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.
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
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.
Author: K. Hagen
Copyright: Elektor Electronics 1998