Optical Interruption Sensor

Using only an 8-pin IC and a few discrete components, you can build the infrared optical interrupter. The NE567
Optical Interruption Sensor - schematic

tone decoder has all the necessary circuit elements: a local oscillator, a PLL decoder, and a 100-mA output-drive capability. The local oscillator, which is tuned to 40 kHz by RT and CT, drives Ql, a universal low-power silicon pnp transistor (such as a 2N3906, BC559, or ZTX500). Ql drives the IR-emit-ting diode. The receiving part of the circuit surrounds the ICs internal PLL input at pin 3. When the pho-todetector, Q2, detects the oscillating IR light beam, the 40-kHz signal appears at pin 3 of the IC, Under this condition, the circuit locks and the ICs output is high. When something opaque comes between the LED and Q2, the 40-kHz signal doesn"t reach the PLL input, and the ICs output goes low. The feedback network between pins 1 and 8 prevents the output from chattering. If you connect this circuit to a high-inertia load (such as a mechanical relay), the output doesn"t tend to oscillate and you can eliminate these feedback components. The circuit works with virtually any LED-photodetector pair, but matched pairs allow for longer distances between the emitter and receiver. The table lists some of the best choices.

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