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I acquired a bread-making machine. See control board image. I want to use the machine for other purposes and control the buttons, paddle, heater & 7-segment display with an Uno. As it stands, the machine provides 0V, -5V & -11V to the control board.

Is it possible to connect 0V to Uno VIN and -5V to Uno GND pins?

My initial idea was to replace the PENTALPHA-BV30K with an Uno and mapping the pins accordingly. I couldn't find any datasheet for the BV30K, however I'm reverse engineering the control board anyway.

Control board findings:

  • -11V is used to drive the A/B/C/D/E/F/G/DP segments of the LED display
  • BV30K only uses 0V & -5V

bread machine control board

I added an image of the transformer based power supply board so that I could check the supply there. The larger heatsink is for the 7905 IC and the smaller heatsink is for the BTA06 Triac.

enter image description here

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  • "0V, -5V & -11V" How do you know your meter probes aren't just swapped?
    – timemage
    Feb 8 at 21:53
  • Voltages measured with Black (COM) meter probe connected to 0V & Red meter probe connected to -5V & -11V on the larger white header on the control board. The header is also labelled as such.
    – Mick
    Feb 8 at 21:59
  • I saw the - on the header silkscreen. It looks just as easily like a poor choice of punctuation. This is part of why I asked. Is there an ground plane in this thing? Is there a place that strapped the metal parts are strapped together that cross checks as showing -5 when you put your positive probe on the thing that says "5V" or "-5V" depending on how you look at it. If there isn't scarcely makes a difference if your UNO is running in the same system, because -5 to 0 is still 5V. The UNO doesn't much care what you label ground as long as whatever it's talking to agrees with it.
    – timemage
    Feb 8 at 22:05
  • 2
    !! CAUTION !! ... the bread maker board may have powerline voltage on it, relative to earth ground
    – jsotola
    Feb 8 at 23:48
  • 1
    I'm not really sure what would make decent answer to this. What can be said about the Arduino in this context is just true of electronics generally. That circuits, independent of any other concern, care what what voltage difference is applied across them. The UNO is not somehow an exemption to this. So if you have any two voltage numbers that are 5V different, say -7805 VDC and -7800 VDC (to pick something absurd) you can put them in the right order to make a 5V "positive" difference. The thing is I get the sense from your comments you're at least that far along in your understanding.
    – timemage
    Feb 8 at 23:48

1 Answer 1

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As you describe the system, you can connect the system's ground to 5V of the Uno, and the system's -5V to Uno's ground.

Please note that you cannot reverse-feed 5V through the pin with the same label, it is an output. You can use the USB connector. You might want to solder wires at its pins instead of cannibalizing an old USB cable. Alternatively you can try VIN, as there is a LDO, but measure the resulting voltage. The specification demands at least 7V there.

If you want to power the Uno during development by the USB connection, you can disconnect any of -5V or ground, but not both, because you need a reference connection.

In contrast, you cannot use the -11V supply and use VIN and GND of the Uno, because the Uno's voltage regulator uses the lower level as reference and will set Uno's "high" (5V) to about -6V with reference to the system's ground. See:

enter image description here


Concerning the separation of power-line and the controller: Well, I'm just worrying about your health.

If you are sure that all is done well, go ahead. With a working separation you can even dare to connect the development PC to your Uno while the power-line is connected.

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