Hacking Sensor Interfaces - MSc Sound Design

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controllers like the Kenton Control Freak have inputs for external sensors. It’s possible to save money bybuying a kit and making an interface yourself. Using an Arduino microcontroller board is another costeffectiveroute, but outside the scope of this tutorial. There’s no “best” way – it simply depends on whatequipment you have access to, how much money you want to spend, and what kind of software you wantto use to access the sensors.The commercial interfaces all have a selection of sensors available, but with a little effort it’s quite easy tohack your own which is much cheaper and more fun. If you want to go all the way you can hack a gamecontroller to turn it into your own sensor interface. Both of these kinds of hacking are covered below.Connecting Variable ResistorsMost of the simple sensors we use are variable resistors. A resistor opposes the flow of electrical current ina circuit – the greater the resistance, the greater the opposition. A sensor converts (or “transduces”) aphysical phenomenon, such pressure or light level, into a resistance. This can then be measured by theinputs on the sensor interface box.The simplest kind of variable resistor is a potentiometer or “pot”. These are very common – the volumecontrol on your stereo, for example – and they’re cheap. The schematic diagram below shows a potconnected to a sensor interface. This is a good starting point for testing your set-up.Resistance is measured in Ohms, written as Ω (the Greek letter omega) or sometimes as “R”. It’s commonto leave out the “ohms” completely, e.g. “a 10k pot” is a potentiometer with a maximum resistance of 10kΩ(10 kilo-Ohms or 10,000 Ohms).red wire+5 volts(other wire)sensor voltageeggplantia5@flickrpotentiometerblack wireground (0 volts)sensor interface(a) typical potentiometer(variable resistor)(b) schematic showing potentiometer connected tosensor interfaceThe middle terminal on the pot is connected to the “wiper”, which moves from one end of the resistor to theother as you turn the knob, changing the resistance between the middle terminal and each end. This is akind of “voltage divider” circuit, as seen below (a). The voltage divider is an important circuit, and allsensors and switches connected to the sensor box will form some kind of voltage divider.R1R2+ve voltage, Voutput voltageground (0V)(a) Basic voltage divider circuitVoltage divider circuitOutput voltage dependson ratio of R1:R2R1 > R2: output lowerR1 < R2: output higherR1 = R2: output is V/2+ve voltage, VVR1output voltageR2ground (0V)(b) sensor as variable resistor in voltage dividerChris Hand 28/1/10 2/10
Connecting <strong>Sensor</strong>sMost sensors are variable resistors, but with two terminals not three. So to connect them to the sensorinterface we add another resistor to make a voltage divider as shown above (b), and in the photographbelow.When experimenting with sensors and switches it’s quick and easy to use terminal blocks instead ofsoldering. The photo below shows a light-dependent resistor (LDR) connected in a voltage divider circuitwith a resistor using a terminal block. A good rule of thumb is to choose a resistor with a roughly similarvalue to the sensor – in this case the LDR’s value varies between 36k and 88k, so we’ve used a 100kresistor (identified by brown-black-yellow stripes).LDR in a voltage divider circuitwith a 100k resistor. The middlewire is the variable output.SwitchesWe also connect switches using a circuit similar to a voltage divider, as shown here.+ve voltage, V+ve voltage, VR1switchto inputgroundswitchR2to inputground(a) switch with pull-up resistor(b) switch with pull-down resistorUsing a “pull-up” resistor as shown in (a) means that the output voltage will be high when the switch isopen, and low when the switch is closed. The set-up in (b) is reversed, i.e. when the switch is open theoutput will be low, going high when the switch is closed. You can use either of these circuits to connect aswitch or push-button to a sensor box. Typically we would use a 10k resistor in this pull-up or pull-downconfiguration.Chris Hand 28/1/10 3/10
Page 1: Hacking Sensor InterfacesChris Hand
Page 5 and 6: especially marker pens - use paint
Page 7 and 8: OutputSome sensor interfaces, such
Page 9 and 10: Websites and Blogshttp://www.tigoe.

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