Those cheap Chinese panel ammeters and voltmeters are nice additions to a project but often require some convoluted wiring. In addition, it appears that people are easily confused about the connections, the number of complaints about poor instructions and dead meters seem to indicate that.
I have been using them for a number of projects and evolved designs to make them easier to use. The AHA! moment came when I realized I could clip a PCB onto the back of the meter. They all seem to use exactly the same plastic bezel so I could standardize the openings in the PCB.
I was building a test setup for my Teensy 4.1 CNC Breakout Board and wanted to monitor USB current. The Teensy 4.1 is fairly power hungry and my board pulls even more. I wanted to know what kind of burden the setup was placing on the USB output of my PC. In addition, I wanted to be able to characterize current draw based on various features in use.
While I had a decent DMM with fairly accurate current measuring capability, I wanted something a bit more permanant without a lot of extra wires. The ammeter I settled on is a generic "5 digit" panel meter that sells for $6-10 USD from the usual suspect sources. I paid about $13 with tax and shipping. Characterizing it, I found it was surprisingly good, within 100 microAmps of my DMM. I designed the back pack with USB A and microUSB recepticles. It connects in-line and doesn't adversely affect USB communication. USB powers the meter itself.
My first PCB had a couple of small errors but was functional. Here is Version 2.21.
Here is the updated version 2.30 installed on the Panel Meter.
And, finally, mounted on a temporary panel. The light you see at the top and bottom of the meter face is light shining through from the back. It does not show when mounted in the panel of an enclosed box.
The second iteration fixed a dumb footprint problem and used differential routing for the USB data lines. I also added silk screened information to make it clear which wires connect where. Kicad files are here. However, this uses a microUSB connector I found on eBay a couple of years ago. A quick check did not show anything availble from the same source. It probably isn't available anywhere else. I am looking for a suitable DigiKey/Mouser sourced part and will post Kicad files and an OSHPark shared project link.
I received the V2.30 boards back from OSHPark and they work great! See the USB Ammeter V2.30 section below.
I liked the design so much, I designed another one for those ubiquitous volt and current panel meters you see all over the place. This one uses a barrel jack and/or a screw terminal for 5-24 Volts. The meter can be powered from 5V to 30V and measure up to 100V. I have it set up to power the meter from the measured voltage. If you want to measure more than 30V, you will need to cut a solder jumper and use the external power pin header on the board. Output is via a screw terminal. I bought my meter from ProtoSupplies. Not the cheapest but they test their meters before sending them out - something those Amazon/eBay/Aliwhatever sites clearly don't do. Protosupplies is one of my go to vendors.
Here it is mounted on the Panel Meter.
The Kicad Design files for Version 2.51 are located here and a shared OSHPark Project is here. You can order three boards from OSHPark for $10.95 USD. The BoM is pretty simple
- J1, J5 - 2 position 3.5mm Screw Terminal - Common, Various Sources.
- J2 Barrel Jack CUI Devices PJ-002A, 2.10mm ID (0.083"), 5.50mm OD (0.217"). Digikey - CP-002A-ND, Mouser - 490-PJ-002A. Many others fit this footprint.
- J3 2.54mm 1x3 Pin Header - Common, Various Sources.
- J4 2.54mm 1x2 Pin Header - Common, Various Sources.
Version 2.21 has been updated to use a more widely available USB Micro connector. I also added a pin header and screw terminal for +5V so the backpack is now able supply power for other projects as well as USB based ones. OSHPark Shared Project is here. Cost for 3 boards is $13.15 USD. BoM is
- J2 USB Micro A/B - Molex 47589-101. Mouser - 538-47589-1001 , DigiKey - WM14335CT-ND
- J3 3.5mm Screw Terminal - Widely Available
- J4 USB A Recpeticle - Amphenol ICC UE27AC5410H. Mouser - 523-UE27-AC54-100, DigiKey - UE27AC5410H-ND ** Update - new Amphenol Mouser Number: 649-87520-0012BLF. Same pinout.
- JP1, JP2 1x2 2.54mm pin headers - Widely Available.
These panel meters all seem to share the same plastic bezel design. To mount them in a panel you need to cut out a rectangle 45.5 mm by 26 mm. (1 13/16" X 1 1/32")
The panel should be at least 1.5mm (1/16") thick but not more than 3mm (1/8"). For thicker panel material, you should make the opening a bit taller (26.5 mm or more)