Meefik's Blog

Freedom and Open Source

Homemade LED Keychain Beacon

29 Sep 2019 | diy

Some time ago I found a curious device on the Internet - a keychain powered by a radioisotope of hydrogen (tritium). The principle of operation there is as follows: there is an isotope in the sealed cavity, the inner part of the cavity is covered with a phosphor, which glows when exposed to electrons emitted as a result of beta decay of tritium. The half-life of tritium is 12 years, which ensures the continuous glow of the phosphor for many years.


I was inspired by this idea and somewhere in 2013, the idea appeared to make a keychain beacon so that it glowed and was visible in the dark for a long time. You can use it for keys, you can use it for something else, I was interested in the idea. However, I decided to use not a radioactive element, but an electric circuit with an ordinary LED. It remained to make the LED glow for many years without replacing the batteries, while maintaining the size of the keychain.

To implement the idea, I found the so-called “Joule thief” circuit to power the LED from 1.5 V pulsating current and selected the components so that the circuit consumed a minimum of energy. It turned out to achieve a consumption of 0.01 mA with a pulsation frequency of 1 Hz. The design life of such a scheme from the LR44 battery was 3 years, in practice the battery lasted about 5 years of continuous operation until the complete cessation of glow.

The JT circuit was as follows:


This is how it looks assembled:

I only got around to assembling the keychain case now. I used an old AA battery, sawed off the bottom of it and took out all the contents. I put the JT circuit inside, powered by an LR44 battery, and filled the circuit with hot melt adhesive for insulation. Then I took transparent epoxy resin and tinted it with green phosphor. Filled the insides with resin and left some resin on the LED side. After the resin cured, I sanded everything and put a loop on the back side to attach the keychain (also filled with resin). The result is a cylinder the size of an AA battery, one end of which glows and the other can be attached as a keychain. The epoxy resin makes the keychain durable and waterproof, and the phosphor adds a nice greenish glow between blinks of the LED.


That’s all, as they say, “Just for fun” :)