So I know this sounds really dumb, but I have a chip that consumes a max of 18ma and will run on 2.5v and I was thinking can I just power and ground connect it via digital pins on an arduino pro mini? So then I can just solder directly to the pro mini pins instead of having to make an interface board that routes the vcc and ground to the vcc and ground pins. I know it isn't hard to do that since I've already done it, but if I simplify the interconnect that would be better. If it will work as expected. I'd guess there is some resistance in the digital pins when high to source 3.3v and to ground when pulled low, but my device can run on 2.5v, so if there is a voltage drop it might be just fine...
I see no reason why that should not work. However you should be aware that if you connect the "ground" of the "chip" through a digital output set to "LOW" the voltage will not be zero, so you wont have a proper "ground" on the chip, and that may affect anything else you connect to it.
A proper schematic and link to the "chip" would be useful.
ok, so I just moved the vcc and gnd pins to pins 16 and 17.
First I just moved VCC to pin 17 and it looks to work no problem, the voltage dropped to 2.6v to the chip but no problem. I talk to it via SPI so that all works. But when I then moved gnd to pin 16, the voltage dropped to 2.2v and the chip was not so happy, still works but not as consistent, which makes sense as 2.5 is datasheet minimum. One could probably attach something with either the gnd or vcc pin in the right place, so just need a wire to attach one or the other. For some chips they will run down to 1.8v, so this might still work for those. In my case since i'm using spi and the pro mini 328 can't remap sdi pins, so i am no choice on where this board must attach.
But then they'll stop working together with 3.3V logic.
Why not connecting vcc to the arduino pin and gnd just to gnd? To the board or arduino gnd pin
For example a circuit board with 4 pins, vcc tx rx gnd
I could just connect it directly to the arduino pins but vcc and gnd will not have two other pins between them. But I could solder it directly to pins gnd, 2, 3, and 4, and make pin 4 be the vcc to power it. Otherwise I'd have to run a wire or make a 'connector board' to attach it that in this case must connect to vcc on the other side.
It would also be very simple to turn on and off the attached board just by taking a pin high or low.
Connect GND to GND, Arduino output pin to VCC of the chip. Output high, the chip is powered, output low, the chip is shut down. There is no power advantage to switching both sides of the device.
I realize, you're trying to avoid 'splitting' your GND wire as well, but that's a bad idea. Wire it properly, GND is the reference for everything in the Arduino world.
By the way, there's a GND and +5V pin on the 6-pin header usually used for ICSP, if you need an extra of each.
@benimble
A digital output pin is not a power supply pin.
The output voltage is NOT regulated.
The voltage will fluctuate depending on the current draw and I’m sure your device doesn’t draw a constant current.
NOT a good idea.
The promini has 3 ground pins and two Vcc.
I needed something similar with a Pro Trinket for an I2C connection. A4 and A5 is I2C. I tied A2 to +5 and A3 to gnd with 26awg bus wire.
You can do the same and assure your digital pins are set as inputs, which they are by default.
Ok, I understandt that you want to plug your small board on top of the Arduino with the 4 pins aligned, like a small hut board.
Then as you suggest I would at least choose the GND pin of the Arduino and the next 3 pins: 2, 3 and 4. So, I think that you should connect both GND's.
I see two problems only. You shouldn't rely only on the pins for the mechanical subjection. Better additionally fix the board to something else, or make sure that it will have no tension. Specially if the pins will be only in one side of the board.
And as Jim said, the digital pins are not good as power source, although in this case it will probably work, but 18mV is significant. I have done it only with sensors that consume less than one mV.
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