There are a lot of digital voltmeter modules on the market today at advantageous prices. Apart from their very high input impedance, they have the advantages of being accurate, versatile, compact, and also quite elegant. The flip side of the coin, however, is that they usually exist in two versions:
• The ‘really cheap’ which require isolation between their supply voltage and the voltage being measured.
• The ‘more expensive’ (expect double!) which accept a common ground.
Considering the low power consumption of these modules (around 1 mA), it is simple and worthwhile to design a little circuit that will allow isolation for this type of equipment.
IC1 is a 40106 CMOS hex Schmitt inverter. Its first gate IC1.A is wired as an oscillator by way of R1 and C1, at a frequency of around 10 kHz. IC1.B inverts the signal, so that gates IC1.C / IC1.D and IC1.E / IC1.F can be driven in antiphase. The signal is output via C2 and C3, rectified by a diode bridge, filtered by C4 and C5, and stabilized by IC2 and C5 to a value of 8 V. The input supply voltage, decoupled by C7, is not very critical, somewhere between 10 and 15 V.
It would be hard to make it simpler… The whole of the circuit fits onto a single sided PCB 24.3 × 27.94 mm, easy to fit behind most display modules.