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Sensors

An electronic control system knows nothing about the outside world unless we give it sensors - its eyes, ears etc. The language it understands is that of electrical signals, voltages and currents.

A sensor has to turn changes in the outside world, such as light level, temperature, sound level, humidity, pressure etc., into changing electrical signals. A different type of sensor is needed for each of these tasks.

Here are some of them.

Light sensor:

The light dependent resistor (LDR) - Its resistance drops when a brighter light shines on it. It is relatively slow to react to changes in light level, though.

The photodiode - A component that passes an electric current only when sufficient light shines on it. It reacts much faster than the LDR to changes in light level.

Temperature sensor:

The thermistor - There are two versions of thermistor. One type, called ntc (negative temperature coefficient) has a resistance that gets less when its temperature goes up. The other type, ptc, (positive temperature coefficient) has a resistance which gets bigger when the temperature rises.

Sound sensor:

The microphone - The common form of microphone generates a voltage when it picks up a sound. The louder the sound, the bigger the voltage!