osvold
Posts: 2
Joined: Mon Nov 03, 2014 10:54 am

RPi and Arduino - power issues

Mon Nov 03, 2014 11:08 am

Hello all,

i am about to connect RPi B+ to my Arduino (UNO R3) with the serial cable. I followed instructions given here http://blog.oscarliang.net/connect-rasp ... usb-cable/ and everything worked fine. So i wrote small programm in Python 2 which reads serial output from Arduino every 15 minutes and stores the temperature and humidity. The Arduino is connected to the RPi just via the USB cable, there is no external power source for the Duino. As sensor, i am using DHT11. The RPi is powered by an mobile phone charger capable of 2Amps. There is no keyboard , mouse or hdmi device attached to the RPi, just ethernet and the single serial cable to Duino. My expectation was, that this should work just fine.
Now, when i run the python programm, everything seems to be fine, except, that the red power diode on RPi is not stable lighting, but blinks in a predictable pattern. So my question is, could it be, tha the Duino with DHT 11 draws so much power, that the RPi power supply is unsufficient? Can i damage the RPi by this?
How do you solve the power supply for these 2 boards, if you connect them via USB cable?
I searched the web, but i did not found a suitable answer. Thanks a lot in advance for any tips.

BMS Doug
Posts: 4423
Joined: Thu Mar 27, 2014 2:42 pm
Location: London, UK

Re: RPi and Arduino - power issues

Mon Nov 03, 2014 2:18 pm

You won't damage the Pi but you might blow the polyfuse (which will self reset with time).

How sure are you of the power supply?
The RPi is powered by an mobile phone charger capable of 2Amps
Mobile phone chargers often make claims that turn out to not be quite accurate, often 5V 2A means either 5V or 2A, because when you draw 2A the chargers voltage has dropped right off.
Also be aware that long leads between the power supply and the Pi can result in voltage drop when under load, another cause of the pi dropping out.

If the power is all OK then check if the Pi will run OK without the arduino connected?


If it is an issue with the Pi being overloaded then you could (if you are confident with a soldering iron) split the USB cable so that the 5v USB power is supplied externally from the pi. (don't forget that you need to keep the Gnd wire linked to the Pi).
Doug.
Building Management Systems Engineer.

osvold
Posts: 2
Joined: Mon Nov 03, 2014 10:54 am

Re: RPi and Arduino - power issues

Mon Nov 03, 2014 4:27 pm

Hello Doug,

thanks a lot for your tips. I checked everything you suggested (BTW, the charger seems to be ok and is not breaking voltage under load).

I borrowed a wattmeter and connected Raspberry to it. Idle it consumes soemthing between 1.6 - 1.7W.
Then i connected the Arduino with DH11 to the whole setup and the consumption went up to cca 1.8 W
After i started the X desktop and python code, the power consumption stabilised at 2W
To my surprise, the red diode is lighting with no blinking etc. So, i checekd every connection at it seems, that the USB connection to the Duino is unstable.
Anyway, problem solved and at least, i know now the power consumption of the device with and without load. Unless you use something big (motors or LED arrays) usage of sensors and powering Arduino throught the USB cable should be no problem at all.

BMS Doug
Posts: 4423
Joined: Thu Mar 27, 2014 2:42 pm
Location: London, UK

Re: RPi and Arduino - power issues

Mon Nov 03, 2014 4:35 pm

Hi Osvald,

glad to help, good to hear that you isolated the problem.
Doug.
Building Management Systems Engineer.

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