Improved single-cell LED flashlight

Being a flashlight builder since age 7, I can't resist the temptation to build more and more of them, even 50 years later. The technology has changed, but the fun remains the same. Flashlights can be large and powerful, or small and convenient to carry. In the latter category, flashlights using a single cell are very convenient, due to their size and usage simplicity. In the old times of incandescent bulbs, all it took was a 1.2V bulb, placing it in a reflector and wiring it up with a battery, so that flashlight building was a purely mechanical task. But in 2015 the device of choice is the LED, due to its far better efficiency and reliability. And since white LEDs need around 3V to operate, they require electronics to be used with a single, cheap AA cell. 

I had already built single-cell LED flashlights before, using several small LEDs in series, but when I stumbled across half-watt LEDs with built-in lens, it struck me that these are ideally suited for homebuilt flashlights using a single AA cell. So I set out to build another nice little lamp.

This one has SMD electronics, on a tiny printed circuit board, and a machined aluminium body. Instead of a switch, the rear end is screwed slightly out to turn the lamp off, by opening the battery contact.

The circuit is about as simple as it can be: A free-running, self-excited boost converter, using a low saturation voltage transistor and a Schottky rectifier, to converter the input voltage to what the LED needs.  This circuit is able to work down to a very low input voltage, making full use of the battery cell. 

I used a factory-made SMD inductor, removed from a junked portable CD player, along with a few new SMD parts. The inductor got a hand-wound secondary winding, for the feedback.  I adjusted the operating power by selecting the proper value for the resistor in the circuit, which controls the transistor's base current, thus defining at what collector current it will desaturate and thus end the inductor charging in each cycle.

A little round single-sided printed circuit board was made, which carried all the components, assembled in a very compact way. The LED, which is a 10mm through-hole type,  goes on the non-copper side, along with the two tantalum capacitors, which make good use of the space between the board and the LED. All other parts are installed in normal SMD style on the copper side of the board.

As you can see, even this small board has a lot of space left, since the circuit is so simple.

For the tests I left one LED pin and another piece of wire stick out, to make convenient connection points for the battery. Later, not shown in this photo, I clipped the right-side wire and soldered a small brass cylinder into/onto the center hole, to act as contact point for the negative battery pole. The wire on the left side, which is the positive connection, was simply bent to act as a spring and make contact to the aluminium housing, whose cap connects to the battery's positive side. That's very crude, but sort of works. Feel free to use a better arrangement, that doesn't use aluminium as contact material...

This shows how small the assembly is. Well, it's a 10mm LED, and the PCB was made 15mm diameter, the same size to which I turned the inside of the housing, to fit the AA battery, which may be up to 14.5mm in diameter.

Playing with the little lamp, before building the housing.

Here is the PCB layout. You have to scale it so that the outer diameter of the outer copper arc is 15mm.

It's seen right onto the copper, with no mirroring, so make sure to invert it (mirror view) if you want to print the image and place it ink-down onto your sensitized board.

And this drawing shows how the parts are located on the board. As usual on my site, you have to work out yourself which part is which.

Two holes are for the LED pins. The center hole is to center the negative connection post, and the last hole has a wire soldered through, that connects to one of the tantalum caps between the board and the LED, and has its other end contacting the aluminium housing.

Seen from the front. I made a reflector-like shape in the aluminium, but that's really not very important, because most of the light concentration is done by the LED's built-in lens, which has roughly a 30° opening angle.

If you really want to copy my flashlight's aluminium body, be my guest. Here is the design, with all dimensions. You can click it to get a larger version. If you prefer, you can also download the file in DXF format, and open it in the CAD program of your preference.
 
In this drawing the electronics are only minimally shown: The PCB, the negative connection post, and the positive cap of the battery.

The drawing is repeated several times, to accomodate all dimensions without making too much of a mess.

Of course you will need a lathe to make this housing.

Note that one mechanical piece isn't drawn here: A foam rubber ring that goes inside the cap, and keeps the battery from rattling around when the end cap is screwed out a turn or two to keep the lamp off.
 


And here is the shootout! The left lamp is an old-fashioned Mini-Maglite that uses two AA cells and a tiny incandescent bulb. On the right, my LED lamp, using a single AA cell. Both  lamps have fresh batteries in them.
 
This circuit runs the LED at roughly 100mA, which should be safe given its 150mA spec. It draws  around 250mA, when the battery is at 1.3V. It works fine both with an alkaline or a rechargeable cell, and should run roughly 7-8 hours on the alkaline, and slightly less on a rechargeable.

I'm writing this page roughly 7 years after having built this lamp. The little lamp has worked well except for occasional contact problems between the positive contact wire and the aluminium body. Someday I will improve that by running a soldered metal foil all the way from the PCB to the positive side of the cell. And it has had one accident: A Duracell leaked, and made a terrible mess!
 
It's a very practical flashlight to carry around for emergencies, because it's small enough to disappear in a pant pocket, without being felt, and still bright enough to find one's way in the dark, work in dark places, etc.

Of course these days everybody runs around carrying a cellphone, and every cellphone has a flashlight built in... but this little lamp is a lot smaller and lighter than a cellphone!
 

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