INFRA-RED LIGHT BARRIER
Infrared light barriers enjoy great popularity as
timing devices at sports venues, as detectors in
alarm installations, as optoelectronic switches in
counting equipment, and many others, because of
their low cost and immunity to electrical interference.
The present light barrier consists of a transmitter 1N4148
and a receiver.
The transmitter, shown in figure 1, consists of a stable multivibrator
(AMV), IC3. The output of the AMV, pin 3, consists of a pulse stream with a
duty factor of about 30 percent. The output is connected to a constant-current source, T2 This source provides infra-red transmit diodes D7 and D8 with a current of just over 20 mA, which pulsates in rhythm with the output signal of the AMV. The infra-red light is, therefore, transmitted in rhythm with the pulse stream also.
The receiver, shown in figure 2, is based on an
SL486 demodulator, 1C1. The output of
logic level at the input of N1 remains low as long as
D4 receives the pulsating infra-red light. Because
which drives a piezoelectric buzzer, is switched off.
Relay Re1 is, however, energized via N4 and transistor T1
When the pulse stream between D7-Ds and D4 is
broken, the logic level at pin 11 of IC1 goes high, so
that the output of N1 becomes logic 0, which triggers monostable N2 via D1. Oscillator N4 is then switched on and actuates the buzzer. At the same time
, N3 ensures that T1 is switched off so that therelay returns to its quiescent state.
When the monostable pulse decays, which with the
stated values of R4 and C10 is after about 5 seconds,
oscillator N4 stops and the alarm tone ceases. Diode
D3 ensures that the relay remains in its rest state,
however, by transferring the high voltage level of
the collector of T1 to the input of N3 whose consequent low logic output continues to hold the transistor off.
The equipment switched by the relay contacts,
therefore, does not only indicate when the light barrier has been interrupted but also when the supply voltage has failed.
The relay is re-energized when resetting
t switch S1 is operated. If D3 and S1 are omitted,
the relay is
Current consumption of the transmitter is about
50 mA; that of the receiver around 10 mA.
The printed circuit board shown in figure 3 is in-
tended to be cut into three along the dashed lines,
although it may not be necessary for some situations
to cut the relay section from the receiver section. If
the latter two are separated, they should on completion be interconnected by a suitable cable.
