The answer is going to depend upon the exact type of sensor in question. For the electric thermostat, the inventor would be Warren S. Johnson. While it might have seemed crude by the modern standards that we have today, this thermostat was able to keep temperatures within a degree of accuracy – something that’s better than some of the low quality thermostats on the market today! The first motion sensor used for an alarm system came about in the early part of the 1950s, and was the invention of Samuel Bagno. His device made use of ultrasonic frequencies as well as the Doppler Effect.