• 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

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