Bluetooth Low Energy is becoming increasingly popular in mobile applications due to the possibility of using it for proximity data. Proximity can be estimated by measuring the strength of the Bluetooth signal, and actions can then be performed based on a user's proximity to a certain location or object. One of the most interesting applications of proximity information is automating common tasks; this paper evaluates Bluetooth Low Energy in the context of using smartphones to automatically unlock a door when a user approaches the door. Measurements were performed to determine signal strength reliability, energy consumption and connection latency. The results show that Bluetooth Low Energy is a suitable technology for proximity-based door locks despite the large variance in signal strength.