There are often situations with a circuit or existing equipment where you want to add a module that can measure and display voltage, current, and perhaps other parameters. You can buy suitable measurement modules in all sorts and sizes, and at prices so low that there’s no point in building them yourself. An example is the JOY-iT VAX-1030. It costs around €40/₤35/$45 and offers a lot of additional features, including communication over a wireless link.

What can you expect at this price? I honestly didn’t imagine very much when I saw the small box, but I was pleasantly surprised when I inspected its contents. They consisted of a measurement module with a lot of connectors, a separate display module with touch keys, a USB cable, a temperature sensor with a short cable, and a separate small cable for powering the display.

A lot of current

The version described here is suitable for DC voltages up to 100 V and currents up to 30 A. There is also a version that can handle up to 100 A, but unless you want to make measurements on car batteries or the like, 30 A should be enough in most cases. The measurement module consists of a small plastic box with a lot of openings on the top. These allow you to connect the cables and the power supply to the screw terminals inside the box.

 
All of this hardware is present in the little box.

The internals of the box consist of a sandwich of two PCBs. The top PCB holds all the screw terminals and a large relay, while the bottom PCB contains the control circuitry and a 2.4 GHz transceiver module for wireless communication with the display module. It looks a lot like an ESP8266 WiFi module, but the chip is hidden beneath a drop of glue. The top part of the enclosure can be easily unclipped to provide good access to the screw terminals.


The idea is that you fit this module in circuit or device where you want to measure the voltage and current. The measurement module requires an external 12 V supply voltage. The temperature sensor, if needed, can be connected to a small connector on the PCB.