20 mA, which a dead 9V battery seems to do on its own
due to its higher internal resistance.
We can make the circuit by simply connecting the
battery to the resistor and LED in series, with a switch to
turn it off by disconnecting the battery from the resistor.
Schematic 1 shows the circuit using the on-off-on switch
with the resistor across the two outputs of the switch.
Photo 1 shows the simple circuit using the on-off-on
switch to either use the resistor or not use it, depending
on the voltage the battery can maintain while driving the
LED. It’s not easy to tell if the 9V battery is weak enough
to drive the LED directly, so the safest
way to tell is to use a voltmeter, or to
wait until the LED is obviously getting
dimmer and try it without the resistor.
After I built this circuit, I came up
with a much simpler way to attach
the resistor. In my new approach, the
resistor is soldered to the two ON
sections of the on-off-on switch, and
then one wire from one of those ends
is connected to the cathode of the
LED. This reduces the amount of wire
needed. I hope you enjoy building this
circuit. Now, you can use those dead
9V batteries as small flashlights or even
as personal nightlights.
The Simple Joule
Thief Night Light
as a Soldering
My use of the term “soldering
islands” here is similar to the
Manhattan style of circuit building. One
can do searches online to find articles on this construction
technique. A good one I’ve come across is by K7QO.
Manhattan Style construction uses small islands for solder
points. These are usually cut from double-sided copper-clad
circuit boards. The bottom side is then soldered to another
single-sided copper-clad board and positioned to follow
the circuit; either to make it easy for the signal, easy to
construct, or both. The top side of each little island is then
used to connect various leads. These islands can be either
round or rectangular.
The difference between Manhattan style construction
and these soldering islands is that there is no way to
connect the soldering islands to a copper-clad circuit
board without shorting out the entire circuit. These islands
must either be glued to something — another board or the
bottom or side of a box, for example
— or float with enough room between
them that they will not short out.
Another method is to use very
high-valued resistors — 1 MΩ or more
— to keep all the solder islands at a
good distance from each other. When
using these with a joule thief circuit,
we have the stiff resistor, LED, and
transistor leads to help maintain the
distance between the solder islands.
I use this island method because
there are times when I need to
connect more than just two wires
or leads together. I find that a small
solder island makes this very easy to
do. Using only the wires or leads is far
more frustrating because the lead or
wire never wants to stay in the position
I need long enough for me to solder
it to the other leads. Or, as I warm up
the other two or more leads already
soldered together, one of them pops
off and I have to fight with all the leads
to get them back together.
Other than using a soldering island,
Post comments on this article and find any associated files and/or downloads at
Photo 1: Dead 9V battery flashlight.
Photo 2: Close-up of resistor attached to the
Photo 3: The plastic wrap box.
September/October 2018 41