3-Digit Voltmeter Dongle

This little project shows a USB dongle which houses a inexpensive little 3-digit voltmeter LED module.

On the front, the box has a cutout for a standard 0.36" 3-digit voltmeter module. These modules are easily available and cost less than $US 2 on eBay. The back of the box is equipped with a sliding cover which gives access to the voltmeter, the USB connectors and/or connection cables. The ends of the box contain cutouts for a female and a male USB connector, or alternatively holes for the corresponding cables.

This box can be used as a testing device for monitoring directly the USB voltage. This is useful to monitor the charging voltage of solar powered USB chargers or also to detect voltage drops in the USB 5V line due to overloading of the USB power source. In particular I use this type of voltmeter with my foldable 5V solar panel to monitor the output level of the solar cells.

Another use of the box is to build a simple USB powered voltmeter. In this case, the voltmeter is powered through the male USB connector on the one end, while at the other end of the box the cables for the PLUS and MINUS probes are attached. Be careful though, always watch the polarity and measure only battery powered circuits, as the voltmeter does not measure negative voltages and shares the ground level with the USB host!!!

The SCAD source file is, as always, fully parametrized and commented, so that you can adapt it easily to your needs. Ready-to print STL files are provided for different configurations of connectors and/or cables:

• male and female usb connectors in the box
• male connector on one side, cable on other side
• female connector on one side, cable on other side
• cabel holes on both sides

The male and/or female connectors are best soldered on a little piece of striped prototyping PCB board with standard 2.54mm spacing. For the dongle with both male and female connectors, a PCB of size 4x19 holes is needed.

Note that there are various versions of the 3-digit led voltmeters available, having different display colors and min/max voltages. Also there are versions with 2 wires (supply voltage is measured) and 3 wires (supply voltage separated from measured voltage). For monitoring the USB voltage, both 2- and 3-wire versions can be used. For building a small USB powered voltmeter with external probes, of course a 3-wire version is required. For monitoring the USB voltage, it's best to use a voltmeter module which displays voltages below 10V with 2 decimals.