![]() ![]() In this article we show how to build a very simple– perhaps even the simplest– darkness-activated LED circuit. (Our own solar circuit collection is here.) Look here for some op-amp based photodetector circuits with LED output, and check out some of the tricks used in well-designed solar garden lights, which include gems like using the solar cell itself as the sensor. ![]() ![]() ![]() The time-honored tradition is to use a 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. Here’s a simple problem: “How do you make an LED turn on when it gets dark?” You might call it the “nightlight problem,” but the same sort of question comes up in a lot of familiar situations– emergency lights, street lights, silly computer keyboard backlights, and the list goes on. ![]()
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