When working with a project on a breadboard I’ve been caught with the supply voltage on the breadboard not being what I expected which resulted in the project not working as expected.
To help prevent this I’ve bought some cheap mini 2 and 3 wire voltmeters. These sell for a couple of (AU) dollars. The 2 wire version just connects across the power source whereas, the 3 wire have an extra wire for measurement source.
2 and 3 wire versions
The 2 wire version I got was advertised as 0-30 volts. Other sellers have advertised them as 3-28 volts which seems closer to what I’m experiencing. With the ones I have the display begins to dim under 3 volts and stops completely at 2.5v. Still that’s ok for most of my needs. As the 2 wire version needs to draw power from the source it is measuring it means that it adds a load to the circuit. I measured a current draw of 18.5mA at 5 volts. I am only intending to use them for measuring the power rails so these should be ok that.
The 3 wire versions not only have dedication connections for power supply, they also have another connection for the voltage to be measured. The 3 wire versions are advertised as being able to measure 0-100 volts. I didn’t try them with high voltages and would not want to use them for that, but it measured down to 0.1V. I checked the current draw on the sense/measurement input and I got around of only 49uA. I measured the current draw on the power supply pins and got a surprising result. One measured 11mA and the other 16mA. I’m not sure why that is, but…
One of the 3 wire versions I bought has segment that does not light, at least usually. Sometimes I get a dim glow, but there is definitely something wrong with it.
I tried reflowing connections, but it didn’t fix it. I had a similar issue with a slightly larger one some time back. I think where I went wrong is buying a cheap one from an ebay seller that didn’t specialise in electronics parts. I’m wondering if they may specialise in faulty parts 🙂
Converting for breadboard use
There are pads on the circuit board but they are not spaced with the usuall breadboard spacing. I was able to solder some 90 degree pin headers for power and added a small amount of red paint to the positive supply pin. I’m not sure they would go if the supply was reversed and I don’t want to involuntary try that.
For third wire I tried cutting one end of a breadboard jumper wire but in the end I didn’t use it. I quite like those wires but mine seem to be mostly insulation with only a very fine cable inside. I don’t know that the connection would have lasted long. I have some wire I pulled out of something that was better quality and used that. I soldered a header pin the the free end and placed some heatshrink over the connetction. On the meter end I placed some clear heatshink around all three connections for some strain relief.
As the solder pads are so small, I don’t know how long they will last. I’ll have to be careful inserting them into breadboards to avoid breaking them.
Here is a shot of them in use
Accuracy and adjustment
The modules are advertised as being adjusted in the factory. Comparing the reading with a couple of my multimeters it was about 0.1 volts different. By adjusting the small potentiometer on the back I got it slightly closer. While they display to two decimal places under 10 volts they appear to jump by at least .02 volts at a time
I don’t expect these are a replacement for a proper meter. I doubt they have much, if any protection circuitry. Their accuracy is not as high as a multimeter either. I’m still happy with them and intend to continue using them.
Is anyone else using these like this? How has your experience been? Anything I should be aware of?