Nuts and Volts - September/October 2018

The Dead 9V Battery Flashlight

I’m sure that by now, most of you have come across a circuit called a “joule thief.” This is a simple self-oscillating voltage booster which most people build to use with either an AA or AAA battery. Any “dead” 1.5 volt battery will do, where the voltage has dropped to about 1.0-1.1 volts or lower. The fact is that you can use a good battery as well, and probably every one of the small flashlights on the market that use only one AAA or one AA battery is using a joule thief circuit.

This will be a great way to use the dead AA and AAA batteries I end up with from battery-operated things around the house. So, what can one do with all those dead 9V batteries? These are generally down to about six volts and seem to be useless since they are rather low current batteries even when new.

The circuit I want to discuss here is one that would be a great project for a newbie to learn about soldering on. All that is needed is an ultra-bright white LED, a 220-330 ohm 1/8-1/4 watt resistor, a “dead” nine volt battery, battery holder, and something to put it all in. You can add an on-off switch or even an on-off-on switch.

A word of warning here for those who use rechargeable batteries: Do NOT use a rechargeable battery in one of these flashlights.

It will be dead when you take it out because the simple version of these circuits will use the battery down to below a half volt. Being one of the more efficient projects I’ve built, this circuit will still be putting out usable light down to about a quarter volt. This is way below where a rechargeable battery would be ruined. I went with an on-off-on switch because an on-off version wouldn’t have the option of driving the 3V ultra-bright white LED directly. You could also build two circuits: one to use the resistor; and one not using the resistor for when the battery is below four volts.

I find that once the 9V battery is reading 3-4 volts when using the resistor and LED, it can be used without the resistor to drive the LED. At this point, the internal resistance of the 9V battery is high enough to limit the current, so no external resistor is needed.

Also, I’ve measured these currents at around 10 mA, which is not even close to the 30 mA max current, or even the recommended 20 mA for longer life for these LEDs.

Remember that it’s the heat generated by the LED that eventually kills it. Two ways to limit the heat buildup are to either drive the LED intermittently — as is done when using the joule thief circuit — or limiting the DC current to below

Super Simple Projects for Lear ning Solder ing and More

I’ve had a couple fun circuit ideas in the back of my head for years and started playing with them, changing things, and finally formalizing them. They’re simple and are a good exercise in soldering and circuit basics. By Phillip Karras