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Topic: basic electronics question (Read 804 times) previous topic - next topic

maxTouzel

Hi,
I have a pretty basic electronics question. I have an adruino board and some external sensor boards. Each of the sensor boards have 5 relevant pins: Vin, grd,  and x,y, and z acceleration data. I've connected them to the arduino's first 3 analog in channels, and the 3V and ground pins. It works great. Now I want to add a second sensor, i.e. 3 more channels (my arduino has 6 analog ins, so that works great) and the Vin and ground wires, and that's what I not so sure about. I can't just add the other Vin wire to the same 3V arduino pin, can I ? and the ground to the same ground? That would mean having the Vin and ground of each in parallel I guess. My arduino gets power from my computer, but will it be able to keep the 3V even when the circuit is now pulling twice as much power? What are my options?

THanks,

max

retrolefty

Quote
I can't just add the other Vin wire to the same 3V arduino pin, can I ? and the ground to the same ground? That would mean having the Vin and ground of each in parallel I guess. My arduino gets power from my computer, but will it be able to keep the 3V even when the circuit is now pulling twice as much power? What are my options?


First it's kind of bad if your external sensor board labeled it's voltage input pin 'Vin' as that is the name of a arduino pin named Vin which may have as much as +12vdc if there is an external voltage sourced plugged into the arduino's external power connector. If one was to wire the modules power input pin to the arduino pin named Vin, damage to the modules would be certain. For our use here lets say the external sensor module uses the name Volt-in for it's power supply pin. As you have it working, obvisly the module wants a 3.3vdc voltage and that is avalible on a arduino shield pin as you found out. There are also 3 ground pins avalible on the shield's connector pins.

So your question is can you use the one 3.3vdc pin from the arduino to power two or more of your sensor modules? Yes you can, as long as the total current draw from all the sensor modules wired to the 3.3vdc pin is below the maximum rated current avalible from the 3.3vdc pin. How much current is avalible from that pin? That depends on what model arduino board you have, some are rated as low as 40ma avalible, others can supply up to 100ma or more. I suspect you will have no problems powering two of your sensor modules from a single arduino 3.3vdc pin, no matter what model arduino you are using, as most simple accelerometer modules need very little current. The correct answer however is to refer to the datasheet for you specific module to find out what it's power current requirements is and do the math that module current X number of modules must be less then the max 3.3vdc current available for your specific model arduino board.

So how do you wire it more then one module seeing that there is only one 3.3vdc pin avalible on the arduino? You have to come up with a wiring method such that allows you to attach several wires to one pin at the arduino end with the other separate ends going to each module, call that a 'branch' wiring. Or you could run a single wire from the arduino to the first module and then daisy chain another wire from that point on the sensor module to the next sensor modules Volt-in pin. You might also use either method to also wire the ground needed between the arduino and the module(s) so as to not use more then one arduino ground pin.

Hope that helps. Wiring drawings, schematics, are the real language that should be used here rather then English words, but we do the best we can.  ;)

maxTouzel

Yeah. the sensors only pull half a mA each. I'll probably just wire them both to the board separately. One more question: I'm using a prototyping shield http://www.ladyada.net/make/pshield/
and can't find documentation as to whether the groups of 3 pins that come off the midline of the shield are isolated or not. I assume so since there seems to be no other designation.

THanks again,

MAx

retrolefty

Quote
and can't find documentation as to whether the groups of 3 pins that come off the midline of the shield are isolated or not. I assume so since there seems to be no other designation.


Yes they are isolated from any traces going anywhere else on the shield, so you are free to use them as you wish.

Lefty


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