Petteri Aimonen had been adding programmable equipment to a home lab: a USB connected power supply, electronic load, multimeter, oscilloscope and a function generator. What to do with all the leads?
In this project I build a simple relay multiplexer for selecting signal sources.
I wanted a board that has banana binding posts for cables, and that switches both negative and positive signal as a pair.
The relays are divided into two groups, with 4 relays each. This allows to use the groups in parallel for two separate instruments or for 4-wire measurements. Alternatively the inputs can be joined with pieces of wire to form a single group with 8 relays.
I chose STM32F042 as the microcontroller handling USB connection, because it does not require an external crystal. TBD62783A handles driving the relays. It has integrated flyback diode and input pull-down resistors, making the schematic easy.
I added two push buttons for convenience: first one cycles through each relay in turn, and second one opens all relays. This can be quite useful when you are making connections to test equipment, as you can check that everything works before starting the measurement script on PC.
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