Raspberry pi as a replacement for an old analog home alarm
GPIO pins used are 21 20 26 and 16 with ground connected to pin 39. The layout of these pins on the raspberry pi can be found here:
https://www.raspberrypi.org/forums/viewtopic.php?f=91&t=181315
The GPIO pins are connected using some jump wires to the ends of reed switches that were used in an old analog home alarm.
2017