A transducer is a device which converts a signal from one form to another.
Most electronics circuits use both input and output transducers:
simple electronic system

Input Transducers

Photograph © Rapid Electronics
Input Transducers convert a quantity to an electrical signal (voltage) or to resistance (which can be converted to voltage). Input transducers are also called sensors.
  • LDR converts brightness (of light) to resistance.
  • Thermistor converts temperature to resistance.
  • Microphone converts sound to voltage.
  • Variable resistor converts position (angle) to resistance.

Output Transducers

Photograph © Rapid Electronics
Output Transducers convert an electrical signal to another quantity.
  • Lamp converts electricity to light.
  • LED converts electricity to light.
  • Loudspeaker converts electricity to sound.
  • Motor converts electricity to motion.
  • Heater converts electricity to heat.

Using input transducers (sensors)

voltage divider with LDR at top
Voltage divider circuit
Most input transducers (sensors) vary their resistance and this can be used directly in some circuits but it is usually converted to an electrical signal in the form of a voltage.
The voltage signal can be fed to other parts of the circuit, such as the input to an IC or a transistor switch.
The conversion of varying resistance to varying voltage is performed by a simple circuit called a voltage divider.
Please see the next page for more information about voltage dividers