Transducers

In this section we will discuss important electrical components known as transducers. Transducers can help circuits gather information, make noise, and do many other things.

There are two types of transducers.

Input transducers
Output transducers

Examples of each:

Input Transducers	Output Transducers
photoresistors	light emitting diodes
phototransistors	piezoelectric buzzers
thermistors	speakers
microphones
speakers
piezoelectric sensors
and many more....

(Speakers work as input or output transducers.)

Photoresistor
Photoresistors are variable resistors. When the light shining on them increases in intensity, their resistance is lowered. Working photoresistors into your circuits will allow you to detect changes in lighting. For example you could build a circuit to beep if someone turned on your room lights.

Thermistor
Thermistors are also variable resistors, however instead of being sensitive to light, they are sensitive to temperature. There are two types of thermistor:

positive temperature coefficient (PTC) thermistors
negative temperature coefficient (NTC) thermistors

The resistance of a PTC device increases with temperature increases.
The resistance of a PTC device decreases with temperature decreases.
The resistance of a NTC device decreases with temperature increases.
The resistance of a NTC device increases with temperature decreases.

Microphone/Speaker
A microphone converts pressure changes (sound) to voltage changes. Speakers are usually used to convert voltage changes to pressure changes (sound), but they can also work the other way. Normally, pressure changes near a microphone do not affect the voltage very much, and the signal must be amplified.

Piezoelectric Devices
Piezoelectric devices contain special crystals. These crystals will produce a voltage if pressure is applied to them in one direction. The crystals will also bend if voltage is applied to them. A common use of piezoelectric devices are "buzzers", which produce a buzzing noise when a voltage is applied.

Light Emitting Diodes
Light emitting diodes, or LED's as they are usually called, generate light when a current is passed through them. You will learn about diodes in the next section. LED's have many widespread uses.