how could i prevent that? Guess that is what is happening.
I have to turn off the serial device (sim800), it then remains connected with Tx, Rx and GND.
I am switching the GSM on and off through a mosfet and a signal wire. The puck has to share the GND with the GSM gnd going to the mosfet's Source pin right.
But I think the messy contact alarm happens even without using the mosfet switching