I have a vintage Weston Model 506 DC ammeter (it has a range of 0 to 20 amps) that has been sitting in my office and I think it would be interesting to use it as a soil moisture indicator for a (long suffering) potted plant. It would be more of a curiosity than something critical, so neither precision or accuracy is a big requirement.
One design that I saw uses an Arduino to indicate the moisture on an ammeter. While interesting, I was hoping for a design that was not microcontroller based--trying to stay in the vintage theme.
Most of the circuits I looked at recommend an AC based design to mitigate corrosion on the moisture sensor. Plus, the designs indicate the moisture level as a voltage, so I would need to put some sort of voltage to current conversion. Given the 20 A range on the ammeter, I probably would need to drop the voltage going to the ammeter to reduce the power demand.
Ideally I would like to avoid having to plug in a wall wart as power source or changing batteries (rechargeable AA) more frequently than once a month. Are there any recommendations for a circuit design or should I go with the microcontroller design?