There are two sensors within the REVIAN cap. One you can see and the other you cannot.
Under the top of the cap, where the internal rechargeable battery is located, there is a printed circuit board (PCB). Mounted to the PCB is a temperature sensor. This sensor protects REVIAN users as it will stop a treatment if the cap becomes too warm. It would be very unusual for the cap to overheat and stop a treatment. This has only been observed when a cap was left for an extended period in a hot vehicle or when the cap was heated for testing purposes.
There is also a capacitive touch sensor that you can see. It is located in the inside front of the cap, as shown below. This sensor must make contact with your forehead in order to connect to the cap and run a treatment. You may simulate this by placing your thumb or finger over the sensor. If you're experiencing difficulty connecting to the REVIAN cap it is recommended that you try connecting while placing your finger over the sensor.