Robot detector

A robot detector was a device the Fourth Doctor had about his person while in Devesham's Space Defence Station.

History
The robot detector was a small, plain metallic box with a red light, which 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, and the real Faraday and Harry had already been captured and replaced.

Opening the office door, the Doctor found an android duplicate of himself outside, armed with a revolver. To escape, he jumped through a window. (TV: The Android Invasion)