Robot detector

A robot detector was a device the Fourth Doctor had about his person while in Devesham's Space Defence Station. It lit up in the presence of androids. While in the company of Colonel Faraday and Harry Sullivan in an office, it did just that, alerting the Doctor to the fact that they were Kraal androids. Opening the office door, he found a duplicate of his person outside. To escape, he jumped through a window. (TV: The Android Invasion)