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.
Sources
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- Paul Hitchcock
[Category:InfraRed](/circuits/infrared)