I recently built an access-control-system, which makes it possible to open our local hack(er)space without a physical key. The core of the system is a Raspberry Pi v2 running Debian. Connected to the RPi’s GPIOs is a big status switch, which is used to announce the hackspace status (open / closed).
More recently it got a couple of MCP23017 I²C port expanders with a couple of bell buttons, a number keypad, door openers, reed switches and other door control related stuff.
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