Ross Mac wrote:

You might add a diode and series 47K ohm resistor to the switch contact
used to measure 120 VAC and leave the meter leads plugged into a wall
socket to monitor house voltage and keep the battery charged.

What's wrong with a spare rechargable battery and a cheap charger.

Well, that way you need two batteries and a charger, and rechargable batteries
are more expensive than plain carbon zinc, and the meter may not work well on
1.2 vs 1.5 V, and this arrangement won't monitor house voltage, and it takes
more human attention, and it's less fun.

The way I see it you are suggesting a 1.5 volt battery be charged at a rate
of 25 ma at 60 volts pulsating DC?

More like 1 mA, altho that's too much for an Eveready 1212 AAA with a 540 mAh
capacity that loses 10% of its energy over a year. That only needs 0.10x540
= 54 mAh over a year, or about 6 microamps on a continuous basis, if the meter
is never used for anything else, but the movement might require another 100
microamps. And why waste power in that resistor, vs a charge pump like this,
viewed in a fixed font?

- C
| \ | | | 1.5 V
------------------| |---------------|--------------------
. | | | | |
120V . --- ---
. ^ -
------------ | |
| | |
| / | | |
- --- --- ---
- _ -

Q = 170C coulombs and I = 60Q = 106 x 10^-6 amps makes C = 0.01 microfarads.
The battery would be a fine smoother and voltage regulator. Harbor Freight
stores sell $2.99 digital multimeters. Maybe they need less than 100 uA.

My flashlight plugs into the wall. Very convenient, because I know where to
find it and don't have to change batteries. My CO and barn heat detectors
work that way too, by design, with audible and remote X10 alarms and "non-
rechargable" batteries that rarely need changing. I hate changing batteries.
I lose my cheap Casio watches with 7 year Li batteries before they go dead.

Nick