• It is possible to adjust the responses of LDRs (whether they cause a HIGH output when light is bright or dim) by changing the position of the LDR in the potential divider:
    • If it is the first element then the output is high when it is bright.
    • If it is the second element then the output is high when it is dim.
    • It is possible to adjust the sensitivity of the LDR by adjusting the resistance of the variable resistor or potentiometer which accompanies it as the remaining element of the potential divider.


  • This is basically an LED on one side and a phototransistor on the other encased within a sealed container with a dielectric separating them.
    • The phototransistor is basically a transistor whose base rail is light.
      • It works just like a transistor does and even amplifies current like a transistor.
        • This gain in current can be worked out in the same way as that of the transistor.
  • Its main purpose is to prevent high voltages or rapidly changing voltages on one side of the circuit from damaging components or distorting transmissions on the other side of the circuit.
    • In many ways they are like relays except they use light instead of electromagnetism to transmit information.
      • This is much less destructive in a circuit.


  • This is just like the opto-isolator except there is no dielectric between the photoemitter (i.e. LED) and the photodetector (i.e. photodiode or phototransistor) and the opto-switch manipulates the path of light and not the photoemitter itself.
    • The photodiode (symbol right) is a photodetector that can convert light energy into either voltage or current.
      • An example is a photovoltaic cell.
  • However, there is a slot between the two components into which an opaque object can be inserted to block the beam:
    • For instance, a computer printer cartridge may be fitted with a projection which interrupts the beam of a slotted switch when it reaches the end of its travel. This allows the cartridge to reverse direction accordingly.
    • Another example is a computer mouse with a rotating ball:
      • The ball rotates, driving chopper wheels turning in the slots of slotted switches:
        • By measuring the rate at which the light stream is interrupted it is possible to calculate the distance the cursor must move and in which direction.


  • Also known as a thermal resistor.
  • It responds very quickly to temperature changes.

Moisture sensor:

  • Should provide an analogue signal:
    • The greater the area of the anodes and cathodes that are submerged in water the stronger the signal.
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