I just got married and came up with the idea of making a blinking commemorative keychain to mark the date. The first thing that came to mind was to make it electronic, to do or be something useful.

I then proceeded to think about how to blink an LED. First thing that came to mind was an Arduino like the Attiny85 or even a ATMEGA328 in TQFP-32. But besided being expensive I would have to design a batttery control circuit for it, so not what I wanted for this moment.

Also I wanted two more things: using mostly SMD components and supplying the circuit with a single 3.3V CR2032 (button). I then thought about using a 555 oscillator, but would mean a bunch of components more.

Schematic diagram

I then was reminded by myself about a schmitt trigger inverter logic IC, that would produce an oscillator with only a resistor and a capacitor. This is because I am tinkering and preparing a new project for you: an inductance meter, where I also need an oscillator. So I put this circuit in the pool of ones I could use.

Schematic diagram for my keychain is below, complete. What I basically do is supply a 74LS14 inverter schmitt trigger straight from a 3.3V CR2032 coin cell. It just goes through a SMD push button so that the circuit is off all the times (except when you press the push button).

I used this website to calculte the frequency and period of the blinking. For 39k Ohm and 10uF for example, I obtain around 3Hz. A LED series resistor of 330 Ohm would allow around 4mA for the 2V LED.

Below you can see the printed circuit board layout, notice I made it simple without any ground plane or anything. It has a diameter of 46mm. The only through hole component is the battery clip (besides of course the actual chain-connecting hole).

Results

Here are a couple more photos of the final build, already connected to my house keys. I originally wanted to make it heart-shaped, but was not able to do so in Kicad. I still have a bit to learn about this software. So I made it a circle, which end up being nice as well.

Final words

For somebody that underestands electronics and know their way around printed circuit board designing, this is an easy project. It is also emotionally fulfilling to give it to your partner for daily use.

Have you ever made something for others using your hobby? comment below and let’s talk.