# Conductive Adhesives?

I'm helping with a kid's Maker's project. We need to attach some LED's to some coin cell batteries for part of a holiday project.

Are there any conductive adhesives we can use for this? Durability is not a factor, since the life of the project will be no longer than the life of the coin cell charge.

• Did you try to Google for "conductive glue"? – Ale..chenski Dec 19 '16 at 2:47
• How about a suitably-sized steel paperclip? They're cheap. Or a small bulldog clip/binder clip, which would be reusable. – Criggie Dec 19 '16 at 11:38
• "...will be no longer than the life of the coin cell charge." Incidentally, I was given some blue LED throwies as described in @HighInBC's answer, and they were still glowing (dimly) more than a year later! – Gavin Lock Dec 19 '16 at 14:06
• @Mark Harrison . What color and how long would you like the cell charge to last in hours? – Tony Stewart EE75 Dec 19 '16 at 16:17
• I am testing my Red and white LEDs on fresh CR3025 cells since this morning and still shining bright at reduced currents. 1.85V red and 2.85V Wh respectively. – Tony Stewart EE75 Dec 20 '16 at 0:39

## 7 Answers

Short Answer: Tuck Tape.

I prefer Tape that holds like a Bulldog in any temperature but allows the LED to be removed with a little effort.

Since my LED's have smooth lead frames the gap between is 2.05mm which would fit a 2.0mm thick coin cell loosely or a 2.5mm thick cell with perfect contact tension and not excessive stress on whisker gold bondwire. Therefore I recommend CR2025 cells and TUCK TAPE ( made in Montreal) used in construction industry for holding anything 1000x times its own weight.

Since the ESR of Energizer and Panasonic CR2025's are perfectly matched to 5mm LEDs it limits the current to 20mA typically, thus Red and Yellow drop from 3.3Vopen to 2.2Vload. Blue and White having a threshold 1V higher than Red,Yellow will cause them to last long but draw much less current but if you have LEDs with 16~30Cd@20mA , but operating at 5~10mA, it is still very bright. All my LEDs range from 10Cd to 30Cd in many colours. I don't make any profit on these custom parts any more as I am retired from that, but if you want them in 250~500 pc/bag, just ask. Compare your specs with 16Cd min.

Most tapes use elastic glue which gradually releases, unlike TUCK tape, which only needs 5 to 10mm wide strip to hold both DMM leads and the LED leads in photo above for a White LED 15 ohm ESR thus I can estimate the current to be 5mA which if you derate the CR2025 to 100mAh yields 20h but of course as current drops with voltage, will glow for months or a year after 20h. Thus Red Yellow will draw the right amount of current. White are better suited to CR123A which far greater capacity and only $1 ea online, albeit bigger. I would bend the wires of the led so that they grip the battery with friction, then tape in place with regular tape. This is a common way to make throw-able LEDs. There are adhesive copper tapes (with conductive adhesive) that can be used for this kind of connection. 3M 1181 tape Price isn't low, though, for a full roll of tape. If you can use aggressive (acid based) flux, it is also possible to solder to a coin cell (short heating time, stay far from the seal). Sn-Ag is the hot-melt glue of choice. • Soldering to a coin cell is not what I would recommend to someone attempting a holiday project. Actually, I wouldn't recommend that to anyone. – Dmitry Grigoryev Dec 19 '16 at 11:26 • Soldering is tricky, that's true. Commercially available coin cells with leads are routinely welded, though. It takes acid flux to adhere to the nickel, and it's hard on the soldering tip. – Whit3rd Dec 20 '16 at 0:16 You can use regular adhesive tape to hold wires against the battery. Copper tape works a little better, although the adhesive layer is non-conductive. Copper tape is sold (around here at least) at Home Depot as a barrier to slugs and snails. There is also copper tape that has the adhesive loaded with silver balls to make the adhesive layer conductive. This tape is expensive, and it would probably better to buy a battery clip or salvage a clip from junk electronics. Yes, you can buy conductive epoxy. Try looking for instructions to make "throwies" for more information. You can also buy copper tape with conductive adhesive. All from normal industrial distributors (in the US McMaster, Newark etc.). A 6 yard roll of 1/4" wide copper tape with conductive tape starts at about$7.50 USD, conductive epoxy will probably be more like $30-$50. All should be from stock.

Battery holders (stand-alone, that don't require a PCB) will run you about \$0.70 USD each.

Maybe use some copper wires and use electrical tape to attach them. Alternatively, there is aluminum tape that could be used, that is conductive.

Connections to small batteries are typically done with spot welding: you'll get a robust connection, both mechanically and electrically, and there's no risk to damage the battery in the process if you do it right.

You may want to check local DIY shops or electronic repair businesses: you may be able to either lease the welder from them for a day or let them do the work. Of course you could also buy such a welder, but it's not exactly cheap.