Martin needed more multimeters in his lab, and instead of buying more, he decided to create his own using about $10 worth of parts with features he deemed necessary — PC connectivity, RMS, power measurement, electronic range switch, capacity measurement, etc. Martin tasked STMicroelectronics’ STM32F1 (Arm Cortex-M3) microcontroller for his Open Source Multimeter, which can handle those functions quite nicely.
Martin designed his multimeter using three individual PCBs — the first housing the STM32F1 microcontroller, the second for buttons, connectors, and SDM test pad, and the third outfitted with a USB interface for onboard Li-Po battery charging. He also included a small LCD display for data readouts and packed everything into a 3D-printed enclosure with a kickstand. It should be noted that there is a switch that connects the power from the USB to the charging circuit, which can be turned off, thus isolating the circuit.
The Open Source Multimeter features six function modes: voltage (±60 V and ±6 V), current (±60mA and ±500mA), power (can do both voltage and current at the same time), continuity/resistance test (sounds buzzer when below 50 ohms), component test (supports resistors, capacitors, and diodes only), and about screen.
Martin states that his multimeter is very accurate — getting within 1% with voltages ranging between 1V and 6V DC, but only within 5% of the real value for the remaining modes, which is impressive, considering he only used around 10 bucks worth of parts.