I was inspired to build this project to wear as a name tag to a job interview. This is my implementation of a novel idea from Bob Blick: bobblick.com/techref/projects/ledsign/ledsign. I stumbled onto Bob’s website while looking for something else and noticed this circuit. I happened to have all the parts to build it in my junk box, so what else would I do? I also had an antiquated Needham’s EMP20 eprom programmer that my Dad gave me, which was just itching to be dusted off and brought back to life. Finding the programmer’s software and getting it to run on a modern operating system proved to be a project in itself.

This was a fairly complex circuit that uses an eprom chip to store the message, but no microcontroller. Seven of the eprom’s eight output bits are used to drive the seven rows of LEDs through a buffer chip. A binary counter indexes the eprom address, and the 10 columns are multiplexed using a CD4017 chip.

I started by making the LED matrix. Hand soldering 70 2mm LEDs in a matrix was quite a feat, but I worked diligently and tested each LED before soldering, hoping to avoid having to rework anything later. After making the LED matrix, I breadboarded the entire circuit for testing and to get a better understanding of it.

Once it was working on the breadboard, I hand soldered the circuit onto a proto board. The small size of the circuit board made the circuit layout dense enough to challenge anyone’s point-to-point soldering abilities.The proto board was sized to fit nicely with a small Lipo battery in a shirt’s front pocket. The display board then clips onto the outside of the pocket with a normal name tag clip.

I didn’t design the circuit, I just built it because I found it fascinating, and I had all of the components.