IR 'slotted switch' sensor

(Circuits -> InfraRed)
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(From Paul Hitchcock)

There is a type of detector known as a "slotted switch" that consists of a phototransistor/LED pair mounted on a solid frame with a small air gap between the two elements. A typical circuit might be:

               o +5v     o +5v
               |         |
               |         |
               \         \
    220 ohms   /         / 4.7K
               \         \
               /         /
               |         |
               |         +-------> Vout
              _|_        /
              \ /      |/
             -----     |\ NPN
               |         \
               |          |
               |          |
               |          |
              GND        GND
                | air gap |

When the air gap is unobstructed, the transistor saturates, pulling Vout to ground; when the gap is blocked, the transistor cuts off and Vout is +5 volts.


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