Every kitchen needs an LED matrix
I bought an ELTEK-737 display off eBay for roughly £10, it was described as for parts / repairs, with the usual "unable to test because no power lead". It arrived a few days later and I stuck current limited 12v into the power jack. It did not do anything.
It looks like the power jack was intended for AC and included a bridge rectifier, large filter caps and a 5v regulator. But that's fine, 12V DC should still work. Probing around the logic I could see that 5v was being pulled down quite a bit. An LED was lit on the keypad (for entering messages), but it wasn't responding to input. The logic board had a small 5v nickle cadmium battery and lifting one lead lit the display up! I'm guessing the dead battery was pulling the voltage down. The very left LED matrix (just out of frame below) was fully illuminated for some reason.
Trying to figure out why the left most 5x7 matrix was lit revealed a very loose transistor and wiggling it caused more problems than I expected. I don't understand why but a shift register further to the right of the display broke and everything to the left of that was lit. I've repaired that transistor's connection with some small wires and removed the now broken shift register. I've connected the input and output until a spare arrives (hence the gap in the photos later).
Sticking my glasgow on the data lines revealed that the protocol was very simple. A clock line, a data line and seven lines indicating which row is active. Very soon after I had it working with an Arduino (Uno R4 WiFi) and pulling messages from MQTT. It is now mounted above a door in my kitchen and shows stats from around the house, such as the temperature and humidity of certain rooms (and fridge), the CO2 downstairs, as well as the current house power usage.
The display is running off a 5V meanwell supply (bypassing the internal filtering and regulation) which sits on top of the kitchen shelves and pulls only 200mA for a normal amount of text at 5v. Most of the code is the same as the departure board at nottinghack, but you can find the code on sourcehut. Once the new shift register arrives the missing 5x7 matrix should be fixed. That was fun!
Related posts:
Wanting to leave a comment?
Comments and feedback are welcome by email (aaron@nospam-aaronsplace.co.uk).