a question on multiple power sources

[BuddingContraptionist]

i have an atmega168 with a 5 volt battery pack, which connects to an LED that i turn on/off every second.  i have an h bridge and a 12v motor with a 12v power supply.  the 168 talks to the h bridge.  when i plug in only the 12v to the h bridge, for the motors, the LED, which is only connected to the 168, lights up dimly.  This is not a good sign, right?  This indicates a short?  I don't have capacitors on the motor leads, or from the 12v ground to the common ground.

[pomprocker]

what kind of resistor do you have on the LED?

[BuddingContraptionist]

330 Ohms

perhaps i shouldn't have used the word "dimly"... its just a little less bright than what the mcu is making it blink, evidence of the 12v somehow coarsing through to my LED

[HDL_CinC_Dragon]

It depends on how the circuit is wired up I guess. Dont suppose you could post a circuit diagram?

[BuddingContraptionist]

i dont have a circuit diagram, i just bought a pcb and some components and have been winging it.  what's particularly disturbing is that if i start it with both power sources, then remove the 5v, the microcontroller continues to run (light keeps blinking) (!!)

[HDL_CinC_Dragon]

well basically that means somehow your regulated 5v line is connected to the unregulated 12v line. Look with a magnifying glass along the path of your 5v regulated lines and make sure nothing is shorting out. Get a high magnification magnifying glass so you can see high detail along the route. Just make sure only the 5v is getting to pin 7, pin 20, and pin 21. Do the same with the 12v lines and make sure there is no way that the 12v lines are touching those pins in anyway

[Admin]

Hmmmm so how exactly do you have the motor driver connected to the 168?