I have a 'transponder as a key to my room door- how does it work?

Question

its a small disk like thing- when I press it close to the sensor in the door the lock opens.

How does this system work? What is a transponder?

Answer 1

Hello sh,

Answering your last question first;  TRANSPONDER  is a word meaning TRANSMITTER  +  RESPONDER.  Transponders are best known in the Air Traffic Control System.  They are the means by which control towers can keep track of airplane traffic.  The tower “transmits” a radio signal, and the airplanes “respond” giving their identity and position data back to the tower.

But a KEY AND LOCK TRANSPONDER SYSTEM does not use high powered radio signals, but rather,  uses magnetic induction to “read” the key, and only works at close range  The key is passive, and contains no battery.

The lock contains an induction coil which emits a magnetic field, usually in the hundred kilohertz range.  Your key also contains a coil which “couples” to the lock’s coil by magnetic induction.  (This is the same principle as a transformer, where two or more coils share the same magnetic field)

When you hold your key near the lock, some of the energy from the lock is transferred to the key by mutual induction of the two coils, and thus, the key receives power from the lock when it is close to it.

When the key receives this electrical energy, the microchip inside is able to transmit its pre-programmed code back into the lock’s coil, and when the locks microprocessor receives the correct code, it will open the lock

These magnetic coupled key and lock systems are gaining popularity in automotive applications, and are becoming standard equipment on some models.

Another closely related technology is the RFID tags.  (Radio Frequency Identification)