Arduino partially powered from analog inputs.

Other Hardware Sensors
1

I have a set up like this:

The PWM signals do not exceed 5V. The configuration to the right of the dotted blue line cannot change. Note that the external power supply is intended to power the input device only, and the ground is tied to the Arduino's ground.

When the Arduino is attached to the computer (and powered) by USB, everything works great.

However, I also need to be able to run the input device independently without the Arduino and a computer. The problem is, when the Arduino is not powered (e.g. USB disconnected) but the device is, the device's PWM outputs partially power the Arduino. There are two effects:

  • Some of the LEDs on the Arduino dimly light, although it does not actually power on completely. This doesn't bother me.
  • More importantly, it draws enough current from the input device to noticeably affect the device's operation. This is the problem.

Is there a way to prevent the Arduino from drawing current from the input device when the device is powered but the Arduino is not? I need the device to operate correctly regardless of whether or not the Arduino is running.

Thanks!
J

2

How about some transistors in the lines on the Arduino side of the line, held "on" by a digital line from powered up Arduino, which disconnects the analog lines when the power goes off?

Edit... or just use one to disconnect the common ground.

3

It's not clear why do you need PWM signals read by analog in? Or may be you using analog as digital?
Answering your question, you cant stop current flow in arduino board, by the way it destructive for AtMega to be powed this way, w/o additional hardware. But you can limit this effect, setting resistors in series with inputs, 10 k or so.

4

Hmm..what is this "PWM Device" ? Any spec-sheet?

5

At the very least add 10k resistors inline with each Arduino input. This limits the current into the Arduino protection diodes to safer levels.

Those LEDs appear not to have current-limiting resistors - they need them.