Did you know you can use an LED as a light sensor? In this video I show you how to use this phenomenon to build a laser tachometer capable of measuring the RPM of mechanical devices.
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This video shows you what a photoresistor is, where to get one, and how to use one to turn a transistor on and off. You can then use the circuit to build a laser tripwire, just like in the movies! When the laser beam gets broken, you can toggle whatever you want.
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