Raspberry PI Alarm
C Python Shell Makefile
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This is a not-entirely-naive python implementation of a RFID alarm. The code architecture is event-oriented to make it clear what's happening and to avoid nasty corner cases (such as activating alarm because the doors were not shut quick-enough when leaving :-)). 2 years in permanent operation prove it's pretty stable.

Basically this is the connection used:

[RPI] --- [Webcam]
[Comm. HUB] --- [Alarm]

[Comm. HUB] is just a level converter It converts a GPIO pin (using a MOSFET in my case) to 12V as a feed for the alarm audio device. It also converts UART from [Node] to 3V3 as to not burn RPI...

[Node] a Attiny2313 device It aggregates the detectors (like movement detector, or a "magnetic detector"), contains a piezo beeper to alert user and talks to a RFID scanner. It talks to RPI via regular RS232 UART. Optionally some RS485 or similar could be used, to improve robustness - but at the speeds of few kbauds, it's hardly needed.

[Alarm] a 12V alarm device - just apply 12V/1A and it beeps real loud :) Activated from RPI via GPIO.

[Webcam] an USB webcam to gater images and send them to email, to alert owner

[RPI] a RPI device. Ideally UPS-backed, obviously.