A Simple and Cheap Dark-Detecting LED Circuit
...circuit with a CdS photoresistor , sometimes called a photocell or LDR, for "light-dependent resistor." (Circuit Example 1 , Example 2 .) Photoresistors are reliable and cost about $1 each, but are going away because they contain cadmium, a toxic heavy metal whose use is increasingly regulated. There are many other solutions as well. Look...
