Specifications

CIRCUIT IDEAS
sary drive
given to triac-
5. Similarly,
when outputs
Q6 through
Q9 become
high, triac-4
through triac-
1 go off one
by one and
the earlier lit
up bulbs go
off last. The
second part
of the se-
quence is also
over and then the
cycle repeats itself endlessly.
PROXIMITY DETECTOR
T
his proximity detector is con-
structed using an infrared diode
detector. Infrared detector can
be used in various equipment such as
burglar alarms, touch-free proximity
switches for turning on a light, and sole-
noid-controlled valves for operating a wa-
ter tap. Briefly, the circuit consists of
an infrared transmitter and an infra-
red receiver (such as Siemens SFH506-
38 used in TV sets).
The transmitter part consists of two
555 timers (IC1 and IC2)
wired in astable mode, as
shown in the figure, for
driving an infrared LED.
A burst output of 38 kHz,
modulated at 100 Hz, is re-
quired for the infrared de-
tector to sense the trans-
mission; hence the set-up
as shown is required. To
save power, the duty cycle
of the 38kHz astable
multivibrator is main-
tained at 10 per cent.
The receiver part has
an infrared detector com-
prising IC 555 (IC3), wired
for operation in
monostable mode, followed
by pnp transistor T1. Upon
reception of infrared sig-
nals, the 555 timer (mono)
is turned ‘on’ and it re-
mains ‘on’ as long as the infrared sig-
nals are being received.
When no more signals are received,
the mono goes ‘off’ after a few seconds
(the delay depends on timing resistor-
capacitor combination of R7-C5). The de-
lay obtained using 470kilo-ohm resistor
and 4.7µF capacitor is about 3 seconds.
Unlike an ordinary mono, the capacitor
in this mono is allowed to charge only
when the reception of the signal has
stopped, because of the pnp transistor
T1 that shorts the charging capacitor
as long as the output from IR receiver
module is available (active low).
This setup can be used to detect
proximity of an object moving by. Both
transmitter and receiver can be
mounted on a single breadboard/PCB,
but care should be taken that infrared
receiver is behind the infrared LED, so
that the problem due to infrared leak-
age is obviated.
An object moving nearby actually
reflects the infrared rays from the in-
frared LED. As the infrared receiver
has a sensitivity angle of 60
o
, the IR
rays are sensed within this lobe and
the mono in the receiver section is trig-
gered. This principle can be used to
turn ‘on’ the light, using
a relay, when a person
comes nearby. The same
automatically turns ‘off’
after some time, as the
person moves away.
The sensitivity de-
pends on the current-lim-
iting resistor in series with
the infrared LED. It is ob-
served that with in-circuit
resistance of preset VR1
set at 20 ohms, the object
at a distance of about 25
cms can be sensed.
This circuit can be used
for burglar alarms based
on beam interruption, with
the added advantage that
the transmitter and re-
ceiver are housed in the
same enclosure, avoiding
any wiring problems.
K.S. SANKAR
RUPANJANA
189