Infra-Red Proximity Detector

The detector is intended for the recognition of obstructions at distances of a few millimeters to a few centimeters. Similar detectors are used in the industry and health services, for instance, to open a water tap via a magnetic valve. The sensor, IC2, is a Type SFH900 optoisolator from Siemens or similar. A phase-locked loop (PLL) in decoder IC1 compares the frequency of the input signal from IC2 with that of an internally generated signal. When the two signals fall within the same band, the output, pin 8, of IC1 changes state (from high to low). The internal oscillator generates a signal at a frequency of about 4.5 kHz (determined by time constant R1-C1). Its rectangular signal at pin 5 switches on the light-emitting diode in IC2 via T1.

Circuit diagram:
The diode then transmits an infra-red light signal pulsed at 4.5 kHz. When the infra-red light is reflected by a nearby object, the photo transistor in IC2 provides a signal to pin 3 of IC1 If the frequency of this signal lies within the same band as that of the internal generator, pin 8 is connected to earth, whereupon diode D1 lights. The comparison by the PLL prevents the circuit reacting to stray light. The sensitivity of the detector may be varied with P1. The detector with components as specified draws a current of 10–30 mA.

As stated earlier, the optoisolator may be one of several types. It may also be built from a discrete LED and photo-transistor, but great care should then be taken to ensure that the photo transistor cannot receive light transmitted by the LED. A suitable solid-state relay at the output enables larger loads to be switched. Circuit IC1 can switch currents of up to 100 mA to earth. Diode D1 should then be omitted.