Relay and burned Arduino Mega

Hello to everybody,
I'm trying to power on and off an Arduino Mega (with an Arduino GSM Shield and an Adafruit data logger on it) with a relay (2 Channels 5V Relay Module - ITEAD Wiki) controlled by a Trinket Pro. Everithing is powered by the same FIAMM 12 V 9Ah battery and it is happening something that I don't fully understand:

  1. if the positive of the battery is connected to the COM connector of the relay and the Arduino Mega takes the positive from the NO (Normally Open) connector of the relay when the relay itself is turned ON by the Trinket the Mega will burn completely (ready for the trash can) and the Trinket will reset itself several times. It seems as all the current is absorbed by the Mega and the Trinket runs out of power;
  2. if the positive of the battery is connected to the NO connector of the relay and the Mega takes the positive from the COM connector of the relay everything seems to work fine.
    Can please somone explain me why is this happening? Is there a better wiring solution?

Thanks a lot!

P.S. The idea is that the Trinket should turn on the Mega for two minutes (ON period, during which the Mega reads a sensor and sends the data to Xively) and then turn it off for 15 minutes (OFF peroid, battery sparing period) in order to to reduce the battery consumption made by the Mega

Post a photo or scan of a drawing that shows every single connection between the boards as well as all gnd and power connections.

Here are the fritizng pictures. The real components such as the battery, the datalogger shield and the battery are the ones described in my first post.

Thanks a lot.

And here is the working solution…

The relay switched side is just that, a set of contacts. It doesn't matter which direction voltage/ current run through them. You did something different after roasting the mega, and it wasn't the C, NO pins on the relay.

You evidently had a short circuit on the Mega board--something laying across pins, wires touching, wrong polarity, ....

Hi,
Did you wire the DC plug going into the MEGA correctly?
Or did you try changing your wiring around with the battery still connected.
With a LiPo you would best to put a fuse in series in the positive wire.

Tom... :slight_smile: