Take me to the Steps
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I’ve always been interested in electronics, but when I started publishing projects professionally, I got interested in photography, too. I soon realized that with BEAM robots and other small “artisanal” circuits, the batteries are one of the largest and most visible components, and which brand you choose — the colors, the text, the design on the label — can make a big difference in how your pictures come out.

017_assemble_and_insulate_handles batteries

So I started hoarding batteries — even dead ones, if they were unusual- or interesting-looking. And now my collection is slightly out of control, which can be a real pain when I just need to swap fresh ones into the video game controllers. Watch batteries and other “coin cells” are particularly annoying because they’re so small and fidgety, and can be really awkward to pinch between voltmeter probes to test.

This tool gives me a better way. The jaws are insulated from each other, and all you have to do is squeeze. The polarity of the cell is indicated by which of the 2 LEDs lights up, and the charge state by its brightness.

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Project Steps

Straighten the heat sinks

These heat sink clips are intended for close soldering work. The idea is you can attach one between the joint you’re working on and a joint you’ve already made, so that excess heat will get drawn off into the large metal body of the clip, instead of re-melting the previous joint. They’re made of thin, plated spring steel that has just the right flex for tweezers.

First, take one heat sink and unhook the jaws. Then cut off the rubber insulation with a hobby knife, and discard it.

Straighten the bend in the heat sink, first with your hands, and then by peening it flat over a hard surface with a hammer. If you don’t have an anvil, a smooth concrete slab works fine for this.

Grab one of the 2 jaws with a pair of pliers, right where it thickens into the handle, and bend it back and forth, fatiguing the metal until it breaks. Discard the small fragment, then smooth off and round the broken edge with a file. Repeat Step 1 with the second heat sink.

Drill the holes

The problem here is getting the holes in both jaws lined up. On the other hand, these really don’t have to be precise, as a coin cell is a big target and it doesn’t matter much if the jaws don’t align precisely. If you’d rather not mess with double-stick tape, you could just measure, mark, and drill the 2 jaws separately.

Stack the 2 flattened heat sinks as in the finished tool, align the edges carefully, and temporarily secure with a layer of double-stick tape.

Mark a centerline 3/16″ from each edge, roughly 2″ in length, starting at the rounded end of the jaws. Measuring from the same end, add one tick mark at 3/16″, a second at 13/16″, and a third at 1-7/16″.

Drill a 1/8″ hole through the stacked jaws at each of the 3 tick marks, then come back and drill out the center hole to 13/64″ for the LEDs. Separate the jaws and remove the tape.

Install the first LED

With LEDs, remember that the long lead gets connected to the current source. In this case, that means each long lead should be connected to the same jaw that the LED is mounted in, and the short lead to the jaw opposite. The crossover leads are insulated with heat shrink tubing so they don’t short if pinched or twisted together.

Assemble one of the jaws, 2 standoffs, 4 ring tongues, and 4 standoff screws as shown. Insert one of your LEDs into the large hole in the jaw from inside, then bend the long lead over and insert it into the forward ring tongue on the same side of the standoffs as the LED and the jaw. Trim the lead as needed, and solder in place.

Slip a 3/8″ length of 1/8″ heat shrink tubing over the remaining LED lead and apply heat from a cigarette lighter to activate it.

Bend the uninsulated part of the lead over and insert it into the rear ring tongue on the opposite side of the standoffs to the LED and the jaw. Trim the lead as needed, and solder in place.

Rinse and repeat

If you need to loosen, rotate, or even remove the ring tongues to get the LED leads inserted, that’s fine. What’s important is to position the LED correctly in the mounting hole and to orient the lugs correctly to the standoffs and the jaw when you finally solder the leads in place.

Mount the other jaw to the standoffs, then temporarily unmount the jaw with the installed LED and set it aside.

Repeat Step 3 to install the other LED.

Reattach the first jaw, tighten the screws down securely, then work a 1-3/4″ length of 3/8″ heat-shrink tubing over each jaw, from the front, and apply heat with the lighter to activate.

Conclusion

To use the tweezers, grip at the insulation and squeeze to close the jaws across the thickness of a coin cell. If neither LED comes on, the cell is dead and can be discarded. If an LED shines brightly, the cell is giving 3V or something close to it. If it shines softly, the cell is giving 1.5V. The LED that lights up is on the positive side of the cell. If you need a more precise measurement, it would be easy to add some wire leads and banana plugs to connect the tweezers to your digital multimeter.