I just finished a little project that need a DVM (digital volt meter) panel meter. You can buy these cheapies for R60.00. The one I got was a 3 ½ digit, 7-segment LCD with ±0.5% accuracy.
You get two types, one has a common GND connection, the other needs the supply and measurement GNDs to be isolated. I can’t find the common GND variety in South Africa (well RS has them but they are R300.00). If you connect the GNDs together the meter just displays the “-1″ overload indication. What to do?
Bellow is a simple circuit that will isolated the DVMs power supply and will allow you to use it in your project. Here’s a quick run down of how it works.
The 555 timer outputs a 70kHz square wave, which is rectified by the two BAT85 Schottky diodes. Capacitors C4, C5 provide the isolation between VCC, GND and the DVM supply. C6 is just a smoothing capacitor.
As you can see it’s very easy to build on a bit of vero-board and could be made much smaller with SMDs.