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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?

Weston Model 506 DC Ammeter

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    \$\begingroup\$ You're probably going to want to crack it open and remove the shunt. \$\endgroup\$ Sep 14 '20 at 15:50
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    \$\begingroup\$ Is there any chance you could find a vintage voltmeter in the same way (or modify this meter as @SpehroPefhany suggests) ? If aesthetics is what you are going for there is no way to get 20A out of a AA cell at any voltage. \$\endgroup\$
    – crasic
    Sep 14 '20 at 15:57
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    \$\begingroup\$ A twenty-amp meter that size may have an external shunt. Look carefully at the back to see if something to that effect is indicated. Also determine the size of the connection posts on the back. \$\endgroup\$ Sep 14 '20 at 15:59
  • \$\begingroup\$ @CharlesCowie The Weston 506 series has an internal 50mV shunt up to 50A. \$\endgroup\$ Sep 14 '20 at 16:22
  • \$\begingroup\$ you should use a micro, hands-down. You can't run the moisture sensor all the time, or else it will corrode the positive completely within weeks, and even after a couple days looses calibration. So if you do it analog, you need an intravalometer and a way to sample and hold a voltage. That's way more complex than a MCU-based setup would be. It's also easier to calibrate and adjust the MCU version, and allow min/max buttons, logs, etc. I would likely give up on batteries, or leave it for the future, but maybe you can find a micro that can leave it's PWM/DAC output on while sleeping. \$\endgroup\$
    – dandavis
    Sep 15 '20 at 8:01
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The Weston 506 DC moving coil meter movement would have a full scale deflection of 1 mA or 50 mV. The coil resistance would be 50 Ω. For a 20A meter, the shunt resistance would be 0.0025 Ω and dissipate 1 W at 20A.

The shunt could be either internally- or externally-mounted across the meter terminals. If the shunt is not found outside, internal mounting may be confirmed by measuring the resistance across the meter terminals.

The shunt would have to be removed before the meter could be used as a moisture indicator. To remove an internal shunt, if any, the movement would need to be taken out by unscrewing 3 screws on its housing.

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