If there are still considerable 

deviations in the intermediate lux values, it is possible to ‘recalibrate’ the particularly poor 

values a few days later. 





Figure 20: Parameter of a controller module 

It is then possible to check whether the curve-specific parameters of the controller (if 

available) fulfil the requirements of the customer. In our example, that would be 

the behaviour after bus voltage recovery; the open-loop control would be switched on 

here 

the minimum On time: if the external brightness should vary within the limit range so 

that the control value for the dimmer can fluctuate around the switching point, then the 

light remains switched on for at least 5 minutes 

the hysteresis limit values shown mean in our example that the light is only switched 

off at approx. 9000 lux (since there is then an underrange in the control value of 20) 

and switched on again at approx. 600 lux because the control value of 30 has been 

achieved again. 





5.6 Installation Notes 

The installation instructions for the open-loop lighting control system are reduced to notes 

about the installation of the sensor head: 

It must be directed outside or be installed outside the building. 

Its recording of the lighting level may not be influenced by seasonal variations such as 

leaves on the trees which stand between it and the sky or snow on the receiving lens. 

Its measurement may (for interior installation) also not be invalidated by the shutter. 

An installation behind the shutter or roller blind should be avoided where possible. 

The presence of shutters require a particular type of control: if proportionally-controlled 

shutters are present, the louvres of the shutters can be adjusted in parallel to the 

external brightness instead of the light. 

For an optimum adaptation of rooms that face different directions, it is advisable to use 

at least 2 differently positioned sensors: one in a south-east direction and another in a 

north-west direction. Large buildings may require even more sensors which should 

then be placed as perpendicular as possible to the respective façade and point 

upwards. 

6 Brightness Control, combined with Master/Slave Control 

6.1 Objective 

With combined open-loop/closed-loop lighting control, you are in general pursuing the aim 

of saving costs without having to relinquish the benefits of a partially true closed-loop 

control system. These can be: 

a simple setting procedure 

optimum lighting conditions in at least one position in the room 

always the correct lighting level in the rooms even when combined with light direction 

and sun protection systems. 

6.2 Principle 

An internal sensor measures the lighting level of a surface that should be regulated – as in 

closed-loop control which was described in detail above. The measured values of the 

sensor are further processed in a control program resulting in a control value which is 

used to trigger the actuator that is responsible for the direct light strip. To eliminate the 

requirement for further sensors (and also their deviation), it transfers a full control curve 

via offset adjustment to all other curves. The required offset adjustment can be 

determined by 2-3 simple measurements: at an artificial lighting level of 25%, 50% and 

75%, the necessary offset of the controlled light strips is determined in comparison to the 

regulated strip to arrive at the required setpoint in lux. The largest recorded differential 

(upwards) per strip is then taken as this guarantees that the minimum lighting level never 

falls below the setpoint. 





6.3 Available Devices 

Since the offset adjustment is derived from the dimming value of a regulated dimming 

actuator, it would be possible for example to use the active status response (8 bit) of this 

type of dimming actuator and simply add it to the established offset in a function module 

or visualisation program. The newly specified control value would then be sent to the next 

controlled light strip via another group address. 

This procedure is however only advisable if the function module or visualisation program 

mentioned are also available or if there is a cost benefit in using them compared to a 

multi-layered lighting control system. 

In terms of device implementation, it is much simpler in any case to use multi-channel 

dimming actuators with an integrated sensor connection and controller application. It is 

also possible here to configure internal offset connections between the actuator channels 

so that fewer bus telegrams can be sent. This solution not only saves device addresses 

but also costs in general. 


In the following example, 3 light strips in one room should be regulated or controller with a 

single dimming actuator and only one light sensor. A time-limited manual operation 

(switching/dimming) of all 3 channels should be possible using a 4-fold push button. The 

fourth rocker should switch all 3 dimming channels back to automatic mode. A special 

‘cleaning light’ function should also be implemented: when a further 

1-fold push button is pressed, the light should be set to full brightness for 1 hour, the 

closed-loop/open-loop control should be deactivated and then everything should be 

switched off. 

Figure 21: Example of a complete 3-channel room control system with an automatic 

‘cleaning-light’ function: only 3 bus devices are required 

The following can also be detected in figure 21: emergency operating mode via local push 

buttons if the KNX should fail. In the case of the device shown, these push buttons have 

the additional function of triggering an automatic calibration of the respective sensor 

channel, if it acts as a master sensor in the closed-loop control. 





6.5 Installation and Solution Notes 

The most important factor in this option of open-loop/closed-loop lighting control is again 

the optimum selection of the installation place and the alignment of the sensors. The 

essential points have already been mentioned above but there is one additional factor: in 

combined closed-loop/open-loop control systems, it must be at least ensured, that in the 

situation of complete outdoor darkness all lights must be equally turned on to the same 

value (which should be ruled by the specification of the lighting system). 

A pure additive / subtractive offset control however means on the other hand, that the 

difference between the master and the slave will always be equal or less than the given 

value. The master dimming value can range between 0 and 100%, but the slave is limited 

to less than this due to the offset. That means: If e.g. slave 1 = master + 20% (master at 

the window side), and slave 2 = master + 40%, then both dependant rows can only be 

controlled in the range 20 – 100 resp. 40 – 100 %. If the master has reached 0%, again 

we have 2 options: the slaves shut off together with the master, then it is too dark there, or 

they stay on, then it is too bright. But at least at full darkness all rows are at or above the 

required lux value setpoint. 

If you look at the same situation, but now with the master in row 3 (darkest area), with a 

negative offset to the slaves, this means, at high outdoor light levels the dimming works 

fine, but the closer you come to full darkness, the more unequal the distribution of light 

would be. In the end row 1 reaches only 60%, row 2 goes to 80%. That this is not the 

optimum, is self explanatory. 

So the only conclusion can be: The sensor must be mounted in the window side row 

which then also must be controlled via closed loop control! 

Underneath a couple of illustrations to this problem: 

Figure 22: optimized situation, but only at one point ! 





Figure 23: same as above, but at more external light – worst case situation ! 

Figure 24: again same configuration, but at complete darkness – rows 2 and 3 “waste” 

energy! 

What is the conclusion of the notes mentioned above? 

If the offset of the slave is positive, then it will always be able to reach 100%, but cannot 

dim down to 0%, it will turn off with the master. If it is negative, it can switch off, but not 

reach 100% of dimming level. 

So there is never a 100% positive solution as with individual light sensors per each row of 

luminaries. However, using the configuration with the master at the window, and the 

slaves with positive offsets in the inner parts of the room, will provide a good compromise. 

The only slight disadvantage is, that the master will either have to turn off late (at more 

than 500 lx, which avoids the “gap”, but causes a much higher level at darkness, or there 

will be a situation, when the master turns off at 500 lx, where rows 2 and 3 still would need 

some light, but will switch of with the master. 





This is an issue, which can be cured when a light controller with a dynamic offset control 

is used (a multiplicative offset instead of a constant one). In this case the slave follows the 

master via a linear function like this: S = M x ( 1 + O ). O : = Offset [%]; S = Dimvalue [%] 

Slave; M = Dimvalue Master [%]. It can easily be seen that at M = 100% and O > 0 the 

value of S reaches 100%. At very small values of M, however, the absolute difference 

between S and M will come close to 0! 

Figure 25: Light controller with multiplicative offset 

7 Appendix Tasks 

7.1 Task 1: Lighting – Control dependent on External Light 

A room that is fitted with two light strips should receive a brightness control system. Up 

until now, the light strips have been switched and dimmed individually. 

The following are used: 

1 x 2-fold switch sensor, switching and dimming objects (4 bit) 

2 x switch/dim actuators, objects for switching, dimming, value setting and value status 

must be present. 

Group addresses for manual control: 

L1 Switch 

L2 Switch 

L1 Dim 

L2 Dim 

First put this simple series circuit into operation. 

The dimmable lighting in the room should now also be controlled by a brightness control 

module dependent on the external light. 





Figure 26: Lighting – Control dependent on external light 

Required devices: 

Brightness sensor e.g. Siemens 5WG1 254 3 AB 02 to record the brightness level 

(external light) 

Brightness control module e.g. Siemens 5WG1 342 1 AB 01 

2-fold push button 

Project design 

Define the following new group addresses 

• Brightness value of sensor (control module, sensor) 

• Calibration request (control module) 

• Dimming value for calibration (control module) 

• Brightness value for calibration (control module) 

• L1 Value set (--> switch/dim actuator + control module) 

• L2 Value set (--> switch/dim actuator + control module) 

• L1 Value status (--> switch/dim actuator + control module) 

• L2 Value status (--> switch/dim actuator + control module) 

• Control L1 enable / block (2-fold push button, automatic) 

• Control L2 enable / block (2-fold push button, automatic) 

Link the bus devices (Control 1 and 2 for L1 and L2). 

Check the read flag in the brightness sensor GE 253. 

Use the default parameter setting of the bus devices. 

Commissioning 

Put the brightness control module and brightness sensor into operation. 

You can now determine the control characteristics using the calibration objects. 





Establishing the control characteristics for L1 and L2 

Switch off the ambient lighting (artificial sun) 

Dim the lighting to the required brightness value. Note that the window luminaire (L1) 

requires a lower modulation (see also the sketch of the characteristic curve below). 

Mark the group address ‘Calibration request’ and select the item “read/write…”. In the 

telegram monitor then check if it is already online, otherwise connect to the bus first 

before the next step ! 

Now be sure the selected group address still is selected; click on the “write” – button 

and enter the value ‘1’ for control characteristic 1 (L1), then click on “OK”. 

Change to the ‘Read value’ page and read out the status of the group address 

‘Dimming value for calibration’. Note the value. 

Now read out the status of the group address ‘Brightness value for calibration’. Also 

note this value. 

Repeat the process for control characteristic 2 (L2). 

An interpolation point of the two control characteristics is now known. Determine further 

points (at least 3) with different ambient light values (simulated external brightness 

values), whereby the last one should be carried out at the maximum brightness level of 

the ‘artificial sun’. Your determined values (hex code) could look as follows: 

Measured value of 

sensor 

Characteristic 1 Characteristic 2 

Dimming value L1 

Measured value 

of sensor 

Dimming value 

L2 

02A3 FF 02A3 B8 

0651 7A 0651 4F 

0F0A 37 0F0A 07 

2714 01 2714 00 

Converting the determined interpolation points of the characteristic curves 

As only decimal values can be entered in the parameters for the interpolation points of 

the characteristic curves, the determined hex values must be converted into decimal 

values. To do so, use the calculator (scientific) in the ‘Programs’ folder under 

‘Accessories’. 

Example: 

Control curve 1 Control curve 2 

Dimming value 

Dimming value 

255 

L1 

255 

184 

L2 

122 

55 

Measured 

value 

79 

7 

Measured 

value 

675 

1617 

3850 

10004 

675 

1617 

3850 

10004 





Load control characteristics 

Enter the established values in the parameters of the brightness control module and 

download the application. 

Functional test 

Test the function. 

• Enable/disable automatic control. 

• Switch lighting on/off manually. 

• Adjust characteristic curve by manual dimming. 

Optimisation 

Modify the parameters of the switch/dim actuators to dim brighter gradually. 





7.2 Task 2: Lighting – Closed Loop Control Master-Slave with 

separate Actuator 

Illumination level at a workplace shall be kept constant at 500 lux. Additionally it should be 

possible to change the parametrised luxvalue setpoint (300 – 800 lux), and the light shall 

be also manually operable (i.e., automatic / manual changeover shall be possible). 

As a device for this task the brightnesscontroller type Siemens UP255 shall be used. 

During the commissioning procedure it has to be calibrated, to adopt it to the prevailing 

light- and reflexion conditions. 

Process description see further below! 

A device to control shall be a 4-gang push button which also can set 16 bit values, or a 

suitable binary input or a push button interface. 

Functions of this 4-gang push button: 

Rocker A: lighting manually on/off + dimming 

Rocker B: Automatic on/off 

Rocker C: Presence on/off 

Rocker D: Upper button setpoint 600 lx; lower button setpoint 400 lx 

Alternatively (and much better) a display and operation unit would be recommendable, 

where beside the normal operating functions you can also see the current brightness level 

indication, and which permits a nearly continuous adjustment of the setpoint. 

Example: Siemens 5WG1585-2AB11 (UP 585) Display- and operation unit. 

Project design 

Create the following group addresses 

• Licht E on/off 

• Light F on/off 

• Light E dimming 

• Light F dimming 

• Sensor setpoint adjust 

• Calibration trigger 

• Automatic control on/off 

• Presence on/off 

• Dimmvalue set auto Licht E 

• Dimmvalue set auto Licht f 

Parametrise and link the bus devices 

Check the necessary read flags in the sensor UP255 and the S/D actuator 

Use default settings of the device if not otherwise defined 





Used devices 

Brightness controller UP255 25 S1 Brightness control 909601 

Push button 4-gang delta profile 12 S4 On/Off/Dim/Blnd/Displ241301 

Dimming actuator like in the basic project 

Commissioning 

Save your work 

Download the devices and test 

7.3 General hints for the calibration of lighting closed loop control 

1) Start ETS 

2) Open the required dim actuators, configure their parameters, create the groups 

and link them, finally download to the actuator 

3) Configure the brightness controller as well: depending on your preference with a 

fixed setpoint (parameter) or a variable one by object. 

4) Don’t forget to link the object “calibration” to a group address! 

5) Now adjustment at 50% operating point follows: Prerequisite: External light level in 

the room >= 50% of setpoint. 

6) To achieve that, switch on the lights and close the shutters 

7) Now place a precision luxmeter underneath the sensor 

8) Then adjust the measured luxvalue at the sensor that it shows approx. 50% of the 

setpoint. Example: >= 250 lx at setpoint of 500 lx. Best results are made when 

direct value control (8 bit) is used to set the required dimming value instead of 4-bit 

dimming. 

9) It must be ensured that daylight level is constant enough 

10) When the luxmeter now shows the correct setpoint, simply send a telegram to let it 

calibrate. (this can be achieved either by ETS, or a test push button) 

11) The controller should respond with the parameterised setpoint. Now the control is 

calibrated and can be used. 

12) If there is no controller feedback, then the reason can be: value out of range, 

caused by too bright or too dark surface, too much side light

Lighting Control - KNX

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