Infra-red Light Barrier Using 555


This is a short-range
light barrier for use as an intruder alarm in doorposts, etc. The 555 in
the transmitter (Figure 1) oscillates at about 4.5 kHz, supplying
pulses with a duty cycle of about 13% to keep power consumption within
reason. Just about any infra-red LED (also called IRED)
may be used. Suggested, commonly available types are the LD271 and
SFH485. The exact pulse frequency is adjusted with preset P1. The LEDs
are pulsed at a peak current of about 100 mA, determined by the 47 Ω
series resistor. In the receiver (Figure 2), the maximum sensitivity of
photo-diode D2 should occur at the wavelength of the IR LEDs
used in the transmitter. You should be okay if you use an SFH205F,
BPW34 or BP104. Note that the photo-diode is connected reverse-biased!
So, if you measure about 0.45 V across this device, it is almost
certainly fitted the wrong way around.

Transmitter

Transmitter

The received pulses are first amplified by T1 and T2. Next comes a PLL (phase lock loop) built with the reverenced NE567 (or LM567). The PLL
chip pulls its output, pin 8, Low when it is locked onto the 4.5 kHz
‘tone’ received from the transmitter. When the (normally invisible)
light beam is interrupted (for example, by someone walking into the
room), the received signal disappears and IC1 will pull its output pin
High. This enables oscillator IC2 in the receiver, and an audible alarm
is produced. The two-transistor amplifier in the receiver is purposely
over-driven to some extent to ensure that the duty cycle of the output
pulses is roughly 50%.

Receiver

Receiver

If the transmitter is too far away from the receiver, over-driving
will no longer be guaranteed, hence IC1 will not be enabled by an alarm
condition. If you want to get the most out of the circuit in respect of
distance covered, start by modifying the value of R2 until the amplifier
output signal again has a duty cycle of about 50%. The circuit is
simple to adjust. Switch on the receiver, the buzzer should sound. Then
switch on the transmitter. Point the transmitter LEDs
to the receiver input. Use a relatively small distance, say, 30 cm.
Adjust P1 on the transmitter until the buzzer is silenced. Switch the
receiver off and on again a few times to make sure it locks onto the
transmitter carrier under all circumstances. If necessary, re-adjust P1,
slowly increasing the distance between the transmitter and the
receiver.


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