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Hello,

I hacked an off-the-shelf blood pressure monitor device that runs on 6V (4 AAA batteries) and connected it to an Arduino UNO R3 to have a program control the device. It is working but I'm having power problems. I have the positive and negative lines from the device connected to the UNO's Vin and GND. I need an Arduino program to ground another wire from the device to control it. However, the 6V from the device is not enough to power the UNO in a stable way, so I started connecting a 9V battery to the UNO's power jack or USB. Doing this works, but I noticed that the 9v power supply is flowing back to the the device from the Arduino. I don't want to fry the external device, but the UNO seems to need more than the 6V the device supplies. If I don't connect the UNO Vin and GND to the device the UNO/device interaction doesn't work. Is there an obvious way to connect the two components with two power supplies so that one won't burn out the other?

Thanks,
andrew
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Generally you only connect the Ground between two systems with different voltage requirements.

If you want to use a single supply, get a 6V regulator and feed both it and the Arduino from ~9V.

Have you tried feeding Vin with 9V and running the "6V" device with 5V from the +5V pin?
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Hello John,

Thanks for your response. I'm already using the +5V pin to power a Bluetooth module. But your 6V regulator idea makes a lot of sense to me. I'm new to electronics and didn't even know there was such a thing. Is there a specific component that comes to mind that you could recommend? I see an Adjustable voltage regulator at Sparkfun but nothing specifically for 6V.

Thanks again,
andrew
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Generally you only connect the Ground between two systems with different voltage requirements.

John, actually I'm ok using two power supplies. Are you saying that connecting the ground and the vin from the 6V device to the UNO while powering the UNO with an additional 9V somehow prevents an over-voltage flow to the device?

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Generally you only connect the Ground between two systems with different voltage requirements.
John, actually I'm ok using two power supplies. Are you saying that connecting the ground and the vin from the 6V device to the UNO while powering the UNO with an additional 9V somehow prevents an over-voltage flow to the device?
You should connect the Arduino Ground to the BPM Ground but don't connect the + side of the two power supplies together.

Try powering the BPM from Arduino Ground and Arduino +5V (NOT Vin).  If the BPM will run on 5V that will make is safer for the Arduino. Six volts is the absolute maximum for an Arduino data pin.
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And the Bluetooth Module? Can I have multiple wires from the 5V pin?  I don't think the 3.3 V pin is enough to power the Bluetooth module.

thanks,
andrew
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Check the bluetooth datasheet- it may well be a 3.3V only part, and 5V would damage it.
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