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1.5V LED flashlight

Most flashlights one can buy today are very bright and powerful. I had a small LED flashlight that I bought 16 years ago. It was one of the first LED flashlights and it is not super bright. I use it at night when I need to go to the bath room and I don't want to wake-up everybody. It produces just enough light to see well but it is not blinding and it is not strong enough to wake-up others. The switch on that flashlight broke recently and it is not possible to buy such a flashlight anymore.

I wanted to have a "not so bright flashlight" that runs on a single 1.5V battery and it should run well even when the battery is older such that I can still use some batteries that are too weak for other applications. All LEDs need more than 1.5V to light up. A white LED needs about 3V DC.

A simple circuit known as Joule Thief (see https://en.wikipedia.org/wiki/Joule_thief) can be used to increase the voltage such that a white LED will light up. This circuit is small and makes for a perfect homemade LED flashlight.

1.5V LED booster circuit, Joule Thief, 1.5V LED flashlight
Fully assembled 1.5V LED flashlight

At the heart of this circuit is a little coil which you wind yourself. You need about 10-30 loops on a ferrite toroid. The LED will be brighter with a coil that can store more magnetic energy. To build this you take two parallel enameled copper wires and you loop them together about 25 times through the ferrite toroid. This makes 2 coils on the same ferrite. The coils are connected in opposite direction as indicated by the dots in the circuit diagram. This means you take the start wire of one coil and twist it together with the end of the other coil. Now you have a kind of high frequency transformer with 3 wires: 2 individual wires and the twisted pair. The twisted pair is where you connect plus from the battery. Scrape off the enamel insulation with some very fine sandpaper.

coils on ferrite toroid, ferrite outer diameter: about 9mm

I have used enameled AWG 30 copper wire and 2 different enamel colors, gold brown and red, to be able to easily distinguish the 2 coils on the ferrite. The brightness of the LED will depend on the amount of energy you can store in the magnetic field (the higher the inductance the more energy). As you can see from the circuit diagram the coil that connects to the base of the transistor is only a "sensor coil". It does not build up the magnetic field. Thus if you want to get more loops and a stronger wire (higher current) onto the ferrite then you can save space by using a very thin wire (e.g AWG 42) for one coil and a thicker wire for the other coil (the one that connects to the collector of the transistor).

My setup with two AWG30 wires and 25 turns for both wires results in a current of 5mA DC through the LED (the voltage will be whatever that LED's voltage at 5mA is; input voltage was 1.4V). This was perfect for my not so super bright flashlight. I know that some readers would want to get more out of this circuit and that is possible. You can not only save space on the toroid by using a thinner wire for the sensor coil but also by using a different turn ratio. I have tried 15 turns AWG30 on the sensor coil and 30 turns AWG25 on the coil connected to the collector. To do this you just take two enameled wires of different length. The setup with 15/30 turns and AWG25 wire for the collector coil gave me 8mA DC current through LED (toroid size as shown above; input voltage 1.4V).

The circuit produces an output signal on the collector pin of the transistor that looks as shown below. The output voltage is just a function of the load you put on. It can produce very high voltages if there is no load and you could ruin the transistor if you run this without any LED attached. The circuit oscillates at a frequency of about 20-40 kHZ dependent on the input voltage and the type of coil. LEDs work at those high frequencies but they are not made for that and their internal PN-layers represent a capacitor. The circuit will have a higher efficiency (output light to consumed energy) if you rectify the signal a little bit. I have therefore added the 1N4148 diode and a small electrolytic capacitor (anything from 1uF to 10uF will work) between the LED and the output of the transistor.

Output signal at the collector pin of the transistor, about 23kHz, voltage depends on the load

It is possible to set-up this circuit first on a breadboard. Breadboards do not work for high frequency but the 20 kHz to 40 kHz frequencies that this circuit produces are OK. It's a good idea to test the LED and coil combination that works best for you before you put this onto a real circuit board.

Breadboard setup
1.5V LED flashlight on a small dot matrix board
Pinout of all polar components
AAA battery holder made from a tin plated steel sheet (bottom of a chocolate powder box). The spring is from a ball-pen.

The Joule Thief will work at very low voltages and a white LED will light up with about 0.4V from the battery but it will produce just a bit of light. The circuit works well with good brightness between 1V and 1.6V battery output voltage. Even already used 1.5V batteries will have 1V. You can therefore still power this flashlight with a battery that you would throw out otherwise.

Fully assembled 1.5V LED flashlight, the battery holder is made from a tin sheet (bottom of a chocolate powder tin box) and a spring from an old ball-pen
Fully assembled 1.5V LED flashlight, with aluminum foil sleeve to focus the light, the aluminum foil sleeve is plastic coated aluminum foil (insulated) from a butter cookie package

Enjoy the flashlight. Merry Christmas.

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