This is a battery powered IR Link which
may be used in more than one room. The standby current
is extremely low - giving a good battery life; and by
shutting down in the presence of extraneous IR radiation
it copes with the problem of excessive output current.
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This circuit is not powered directly
from the battery. When a remote control signal is
received, the energy stored in C2 drives the emitter
diode. At the same time, Q1 switches on briefly to allow
the battery to recharge C2. The green LED shows that the
circuit is transmitting; and the yellow LED confirms
that C2 has been topped-up.
There is unwanted IR radiation in both
daylight and tungsten lighting. To minimize its effect
use an opaque housing and do not make the opening too
large. (Try a horizontal slot measuring 2 cm X 1.5 cm.)
Shade the receiver diodes by mounting them side-by-side
a few centimetres deep, inside the case. The depth of
shading required will depend on the lighting conditions.
(Try 5 cm to start with). To reduce the effects of
visible light, use receiver diodes with a built-in
daylight filter ( Maplin CY91Y). Or cover the opening
using a small piece of dark transparent plastic. Part of
the display panel from a scrap VCR is ideal. Position
the unit out of direct light and avoid reflective
surfaces. If all else fails, adjust VR1 to reduce
sensitivity. What you are aiming for is to ensure that
in standby mode Q2 remains switched off so that C2
retains its charge. If unwanted radiation does reach the
receiver it will not result in a large output current.
C2 simply discharges and the circuit shuts down. When
the source of the unwanted radiation is removed the unit
may be reset by interrupting the power supply for a few
seconds or by pushing the (optional) reset button. If
you do neither then it will reset itself after about an
hour when C2 has recharged through R7. With two receiver
diodes wired in parallel, the operating range is up to
about 1 meter. The exact distance depends on the remote
you are using and on the position of VR1 (start by
setting it about halfway). Correctly focused, a plastic
lens from a small magnifying glass will extend the
distance.
I used the high gain version of the
BC337 because that was what I had available. However,
the only transistor whose gain is likely to be important
is the BC547C. For the infrared emitter I used a TIL38 (Maplin
YH70M) at the end of 12 meters of alarm cable. However,
the diode from a scrap remote control should be worth
trying also. Two diodes wired in series will give
improved output performance.
The circuit was designed with a small
9-volt alkaline battery in mind (PP3, MN1604, 6LR61) but
the prototype worked well at 6-volts using four AA
batteries. The standby current was too small to measure
reliably. An earphone socket makes the unit portable; so
it can be used in more than one room. If you can obtain
the style of socket in the diagram (Maplin HF82D), its
normally closed switch can be converted to a normally
open switch by releasing the inner contact as shown.
This means that it will act as an on/off switch when you
unplug the lead; and because it allows you to interrupt
the power supply, there is no need for a reset button.
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