A very useful feature of any multimeter is the ability to output its measurements to a computer, because then, with simple scripts, you can automate many measurements. The BM867s/869s is a great multimeter, the only pity is that the USB adapter is sold separately and costs 50 euros (60 USD), which is just ridiculous.

So I made this adapter. It communicates with the computer via HT42B534 (USB-serial chip), so it acts as a serial port. The brain is an STM32G030F6, which decodes the output to a human readable form - the original cable does not even do that! So all you need is a serial terminal of your choice, you are not stuck with the proprietary firmware from Brymen (this, for example, allows you to really easily input data into MS Excel or use them in any script you want). And if you want to read the data into a program of your own, all you need to use is the scanf function (which is much much simpler then decoding the raw output - the STM already did the hard work for you!).

Update 27.4.2021: I've reworked the entire product to use an STM32, redesigned the case from scratch & the firmware is also mostly new.

Update 9.3.2022: The chip shortage forced me once again to rework this entire thing to use STM32G030. The firmware is also brand new with some improvements, this time it is written using ST's HAL, so it should be pretty easy to modify it to your own needs.

Features

• reads data from Brymen 867/869 via infrared
• sends the measurements at 115200 baud via serial port
• output data rate and format adjustable (5 readings/s, 1 reading/s, single reading); settings are stored in EEPROM and remembered on next power up
• BOOT0 pin available as a button, so you can update the FW via USB without disassembling the case
• activity LED
• open source
• shipped as a populated PCB with uploaded firmware
• optional: 1 metre long USB-A cable attached
• optional: 3D printed case - you need this to reasonably use this product - print it yourself or order it as an extra

Controls

On power up, it starts in the default mode, which can be changed (see below). You can then send it the following commands: - "F": it reports data at full speed, roughly 5 times a second - "O": it reports data once per second - "S": it stops reporting automatically - "D": it sends a single report (in stop mode) - "R": toggles between raw data and decoded output - "E": saves current mode as default - "H": displays all available commands & current FW version

Here you can see it in action:

Case

The case is 3D printed from black PLA and put together with two M2 screws. You can either order it as an optional extra or print it yourself. But note that the case needs tight tolerances to fit 'just right', so you might need to adjust it according to your printer.