Hacking Sensor Interfaces - MSc Sound Design

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Types of SwitchThe most common switches we encounter from day to day are push switches (push on – release off, or viceversa) and toggle switches (flick it over – it stays over, e.g. on/off switch). These are designed for control byhuman fingers and are not well-suited to sensing objects. The switches listed below are adapted for specialsensing applications and are much more useful for hacking purposes.Tilt SwitchMakes a connection when tilted beyond a certain angle. Easy to use, cheap and good for detecting generalmovement. Might be used on a motorcycle anti-theft alarm. Get more sensitivity by using several togethermounted at different angles.Reed SwitchConnection is made or broken by moving a magnet close to – or away from – the switch, whose contactsare extremely light and hence moved by the magnetic field. Typically used in burglar alarm systems todetect opening of windows and doors.Pressure Mat/Dance MatA pressure mat is a large flat switch that makes a connection when you step on it. Used in burglar alarmsby hiding them under door-mats and stair carpets where intruders won’t see them. A simple way ofdetecting someone entering your installation space. Dance Mats such as those used with game consoles(e.g. Dance Dance Revolution) are similar but feature multiple switches.MicroswitchSmall, sensitive switch capable of detecting small movements mechanically via a lever mechanism. Typicaluse is again on burglar alarms, to detect someone opening the case. Useful for detecting the opening of abox lid or the arrival of a sliding/moveable object.Types of SensorThere are many different kinds of sensor you can connect in the configurations shown above, and someonline lists are provided in the Further Reading section. O’Sullivan and Igoe’s book also has a very goodoverview.Light-Dependent Resistor (LDR)As the name suggests, this is a component whose resistance changes depending on the amount of lightfalling on it. These are cheap, reliable and very versatile. Put one in a cupboard and detect the opening ofthe door as light falls inside. Put one under an opaque object and you can detect when it’s lifted up. Canalso create a simple interface with existing equipment – stick one on an indicator LED or display to sensewhen something has happened (e.g. screen of mobile phone lighting up when someone calls).Infra-red DistanceSends out a beam of invisible infra-red light which bounces off a nearby object and provides a measure ofdistance from the sensor. This is typically short-range (less than a metre), but varies with different models.Output may be analogue or digital. Sharp have a good range of these, and GP2D12 has an analogueoutput. These are used in Roland musical instruments such as the MC-505 and known as the “D-beam”.Infra-red ProximitySimilar to Infra-red distance, but the aim is to detect the presence or absence or an object, over a largerdistance. The classic one is a Passive Infra-red Sensor (PIR) which is used in alarm systems high up in thecorner of a room, and to trigger automatic lights in driveways and gardens. Output is digital: on or off.Infra-red ReflectorAnother kind of infra-red sensor whose purpose is to detect linear or rotational movement. The object thatmoves typically has a wheel with slits in, or has black and white stripes, and the IR sensor detects blackareas or the slots, each of which will result in a digital pulse. Counting the pulses gives an measure ofposition, while counting the rate of pulses measures speed. Used in car rev-counters and the older ballbasedcomputer mice. Can be fiddly to set up right since not all blacks appear black to an IR photodiode,Chris Hand 28/1/10 4/10
especially marker pens – use paint or print the pattern on a laser printer. If the movement is large you canmake a crude but similar setup using an LDR and an LED.Ultrasonic distance/rangefinder.Similar to infra-red distance sensor but uses ultra-sonic pulses. Capable of measuring much largerdistances. Main drawbacks are that a faint clicking sound may be audible, and that indoors the pulses willreflect off hard and shiny surfaces and can cause false triggering. Usually digital output.Hall Effect SensorOutputs a voltage that varies with the strength and direction of magnetic field hitting it, e.g. from a magnetmounted on an object you want to detect. Can be unreliable unless you can guarantee the absence ofinterfering magnetic fields such as those generated by nearby electric motors or TV sets.Electronic CompassGives a reading proportional to its orientation towards the Earth’s magnetic North. Usually digital output.Thermistor/ThermocoupleA thermistor changes resistance according to temperature, while a thermocouple outputs a voltage thatvaries with temperature. Thermocouples are used in electronic thermometers.Flex sensorA flexible sensor that changes resistance according to how much it is bent. Typically used on the fingers ofvirtual reality gloves. Fun to play with but not so cheap (around £5 each) and hard to find.Force-Sensing Resistor (FSR)Similar to flex sensor but detects how much force is applied, e.g. when you press it. Could be used todetect whether or not someone’s sitting on a chair, for instance. Also not that easy to find.AccelerometerMeasures acceleration in 2 or 3 axes. Famously used in the Nintendo Wii remote control, as well as indigital cameras to detect rotation of the camera and mark the photo accordingly. Also used in the AppleiPhone to auto-rotate the screen. Relatively expensive (£30 or more) – hence the popularity of Wii remotesas gestural controllers for music etc since they cost around the same and you get Bluetooth and somebuttons built-in.GPS receiver (Global Positioning System)Gives longitude, latitude and altitude readings as well as date and a very accurate measure of time. Somewill also output speed. As used in car satellite navigation systems. Not really a sensor as such – typicallyconnects to a computer by Bluetooth or a USB cable. Pretty cheap these days, and increasingly being builtinto mobile phones which makes them ripe for interesting software hacks and mash-ups with the likes ofGoogle maps. Downside is that the current generation of devices only works outside and needs to be ableto “see” a number of satellites in the sky to give a reading, so built-up urban areas can be a problem.Chris Hand 28/1/10 5/10
Page 1 and 2: Hacking Sensor InterfacesChris Hand
Page 3: Connecting SensorsMost sensors are
Page 7 and 8: OutputSome sensor interfaces, such
Page 9 and 10: Websites and Blogshttp://www.tigoe.

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