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If all you need to do is replace a potentiometer with electronic control, it may be possible to do that with a vactrol. It's more invasive perhaps, but then we'

If all you need to do is replace a potentiometer with electronic control, it may be possible to do that with a vactrol. It's more invasive perhaps, but then we're not dealing with anything mechanical. A vactrol is a device which combines LED and photoresistor in one package (sealed against the leakage of light). The photoresistor has a "dark resistance" that is typically very high, like hundreds of thousands of ohms. When it is illuminated by the LED, the resistance goes down: you control how much via the current to the LED. Another possibility is using a digital potentiometer, like this: https://ww1.microchip.com/downloads/en/DeviceDoc/20005207B.pdf https://ww1.microchip.com/downloads/en/DeviceDoc/20005207B.p... A digital pot is an integrated circuit that switches a bank of resistors with FETs to create a desired resistance, or combination of resistances to simulate a wiper position along an analog pot's track. They are programmed with I2C and such. These things have certain disadvantages; like the one I linked to requires that power rails be provided for the analog domain where the resistors "live", and the voltages on them must be within this range. The vactrol has no such issue; it is a passive resistor with light shining on it.

dlenski 2mo ago
> If all you need to do is replace a potentiometer with electronic control, it may be possible to do that with a vactrol. It's more invasive perhaps, but then we're not dealing with anything mechanical. This is what I was wondering about as well. The knobs on the unit are probably potentiometers, so it ought to be possible to replace them with a digital device of some kind, and avoid the mechanical coupling altogether. I imagine that the author did consider this possibility, but decided that disassembling the control panel in order to replace or byypass them was just too difficult and dangerous, especially since everything is at AC line voltage as he notes in the write-up. The modern smart thermostats that I've looked inside have a clean separate between the low-voltage digital side, and the AC-line-voltage power control side… but evidently these older purely-analog devices didn't.

dlenski 2mo ago

> If all you need to do is replace a potentiometer with electronic control, it may be possible to do that with a vactrol. It's more invasive perhaps, but then we're not dealing with anything mechanical. This is what I was wondering about as well. The knobs on the unit are probably potentiometers, so it ought to be possible to replace them with a digital device of some kind, and avoid the mechanical coupling altogether. I imagine that the author did consider this possibility, but decided that disassembling the control panel in order to replace or byypass them was just too difficult and dangerous, especially since everything is at AC line voltage as he notes in the write-up. The modern smart thermostats that I've looked inside have a clean separate between the low-voltage digital side, and the AC-line-voltage power control side… but evidently these older purely-analog devices didn't.