Quantity   Component name
1 × Raspberry Pi Version 2 or 3 // $20~$35
1 × 8Gb SD card ~$6
1 × GPIO breadboard + wires and button ($5)
1 × An old webcam $0
1 × [OPTIONAL] MicroBot Push - wireless button pusher To automate your door - https://prota.info/microbot/push/index.html
1 × [OPTIONAL] Philips Hue Light - for visual warning https://www.amazon.com/Philips-White-Starter-light-bridge/dp/B014H2OZAC