I decided to design and build my own digital multimeter for this project because I couldn’t find a suitably cheap USB multimeter. Designing my own also let me flex my analog circuit design muscles as well as control the size and shape of the board. Below are the different sections of the circuit and a brief explanation of how each works.
This is the voltage measuring circuit. It operates by measuring the voltage between the two resistors in a voltage divider circuit with three selectable voltage ranges. Turning on the 10kOhm resistor allows for a maximum of 10V, the 2kOhm resistor allows for a maximum of 25V, and the 470Ohm resistor allows for a maximum of 100V. Details on how the resistor values were calculated can be found in the Instructables article where I found this design.
The current measurement circuit uses a combination of Ohm’s law and a shunt resistor with a low resistance. The voltage generated over the resistor is then amplified by a non-inverting amplifier. With a gain of 5x from the amplifier, a 1 Ohm power resistor, and two 500mA resettable fuses, I can measure a maximum of 1A of current.
The resistance measuring circuit is the only thing I decided not to use from the multimeter Instructable. The original circuit used a constant current source and Ohm’s law to determine resistance, but because I wanted a simpler circuit I just used another voltage divider with selectable ranges. The only downside of this method is that the resolution of the measurement will now be non-linear, but I don’t think accuracy will be a huge deal for most of the circuits I’ll be working on so I’m not that concerned.
Above is the final board layout that I’ll be using. I kept most of the components as surface mount to minimize board size. Now all that’s left is checking to make sure it actually works!