test out some part-saving ideas.
Generally speaking, the more parts we try to save,
the more complex a circuit becomes. This was definitely
the case with this combined project. It’s no longer super
simple, but it’s not all that difficult either.
I’ll explain a bit first, and then show the photos of the
two different approaches. I’ll include the schematic as well.
The first circuit used two switches: an on-off-on single
pole switch for the 9V dead battery side of the project;
and a double-pole-double-throw (DPDT) switch for the two
AAA and AA battery holders. This powers the joule thief
circuit and also disconnects the 9V battery section to the
off joule thief circuit. In this circuit, the two battery holders
are connected together in parallel so only one battery at a
time can be in a holder; refer to Photo 7.
In the second circuit, I used two LEDs to separate the
9V battery from the joule thief circuit. This was necessary
because I removed the DPDT switch and replaced it with
a second on-off-on switch. This meant there was no way to
separate the 9V from the joule thief circuit via the common
positive lead of the LED. By adding a second LED in series
with the first LED, this LED could be
used as a blocking diode, restricting the
current path from the 9V battery to the
joule thief circuit.
The joule thief circuit is now
connected to the second LED in the
series and so it drives both LEDs. This
means that the joule thief circuit had
to boost the voltage to 6V to turn on
both LEDs in series. The only reason the
joule thief stops at 3V is because that’s
when the LED turns on and stops the
voltage increase of the oscillator. So, the
oscillator boosts the voltage to what is
needed to turn on both LEDs.
By removing the more complex
and expensive switch, I had to add the
blocking diode. I figured I’d use another LED to do the
blocking since I could then get light out of it as well. This
saved the expense of the DPDT switch but added a minor
cost for the second LED. This saved a little money probably
at the expense of a shorter battery life.
You’ll also notice that I removed the AAA battery
holder. This was because I found that the AAA battery will
work in a double-A battery holder. It’s not as secure, but
as a night light it doesn’t need to be. As a flashlight for
children, I’d keep the AAA battery holder.
With an on-off-on switch, I found another way to
hook up the double- and triple-A holders. Use the center
connector of the switch for the output to the joule thief
circuit, and then each battery holder could attach to the
switch on either side and could be selected to supply
In this way, all three batteries could be installed. As
one starts to get too dim, the weak battery could be turned
off and then one of the other batteries could be switched
in. Check out Photo 8.
Schematic 2 shows the two LED AA and AAA battery
9V + joule
Photo 8: Using
two LEDs to
Schematic 2: Two LED version
of the dead 9V/dead AA/AAA
September/October 2018 43