siliconchip.com.au Australia’s electronics magazine April 2019 77
voltage to the multimeter terminals
while pressing S1.
The second pole of double-pole
pushbutton switch S1 disconnects the
circuit from the test terminal when it
isn’t in use, so that it does not interfere
with the other functions of the DMM.
Assembly
T1 was wound on a toroidal ferrite
core of 9mm OD, 5mm ID and 6mm
thick. It was bifilar wound with 18
turns of 0.3mm enamelled copper wire.
Make sure you connect the start and
finish of each winding with the orien-
tation shown in the circuit diagram. If
the oscillator doesn't work, try swap-
ping the start and finish of either wind-
ing but not both.
The size of the toroidal core is re-
stricted by the space available inside
the DMM. If you don't have a suitable
one in your junk box, you could try
stacking two Radio Spares Cat 467-
4239 cores. You may need to modify
some of the component values to get
the desired performance.
When building the board, the lay-
out is not critical but keep the leads
to T1 short.
For S1, I used a tactile membrane
switch, mounted on the outside of the
DMM (see photo) and wired to the bat-
tery and veroboard. You only need to
make three connections to the board:
the ground and V+ test lead (via S1)
and the +9V supply from switch S1.Using it
The zener to test is connected like a
resistor when you measure resistance,
but the polarity matters. If connectedthe wrong way around (with the anode
to the positive lead), you will simply
measure the forward voltage of around
0.6-0.7V.
Switch the DMM to a suitable DC
voltage scale and press S1 for 1-2 sec-
onds. I find the accuracy to be surpris-
ingly good.
You should keep presses of S1 brief
since the circuit draws about 60mA
from the battery during the test. But
you would have to use this mode a lot
to significantly reduced the battery life,
as long as you keep the presses short.
You can check the condition of the
battery using the same procedure but
with no device connected. If the bat-
tery is good, you will get a reading
above 50V.
Colin O'Donnell,
Adelaide, SA ($70).If you have the same problem that I
do, of falling asleep in front of the TV,
you will find this simple modification
to an existing kit very handy indeed.
Some TVs include a sleep timer
function – mine does, but I find it very
awkward to use. So I came up with this
design as a simple way of switching
the TV off automatically.
It’s based on the PIR-Triggered
Mains Switch project, published in
the February 2008 issue. That’s still
available as a Jaycar kit, Cat KC5455.
You only need a few extra parts to
modify the kit, which you can also
get from Jaycar.
The result is a mains timer which
can be set for a timeout of between 7.5
seconds and about two hours, which
is automatically reset each time you
press a button on your TV remote
control.
So as long as you use the remote at
least once every hour or so (to change
the volume, channel, whatever), the
TV will remain on. If you fall asleep,
unless you’re pressing buttons in
your sleep, the TV will eventually
switch off!
If you wake up later and still want
to watch TV, all you have to do is hit
a button on the remote to reset the
timer and switch the TV back on. (Or
you could leave it off and go to bed...)Automatic sleep timer for TVs
There are just five parts that need
to be added to the circuit, shown in a
cyan-shaded box.
REG1 and its output filter capacitor
provide a regulated 5V rail to run in-
frared receiver IRD1.
When you press a button on your
TV remote control, the signal is picked
up by IRD1 and its output goes low.
That results in pin 4 of CON1 being
pulled low, via 1N4148 diode D7 and
the 3.3kW resistor.
The lowest negative-going voltage
threshold for the input of a 4093B (pin
2 of IC1a) with a 12V supply is 3.5V
(the highest threshold is 5.4V).
Taking into account the forwardGot an interesting original circuit that you have cleverly devised? We will pay good money to fea-
ture it in Circuit Notebook. We can pay you by electronic funds transfer, cheque or direct to your
PayPal account. Or you can use the funds to purchase anything from the SILICON CHIP Online
Store, including PCBs and components, back issues, subscriptions or whatever. Email your cir-
cuit and descriptive text to [email protected]Circuit
Ideas
Wanted