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How to Use Light Dependent Resistors

Light dependent resistors --- also called photoresistors --- are inexpensive tools for detecting light level. They can be made of several materials, such as cadmium sulfide, silicon, or germanium. They have different values of resistance in the dark and they also change their resistance differently with light intensity. You need to select a resistor that matches your application. The way you use the resistor will also vary depending upon your application, but there is one common start.

Things You'll Need

Photoresistor
Fixed resistor
Voltage source
Multimeter

Instructions

- 1
  Measure the resistance of the photoresistor over the range of light expected when the circuit is in use. Just hook up the multimeter to the photoresistor and measure the value in the expected illumination conditions. For example, you might have a photoresistor that starts off with a dark value of 100 kilo-ohms, but goes through the range from 50 kilo-ohms to 5 kilo-ohms.
- 2
  Select a fixed resistor to build a voltage divider. You want the voltage range to be in a reasonable measurement range as the resistance changes. The voltage range is given by the maximum voltage x (fixed resistor/(photoresistor + fixed resistor)). For the example resistor, you could build a bridge from a 5 volt line to the photoresistor to a 10 kilo-ohm resistor to ground. The voltage is divided at the point between the two resistors. The voltage at that point will be about 0.8 V at the dark end of the illumination range and it will be about 3.3 V under bright illumination.
- 3
  Connect the voltage divider location to the input of the next circuit stage. This could go directly to an analog-to-digital converter as the input for a digital control, or it can go into a comparator to switch a relay and turn on an outdoor light, or it can go into an amplifier to drive an audible indicator of the amount of light on the photoresistor.

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