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Project Notes

#181 VoltmeterModule

Test a 3-wire voltmeter module


I obtained a 3-wire voltmeter module as part of a kit, so a chance to test one out!

front rear

There are two and three wire modules in the market.

Two-wire modules are the simplest (and generally cheapest). For most applications where a simple readout of a power supply is required, they are most convenient since a separate power supply connection is not required. It does mean of course that the meter draws current from the circuit under test, and will only work down to a certain voltage (usually ~4.5V).

For more precision measurement, the three-wire modules are better suited:

  • they present relatively high input impedance (> 100kΩ)
  • can measure voltages from 0V and up

I can’t find a specific datasheet for the 3-wire module, but from product listings, the specifications are as follows:

  • Measuring range: 0~30V;
  • Power supply: DC 4.5~30V;
  • Max. input: DC 30V;
  • Tolerance: +/-1%, +/-0.3V (>=10V), +/0.1 (<10V);
  • Input impedance: >100Kohm;
  • Working current: red: <23mA;
  • Refresh rate: 300ms;
  • Display: 3-digit 0.36” LED;
  • Display color: red;
  • Cable length: 15cm

These modules do not have any kind of digital output that could be used to integrate with a microprocessor, but as a stand-alone meter, a useful device for power supply circuits.

Test Circuit

Schematics below are for a simple test of the meter using a 10kΩ pot to vary the input voltage and compare the reading with a multimeter.

Using my DMM as a reference, it appears accuracy is within 40mV, good enough for general purposes.


NB: the Fritzing part used here is not quite right (it’s actually a 4-wire ammeter) but does well enough for a simple schematic:


The Schematic

The Build

Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.