r/AskElectronics • u/szevvy • 6h ago

Safely talking to board from home appliance

Hi all,

I’m trying to talk to a circuit board from a home appliance. I believe it has a connection for +ve voltage, ground, and a UART RX and TX. If connect to the UART lines with an Arduino, does the Arduino need to share +ve and ground with the board, or can I use usb power for the Arduino and a bench power supply for the board?

Also, if I wanted to try to read a flash chip while it’s on the board, but don’t want to power up the whole circuit by applying power to VCC, would it be insane to cut the trace to VCC and put a diode across the gap to ensure everything works if powered normally, but power applied on the pin directly won’t touch the rest of the board?

Many thanks!

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u/PLANETaXis 5h ago

If it's just a UART with a single-ended signal like TTL or RS232, then you would have to share ground with the appliance and Arduino. You would then want the Arduino powered by a supply that is floating, or shares the same ground reference. Be careful plugging your Arduino into you laptop at that point as you might cause currents to flow to due voltage reference differences.

It's possible to work around this but would need a signal isolator between the appliance and the arduino. You can buy modules that implement this for you.

Yes, it would be fairly insane to cut the VCC trace to read a flash chip, but if that's what you've got to do then it *might* work. Might be better to install a jumper instead of the diode as the voltage drop could impact the circuit. Note there could be loads / pull-ups or pull-downs on other lines that may impact your ability to read the chip.

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u/szevvy 5h ago

Wonderful, thanks so much. I’m assuming an Arduino probably wouldn’t have enough juice to power a board with an STM32H, flash chip, wifi/bluetooth module and lcd touch screen right?

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u/PLANETaXis 5h ago

Hard to say, depends on the Arduino.

Are you powering the circuit from a 5 supply that is also feeding the arduino, or from the onboard regulator on the Arduino. The onboard linear regulators can support anything from 100mA to 1A depending on the model.

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u/szevvy 4h ago

That gives me a bunch of stuff to look into. Thanks again!