TLC555 Timer Notes

Physics 116B
TLC555 Timer
Circuit
Physics116B, 1/17/07
D. Pellett
1

TLC555 Timer Circuit
• Variation on widely-used 555 timer using MOSFETs rather
than BJTs
• Can be used to make (among other things):
• Schmitt trigger
• astable multivibrator (relaxation oscillator)
• monostable multivibrator (“one-shot” circuit) – a simple
timer based on an RC network
• The basic circuit operation can be described without delving
fully into its internal logic
2

TLC555 Functional Description
Functional Block Diagram
VDD
Function Table
Assumes Control input not connected
Reset Trigger Threshold Output Discharge
2 VDD/3
VDD/3
Latch
Inverter
(circles indicate
logic negation)
Low
High
Don’t
Care
< VDD/3
Don’t
Care
Don’t
Care
Low
High
Grounded
Floating
High > VDD/3 >2 VDD/3 Low Grounded
Comparators
(circle is “-” input)
n-channel
enhancement-mode
MOSFET
High > VDD/3 < 2 VDD/3
Based on figure from TLC555 data sheet © Radio Shack
As previously
established
• We will set VDD = 5 V so logic levels will be 0 V (Low) and 5 V (High)
• Connect Reset to ground, leave Control unconnected
• Circuit performance described by shaded area in Function Table
• Threshold and Trigger are analog inputs in range 0 to 5 V
• Output is a logic level: 0 V or 5 V
• Discharge is connected to an n-channel MOSFET drain whose source is grounded.
• Discharge is grounded only when Output is Low. Otherwise, it is an open circuit
(“floating”).
3

TLC555 as Schmitt Trigger
Functional Block Diagram
VDD
Function Table
Assumes Control input not connected
Reset Trigger Threshold Output Discharge
VIN
2 VDD/3
VDD/3
Latch
VOUT
Inverter
(circles indicate
logic negation)
Low
High
Don’t
Care
< VDD/3
Don’t
Care
Don’t
Care
Low
High
Grounded
Floating
High > VDD/3 >2 VDD/3 Low Grounded
Comparators
(circle is “-” input)
n-channel
enhancement-mode
MOSFET
High > VDD/3 < 2 VDD/3
Based on figure from TLC555 data sheet © Radio Shack
• Connect Threshold and Trigger together to form a single input VIN. We get an
inverting Schmitt trigger with VD = 2 VDD/3 and VU = VDD/3
• You can see this from the table.
• Start with VIN = 0 V. Since Trigger < VDD/3, Output is High (VOUT = 5 V)
• Now increase VIN. Output does not go low until VIN crosses VD = 2VDD/3
• Output remains “as previously established” (High) for VDD/3 < VIN < 2VDD/3
• It remains low as VIN increases to 5 V.
• Now decrease VIN toward 0. VOUT does not go high until VIN falls below VU = VDD/3.
As previously
established
VOUT
5 V
0
0 VU V D
Schmitt trigger
response curve
5 V
VIN
4

TLC555 as Relaxation Oscillator
VDD
Function Table
Assumes Control input not connected
R
‘
Reset Trigger Threshold Output Discharge
VIN
2 VDD/3
VDD/3
‘
Latch
VOUT
Inverter
(circles indicate
logic negation)
Low
High
Don’t
Care
< VDD/3
Don’t
Care
Don’t
Care
Low
High
Grounded
Floating
C
‘
Comparators
(circle is “-” input)
n-channel
enhancement-mode
MOSFET
High > VDD/3 >2 VDD/3 Low Grounded
High > VDD/3 < 2 VDD/3
Based on figure from TLC555 data sheet © Radio Shack
• Again, connect Threshold and Trigger together to form a single input VIN and
get an inverting Schmitt trigger with VD = 2 VDD/3 and VU = VDD/3.
• Connect a capacitor C between VIN and ground and a resistor R between
VIN and VOUT.
• Just as we showed in class, we have converted an inverting Schmitt trigger
into a relaxation oscillator.
• If the output is High, C charges toward 5 V until VIN reaches VD, at which
point the output goes Low. Then C discharges toward 0 V until VIN
reaches VU and the output goes High. The process continues.
As previously
established
VOUT
5 V
0
0 VU V D
Schmitt trigger
response curve
5 V
VIN
5

Observe the pulse on the trigger input of the timer and sketch it to include in your report. Observe the
operation of the timer circuit for timing resistances of 100 kΩ and 1.0 MΩ. Measure the periods. Analyze
the operation of the circuit and compare the measured periods from what you expect from the analysis.
Relaxation Oscillator Circuit
TLC555 Pinout
• Note Reset is connected to VDD (“active low” input)
Figure 1: Relaxation oscillator diagram with thermistor resistance chart.
• The Control input is bypassed to ground with a .01 μF capacitor to
prevent pickup of interfering signals (glitches)
• It is also a good idea to put a .01 μF capacitor between VDD and ground
at the chip to filter out glitches on the power supply connection
6

TLC555 as “One-Shot” Pulse Generator
R
VDD VDD
‘
Function Table
Assumes Control input not connected
Reset Trigger Threshold Output Discharge
VC
VIN = 5V
C
2 VDD/3
VDD/3
‘
‘
Latch
Comparators
(circle is “-” input)
VOUT
Inverter
(circles indicate
logic negation)
Low
High
Don’t
Care
< VDD/3
Don’t
Care
Don’t
Care
Low
High
Grounded
Floating
High > VDD/3 >2 VDD/3 Low Grounded
High > VDD/3 < 2 VDD/3
As previously
established
Based on figure from TLC555 data sheet © Radio Shack
• We do some rewiring of the relaxation oscillator to get a One-Shot (monostable
multivibrator). Trigger has been disconnected from Threshold and R connects to VDD (5 V)
instead of Output.
• Threshold is connected to Discharge, which shorts the capacitor and forces VC = 0
when Output is low (see table).
• Now Trigger serves as the input VIN, normally set High (5 V).
• If VIN remains high and the output is high, C will charge through R toward VDD (5 V) until
Threshold reaches 2VDD/3. Then Output goes low, Discharge and Threshold are grounded, and
VC = 0 V. This is the stable state (VIN high, VOUT low).
7

TLC555 as “One-Shot” Pulse Generator
R
VDD VDD
‘
Function Table
Assumes Control input not connected
Reset Trigger Threshold Output Discharge
VC
VIN = 5V
C
2 VDD/3
VDD/3
‘
‘
Latch
Comparators
(circle is “-” input)
VOUT
Inverter
(circles indicate
logic negation)
Low
High
Don’t
Care
< VDD/3
Don’t
Care
Don’t
Care
Low
High
Grounded
Floating
High > VDD/3 >2 VDD/3 Low Grounded
High > VDD/3 < 2 VDD/3
As previously
established
Based on figure from TLC555 data sheet © Radio Shack
• Start in the stable state: VIN high, VOUT low, Discharge and Threshold
grounded, and VC = 0 V.
• A short pulse on the input causing VIN to fall below VDD/3 will set
Output high, ungrounding Discharge and allowing C to charge
through R toward 5 V. The output remains high while C is
charging.
• When VC (Threshold input) reaches 2VDD/3, Output goes low
again, shorting C to ground, terminating the output pulse and
returning the one-shot its the stable state. The result is an output
pulse whose width T is related to the RC time constant.
8

One-Shot Circuit for Lab
Figure 1: Relaxation oscillator diagram with thermistor resistance chart.
TLC555 Pinout
10 μs positive
input pulse
(TTL level)
Pulse inverter
and stretcher
• Suppose In is held at 0 V for a long time. Since the BJT is cut off, the 555 Trigger
input is pulled high by R′ (charging FigureC′ 2: One-shot so Trigger (timer) →5 circuit. V). Now the one-shot is in its
stable state with Output low.
• A short positive input pulse forces the BJT 2into saturation (like a closed switch)
discharging C′. This pulls the Trigger input low momentarily and starts the TLC555
output pulse. The R′C′ time constant is long enough to assure that the TLC555 is
triggered (input circuit acts as pulse “stretcher” as well as an inverter).
• The width of the one-shot output pulse T equals the time required for the
voltage VC across the timing capacitor C to go from 0 V to 2/3 VDD.
VC
One-shot
circuit
T
Output pulse of
width T
9