Description
Many households are still graced by tube-type television sets. If you
want to connect one of these large tellies to your stereo system to
improve the sound quality, this is usually not a problem because there
are plenty of SCART to Cinch adapters available in accessory shops.
However, with some sets your pleasure is spoiled by the fact that the
audio outputs of the SCART connector are not muted during channel
switching.
This can sometimes lead to nasty signal spikes, which can cause the
loudspeakers of your stereo system to emit irritating popping and
cracking noises. In such cases it is a good idea to fit your system with
a mute circuit. Fortunately, the right time to activate the mute
circuit is defined by the fact that the happy zapper presses buttons on
the remote control to switch channels, and the remote control emits IR
signals.
There are even inexpensive ready-made IR receiver modules available,
such as the TSOP1136 used here, which produce trains of active-low
pulses in response to such signals. About the circuit: when no IR signal
is present, a capacitor is charged via P2 and a diode. IC1 is a
comparator that compares this IR voltage (applied to its non-inverting
input on pin 3) to a voltage applied to its other input on pin 2.
Circuit Diagram:
This reference voltage, which can be adjusted with P1, determines the
switching threshold of the comparator. If IC2 receives an IR signal, T2
conducts, and as a result the voltage on C1 drops rapidly below the
threshold level set by P1. This causes T1 to change from its previous
‘on’ state to the ‘off’ state. As a result, the relay drops out and the
audio link to the stereo system is interrupted for the duration of the
noise interval.
It’s all quite simple, as you can see. If you do not have a
stabilized 5-V supply voltage available, you can use the circuit at the
of the schematic diagram (with a 5-V voltage regulator) together with a
simple (unstabilised) AC mains adapter that supplies a voltage in the
range of 9 V to 12 V to the 7805 (IC3). You can also use a relay with
normally-closed contacts instead of normally-open contacts.
In this case, simply swap the signals on pins 2 and 3 of IC1 so the
relay pulls in when an IR signal is received instead of dropping out.
This saves a bit of power because the relay is only energized during
zapping. If you can’t find any worthwhile use for the second comparator
of IC1, it’s a good idea to connect pin 6 to +5 V and pin 5 to ground.
To improve noise immunity, you should shield the IR sensor so it is not
exposed directly to light from a fluorescent fixture.
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