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The detector is designed to recognize obstacles at distances from a few millimeters to several centimeters. Similar sensors are used in industry and medical services, such as to open faucets via magnetic valves. Sensor IC2 is a Siemens SFH900 optical isolator or similar. The phase-locked loop (PLL) in the IC1 decoder compares the frequency of IC2's input signal with the frequency of the internally generated signal. When two signals fall into the same band, the output, pin 8 of IC1 will change state (high to low).
The internal oscillator generates a signal with a frequency of about 4.5 kHz (determined by the time constant R1-C1. Its rectangular signal on pin 5 turns on IC2's light-emitting diode through T1. The diode then transmits a pulsed infrared light signal at 4.5 kHz When infrared light is reflected from a nearby object, the phototransistor of IC2 will provide a signal to pin 3 of IC1. If the frequency is of this signal is in the same band as the internal emitter frequency, pin 8 is grounded, then diode D1 lights up. PLL comparator prevents the circuit from reacting to stray light, can be changed by P1.
Detectors with specified components consume 10-30 mA current. As indicated above, opto-isolators can be of many types. It can also be made from a discrete LED and a phototransistor, but great care must then be taken so that the phototransistor cannot receive the light transmitted through the LED. A suitable solid state relay at the output makes it possible to switch larger loads. The IC1 circuit can transfer currents up to 100 mA to ground. Diode D1 must then be bypassed.