Metabolism detector

The metabolism detector was a mechanism in the Doctor's TARDIS which allowed no one to enter it with the TARDIS key except for the Doctor. Brigadier Lethbridge-Stewart failed in his attempt to open the TARDIS door due to the detector. (TV: Spearhead from Space)

In a parallel universe, the Doctor's TARDIS had a metabolism detector in the lock. Lee was unable to gain entrance into the TARDIS. (AUDIO: Full Fathom Five)