78 Silicon chip Australia’s electronics magazine siliconchip.com.au
voltage of D7 and the voltage divider
effect of the 3.3kW and 10kW resistors,
with the output of IRD1 low, the volt-
age at pin 2 of IC1a should be 3.4V
(0.6V + 11.4V × 3.3kW ÷ 13.3kW), just
below that threshold.
We don’t want IC1a’s output to go
high (because of its input voltage be-
ing low) when there is no IR activity,
ie, when pin 1 of IRD1 is high.
In that case, the voltage at pin 2 of
IC1a would normally be around 7.2V
(5V [IRD1] + 0.6V [D7] + 6.4V × 3.3kW
÷ 13.3kW), well above the 5.4V worst-
case threshold which might trigger the
timer reset.
Note that the PIR-Triggered Mains
Switch PCB has provision for mount-
ing a 3-way terminal block in place
of CON1, which provides all the con-
nections you need to add the extra
circuitry.
Its terminals connect to the 12V sup-
ply, pin 2 of IC1a and ground.
So you could wire up that extra cir-
cuitry on a small piece of veroboard,
mount it so that the infrared receiver
pokes through a hole drilled in the
front panel, then just run three wires
back to that terminal block (or solder
them straight to its pads).
All you need to do then is power up
the unit, plug in a mains load (a lamp
will do), set it to a short delay (eg, 7.5
seconds), switch it on and check that
the lamp goes off after the set time.
Then point your TV remote at the
IR receiver and hold down a button.Check that the lamp switches back on.
If it doesn’t, you may need to lower
the value of the 100nF capacitor shown
in red on the circuit diagram.
That’s because infrared receivers
generate a short burst of pulses and
it’s possible that the RC filter formed
by the added 3.3kW resistor and the
100nF capacitor could have a long
enough time constant that the pulses
are effectively filtered out. Try drop-
ping it to 10nF and if that doesn’t
work, try 1nF.
Once it's working, you can then set
the timeout to a more sensible value
(32, 64 or 128 minutes) and plug your
TV in. Happy snoozing!
Eric Richards,
Auckland, New Zealand ($55).