Single chip detects optical interruptions

-March 01, 2001

Setting up a light beam and detector to count objects on a conveyer belt, sense security intrusions, or drive a tachometer is simple. However, the task is no longer trivial if you add ambient light or multiple beams, limit optical power, or extend the distance of the light beam more than a few inches. You can use optical lenses and filters and high-power optical sources on the light-path side to improve performance. On the electronic side, servo-bias control of the detector and electronic modulation and filtering of the light beam can add considerable range. The circuit in Figure 1, which you can use with these performance improvements, economically provides a minimal-parts-count circuit with negligible power requirements to achieve approximately a foot of useful range even under varying ambient-light conditions.

The venerable LM567 PLL is the only IC in the circuit. The 567's oscillator directly drives an infrared LED on the optical-transmitter end. When the pulsed light returns to the IR phototransistor, a single-stage 2N2222 transistor amplifies the resultant signal to drive Pin 3 of the LM567. Thus, the circuit essentially directs the PLL to lock to itself, which makes Pin 8 go low. The values of R1 and C1 provide operation of approximately 3 kHz, and the filters set by C2 and C3 provide a clean output from the 567. Operation from 2 to 5 kHz works best. Lower frequencies require more conditioning and thus larger and more critical values of C2 and C3, resulting in longer response times and possible jitter. Higher frequencies result in lower efficiencies for the cheap LED and phototransistor. However, tachometers may require higher frequencies. IR components are unnecessary. Two same-color LEDs (one for the photo detector) also work to a degree.

Ambient light or another beam breaker's IR light doesn't false-trigger the circuit unless significant near-frequency light content exists. However, ambient light can swamp the detector, so you may need to adjust the R2 bias for your application. Of course, using a self-adjusting module with IR filters can easily increase the range by two orders of magnitude.

One interesting variation of the circuit is to use two or more devices on the same frequency, forming a ring. All devices lock, and both ends detect a break in any beam or a modulation of the frequency of any device for communication.

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