An Arduino Nano connected to a light sensor (external to project box), a 5V relay, and a Z-Wave door sensor. Replaced the door sensor's 3V CR123 with a hard wire connection to the Nano's 3.3V output. This allows everything to be powered off the mini USB interface. The circuit is wired such that the relay is de-energized when there is no power. This prevents false-positives in the event power is lost, either by reconnecting the USB cable or when house power goes out and later comes back online.