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I picked up this old G.E.C. switchboard voltmeter at a garage sale. It’s cast iron, very heavy and probably from the 1950s.
Now an ESP32 tells it what to show over Wi-Fi. It listens to MQTT, so it can show anything I can get a number for. For now it shows how much power our solar panels are making.
Getting an old needle to behave turned into a little physics lesson. There’s no spring: gravity pulls the needle back to zero, so it swings like a pendulum. The iron inside remembers where the needle came from, and the copper coil loses current as it warms. The fixes are a slow S-shaped glide, pulsed current, and an op-amp that holds the current steady.
It lives in an old whiskey display cabinet I had lying around, with the electronics hidden underneath, and is powered vis a USB cable.