[Tom] needed 8 displays for a project. He wanted to to control them over I2C, and was trying to reduce cost. Some vendors make I2C controllable seven segment displays, but they cost about $10 each. [Tom] figured he could hack cheap voltmeters to get the same results for about $3 a pop.
The voltmeters that [Tom] bought used a 8 bit STM8S003F3P6 microcontroller. He reverse engineered the device and re-created the schematic to find out where the I2C and programming pins would be. Then he hooked it up to a STM8 Discovery development board, which has an integrated programmer.
With the hardware figured out, it was time for new firmware. Fortunately, [ba0sh1] had already written firmware for a similar purpose which could easily be adapted. The code implements a software I2C slave, which reads data off the bus and displays it. It’s all available on Github.
The end result is a I2C controlled display for a third of the cost. Next time you need a bunch of these in a project, consider picking up some cheap voltmeters.
I would just have used a (few) shift register(s) (74HC595) and a 4-digit 7-segment display, both available on Ebay for even less money (2$). You just don’t have that much fun reverse-engineering the thing! Well done, though :).
You mean shift registers, display, resistors, pcb, headers, and possibly even voltage regulator. More than 3 shipped.
Well, I have all of these lying around, so I’d prefer it that way :). If you compare working hours per display, Tom will win ;).
Whoa! Dude re-wrote the code for the voltmeter controller chip??! Sweet hack, Tom Price!
Very impressive piece of work!
ebay: Max7219
But its a nice hack!
99% of eBay are fakes. Not worth it.