“A multimeter, based on the STM32F103, which can measure voltage, current and power, both bipolar and in DC and true RMS.
In short, I needed more multimeters at my lab. You can get these cheap Chinese “multimeters” from Ebay, but I don’t trust them at all and they lack some of the features I want (PC connectivity, RMS, power measurement, electronic range switch, capacity measurement, …). So I wondered, what can I achieve with circa 10 dollars worth of parts?Well, turns out the STM32F1 is pretty powerful and I managed to cram all of the above functions into this little thing! And as a little bonus, it pretty accurate (for a day-to-day prototyping at least, it’s not a 200 dollar Fluke).
DETAILS
Version 1.2B released
So, after a few revisions I decided to publish this project. Now I am not going to describe how everything works and all of the details, if you are interested in that, read the article on the project homepage. Here, I want more to summarize what it can do in short (more in-depth description on the project homepage)
So, what does it do? It has six modes:
Voltage mode - measures voltage with two ranges, ±6 V and ±60 V. Ranges are switched electronically, there’s a hold function (to freeze the display) and you can select between DC (or average) mode and RMS (bandwidth about 3 - 10 kHz).
Current mode - basically the same as voltage, ranges are ±60 mA and ±500 mA, again switched electronically, again with DC/RMS.
Power mode - yep, it can do both voltage & current at the same time, so I wondered, why not make a power mode? This comes in really handy when working with power supplies (or, just today, I used it for testing a bunch of small solar panels). All 4 combinations of ranges are possible, and you can again do RMS or DC.
Continuity screen - basically it tests resistance and if it’s below 50 Ohm, it sounds the buzzer.
Component test - this can measure resistors, capacitors and diodes (so far). Basically I use it mostly for sorting parts after prototyping or when I need to clean a pile of parts (then it comes really handy).
About screen - well, it tells you the name of this project.
Furthermore, some other features:
fully isolated USB interface (capable of both receiving commands and transmitting data)
powered by li-ion (protection & charging included) or anything with a voltage of 3.2 to 5.5 volts
measures battery voltage and signalizes low battery
RTC & memory, so it remembers the mode you last used
voltage & current inputs protected against overrange
What it does not do:
it does not measure mains, nor is it meant to; I rarely use mains, so I don’t need this function, and because of safety I did not even try to implement it
What it could do in the future:
frequency measurement (it’s just a matter of writing the software for it)
additional voltage range (± 60 mV), coming in rev. 1.3, hopefully
and some other fancy stuff, it’s described more in detail on the project’s homepage”