1 - An Introduction to Sensors P roduced at Warwick University by Margaret Low Simon Leigh Bob Hodge Funded by Bloodhound SSC Parachute Engine Thrust Vibration Throttle Acceleration Downforce Fuel Speed Distance Temperature In order to control Bloodhound at 1000mph, Bloodhound’s driver, Andy Green, needs a continuous stream of data about how the car is performing and its external environment. The data has to be accurate and, particularly at these speeds, readily understood. All this data needs to be extracted from the car’s environment, analysed and delivered to the cockpit display. These are the functions of Bloodhound’s sensor systems. The task of a sensor is to convert a physical stimulus, e.g. temperature or pressure, into a form that can be interpreted and displayed. The term sensor is often used as if there were a single device involved. Whilst this is true for a simple sensor, such as a temperature sensor that takes the form of a mercury thermometer, if there is a need to display temperature on a computer screen then there are a number of functions involved, as shown below.

Switch A switch is the simplest form of force measuring device, changing its resistance significantly when the applied force reaches a threshold. Variable Resistor A variable resistor changes its resistance according to the position of the slider. Force/Pressure/Movement Measurement Force Sensitive Resistor A force sensitive resistor changes its resistance according to the pressure applied to its surface. Piezo Electrical Device When some solids, notably crystals, are subjected to a varying pressure, they generate a small voltage. This is known as piezoelectricity. Electromagnetic Device Electromagnetic induction is the production of a voltage across a wire coil, typically made of copper, by exposing it to a constantly changing magnetic field. This effect is used in a range of devices including electrical generators and microphones. The Transducer The function of the transducer is to convert a stimulus from one form of energy into another. These are some examples: 2 P roduced at Warwick University by Margaret Low Simon Leigh Bob Hodge Funded by Bloodhound SSC

Thermocouple A thermocouple comprises two dissimilar metals in contact that produce a voltage when heated. Thermistor A thermistor is a form of resistor, the resistance of which varies significantly with temperature, albeit not in a linear manner. The name thermistor is derived from thermal and resistor. Temperature Measurement Light Measurement Light Dependent Resistor, LDR An LDR is a resistor whose resistance decreases with increasing light intensity. Photocell A photocell, or solar cell, uses a photovoltaic effect to convert energy from light to create an electrical current that is proportional to the intensity of the light. Such devices are generally intended to work with sunlight and are unlikely to be applicable to Bloodhound. The Transducer (continued) 3 P roduced at Warwick University by Margaret Low Simon Leigh Bob Hodge Funded by Bloodhound SSC

Analogue to Digital Converter This device takes the analogue output from the transducer and converts it to a digital number that the computer can understand. The conversion is generally carried out in an Integrated Circuit. Note that the ‘opposite’ device, a Digital to Analogue Converter, also exists. It takes a number from a computer and turns it into an analogue voltage. PicoBoard A PicoBoard is an electronic circuit that has been designed to make experimenting with sensors easy. It contains an Analogue to Digital Converter and several transducers: - Slider Resistor - Light Dependent Resistor - Microphone - Switch (actually a button) Slider Resistor Analogue Inputs Microphone A-D Converter Switch USB Computer Interface 4 P roduced at Warwick University by Margaret Low Simon Leigh Bob Hodge Funded by Bloodhound SSC

Software Interpretation and Output Most of the modules in the Bloodhound series use Scratch as the programming language. This is part of a Scratch program that is the subject of a later module. The code is reading the slider value and using its reading to control a timer. This is the output display from one of the Bloodhound modules. The arrow is moved around the disc to indicate the output of the sensor. 5 P roduced at Warwick University by Margaret Low Simon Leigh Bob Hodge Funded by Bloodhound SSC

References Sparkfun ProtoSnaphttp:// Acknowledgements Thermistor imagehttp:// Switch imagehttp:// Slider resistorhttp:// Piezoelectric imagehttp:// Motor imagehttp:// Microphone imagehttp:// Thermocouple imagehttp:// LDR imagehttp:// Photocell imagehttp:// PicoBoard imagehttp:// 6 P roduced at Warwick University by Margaret Low Simon Leigh Bob Hodge Funded by Bloodhound SSC