Chapter 31. Experiment 31: Electronic Optics

Two basic types of sensors are triggered by variations in light. There is the active type and the passive type.

The Chronophotonic Lamp Controller that I described in Experiment 7 used a phototransistor, which is a passive sensor. It just sits and waits, measuring light from outside sources and changing its effective internal resistance accordingly.

Another common passive light-sensing component is a PIR motion sensor, where the P in the acronym stands for “passive” and the IR stands for “infrared.” These are commonly used to switch on lights or trigger alarm systems when they sense body heat from a person moving around.

PIR motion sensors are useful, but the complete units that you find in hardware ...

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