One of my Elektronika MK-52 (Электроника МК-52) Soviet era micro calculators decided to show its age by turning off a couple of VFD positions. In this case the ‘minus’ sign and the first digit are almost invisible, so instead of ‘-123’ you can see only ‘ 23’. I have a few spare VFDs and could just replace the faulty one, but that would be a bit boring. Plus, all spare ones I have are from Soviet era as well and there is no guarantee that they will last.
So I’ve decided to replace original VFD with something else, preferably some ready to use parts. Having a few STM32F103C8T6 boards laying around it was the obvious choice – use STM32 to scan VFD pins to read the values being displayed and send them to anything connected to the STM (to be yet decided).
MK-52 drives its VFD using -27V, so direct connection with STM is not possible. But simple resistive voltage divider works just fine. There is how I’ve connected VFD to the STM:
Note that only of VFD grid controls is connected, below I’ll explain why.
This is how VFD digits multiplexing works: