Fixing a Very Dangerous Old LED Matrix Display

A proper solder joint will be quite strong. But we’ve all had solder joints fail, so clearly they aren’t immune to stress and vibration. What if that failure caused mains AC voltage to short to a device’s enclosure? And what if the device in question is in your home? The possibility should make you shudder and Clem Mayer encountered that potential while digging into this old LED matrix display.

This is just a big grid of red LEDs used to display text, as was very common at the end of the 20th century. Mayer acquired the display and did the same thing every one of us would: he plugged it in and turned it on. Fortunately, nothing bad happened. In fact, it worked perfectly and even showed its last programmed message in glowing red dots.

But when Mayer opened up the enclosure, he found something that you will cause all of us to proceed with caution in the future. The display’s power supply, an AC-to-DC brick, had its AC input wires soldered directly to the pins where a plug should have been. If either of those solder joints had failed, a live wire could have easily made contact with the enclosure and given Mayer a potentially fatal shock.

That’s a real concern when working with old electronics of unknown provenance.

In this case, the fix was easy and Mayer simply replaced that fire hazard with an external power supply that outputs a benign 5V DC to the display. But it could have been very bad and proves that it is a good idea to inspect old electronic and electric devices before using them.

Crisis averted, Mayer still had to figure out how to control the display. It turns out that its designers intended for users to program messages though input on a PS/2 keyboard. Entering the proper commands and desired text string is all the programming it needs.

Because Mayer wants to eventually connect the display to his network, he programmed an Arduino UNO R4 WiFi with a PS/2 library and a simple sketch that “types” out the required commands and the desired text string. It has to type slowly for the display to keep up, but that was easy to accommodate.

Now the Arduino can tell the display to show any text and all Mayer has to do is connect it to his network and program the desired behavior. He could make it so he can enter messages from a web interface, or have it work automatically to display any information from the internet.