What Is a Photoresistor? - dummies

By Cathleen Shamieh

A photoresistor, which is also called a light-dependent resistor (LDR) or photocell, consists of a piece of semiconductor material that exhibits an interesting characteristic: It acts like a resistor except that the value of resistance depends on how much light is shining on it.

You can see an assortment of photoresistors in this figure, left, and a close-up of one photoresistor on the right. Note that these little devices have no identifying marks, such as a model number, which makes working with them a bit sketchy, er, loads of fun!

Photoresistor
An assortment of photoresistors

Pinning down the exact resistance of a photoresistor is like trying to catch a feather as it floats down through the air. Part of the problem is that you never really know exactly how much light is shining — that is, unless you happen to own a special device called a light meter, which measures light energy. (Add it to your holiday wish list or borrow one from a photographer friend!)

In general, a photoresistor works like this:

  • In bright light, its resistance is relatively low (usually less than 10 kΩ).
  • In darkness, its resistance is relatively high (usually more than 1 MΩ).