# Coin cell to power a 5mm UV LED?

I am a beginner with LED lights, but would like to use a 5mm UV LED to make a pendant with a small fluorescent crystal. (This is what I purchased (https://www.amazon.com/uxcell-365-370nm-Ultraviolet-Electronics-Components/dp/B07XKPR25C/ref=sr_1_22?dchild=1&keywords=Uv+bulb+5mm&qid=1607026696&sr=8-22)).

I would like to create a very simple, very compact circuit with a coin cell (watch battery), and an on/off switch. What would be the easiest way to do this?

Also, would using 2 batteries fix the voltage issue?

• Welcome to the Electrical Engineering Stack Exchange. We can help you with the circuit here, however, you have to do your own shopping around to find parts (such as the switch). That is outside the scope of this website. Dec 3, 2020 at 20:42
• Lithium coin cells are limited to 3V so a 3.4V diode isn't ideal. I think you can get 2.9-3V 395-405 nm diodes, so those might be a better choice. Dec 3, 2020 at 20:47
• Thank you Math Keeps me Busy and user 1850479! I’m sorry about the supply question, I’ll see if I can edit my post. As for the light, if I switch to the 2.9-3.0 bulb, what would be the best mechanism to use for a switch? Dec 3, 2020 at 20:56
• Would using a pair of batteries and a resistor solve the voltage issues? Dec 3, 2020 at 21:04
• Depending on how small "small" is, two coin cells and a resistor would work.
– W5VO
Dec 3, 2020 at 21:04

Because you specified a coin cell, the circuit becomes really simple:

simulate this circuit – Schematic created using CircuitLab

This makes use of a couple of dirty tricks. Do not use it for anything but coin cells.

Dirty trick number 1:

Normally, you need to limit the current to an LED. A typical 5mm colored LED is rated for 20 milliamperes of current. A coin cell can't deliver 20 milliamperes of current. It can only deliver a couple of milliamperes at most. It can't deliver enough current to damage a typical LED.

Dirty trick number 2:

The ultraviolet LED you've picked out is rated for 3.4V at 20 milliamperes. That's at the high end of the voltage a coin cell can deliver. But, that's at 20 milliamperes. Since the current is lower, so is the forward voltage. LEDs are much like normal diodes in that the forward voltage drops when the forward current drops.

I expect it will light up just fine from the coin cell, though it won't be anywhere near its full brightness. It should work for what you are doing.

The coin cell won't last long, hours at most. You'll have to see if it will last long enough.

You'll probably want to get a holder for the coin cell - you're going to have to replace it often. The holder can double as a switch - just take the cell out to turn off.

If you need a switch, you'll want to get a single pole, single throw (SPST) on/off switch. Any electronics dealer can sell you one. You might even find one on amazon that you like.

Finding parts is your task. Product recommendations are off topic on this site.

Those two "dirty tricks" are the secret behind LED throwies.

A throwie is nothing but an LED soldered to a coin cell and glued to a magnet.

Cheap and simple.

This is a current and voltage chart I made of a blue LED rated for 3.4V at 20 milliamperes:

As you can see, it begins to conduct at about 2.5V. At the top end there, with only 2mA of current, it was uncomfortably bright.

I expect your UV LED will be similar.

• Thank you JRE! And I’ve already been warned about parts requests. I’ve edited my original post and dropped that question. Dec 3, 2020 at 23:03
• @DevikaMenon A 375nm photon has an energy of 3.3eV, you'll start getting light at more than 3V. A 405-410nm LED would be a better choice (bandgap of about 3V) if you want to try this idea. Dec 4, 2020 at 17:50