electronut Programming & Embedded Systems - by Mahesh Venkitachalam

Touch Activated Blinky Badge

Touch Activated Blinky Badge

I wanted to create a simple project to teach soldering and electronics to kids (10+ years) and beginners. I first thought of a simple two transistor LED flasher, but then decided to make it interactive by adding a touch sensitive element to it. The idea is that once made, this PCB can be worn as a badge.

I had initially designed this using through-hole components. Here’s a photo of the breadboard prototype. I was using a couple on BC547 and 2N2222 transistors.

BB Proto

With SMD becoming more and more common, I decided to design the PCB with. Plus I though SMD would have a much flatter profile - more suited to a badge.

Here’s the schematic for the Blinky Badge:

BB Schematic

The touch plate consists of a few closely spaced copper tracks, and when you put a finger across a track, it acts as a resistor, sending a small current into the darlington pair Q1 + Q2, which amplifies the current and switches on the bistable multivibrator circuit formed by Q3 + Q4. The circuit is powered by a CR2032 coin cell.

I designed this project using kicad, and the most interesting part was creating the touch pad. Since all kicad project files are plain text files, it was easy to figure out the structure and write a small Python script to generate the pad. Here’s a glimpse of the process:

BB kicad

Soldering the badge was mostly very easy, except for the transistors, which had very small pads. I have now changed the design to use larger pads. My next goal is to teach my 10 year old to solder this board.


You can find all design files for this project, including the Bill Of Materials (BOM) and gerbers, at my github link below:


Order From OSH Park


I’ve uploaded my gerbers to OSH Park. You can order PCBs from them using the link below.


If you liked this article, please consider supporting my efforts by purchasing my book.

Python Playground, published by No Starch Press, USA, is a collection of imaginative programming projects that will inspire you to use Python to make art and music, build simulations of real-world phenomena, and interact with hardware like the Arduino and Raspberry Pi. Do check it out!