# Key Fob

#### Nathan Hale

Joined Oct 28, 2011
159
Hi everyone! The building I live in has doors which open only when i touch my " key fob " to a special box attached to the door. The key fob is a tiny oval grey device the size of a tootsie roll. My question is how does the key fob work? I broke open one of them and didnt see any battery. Nor did i see any antenna to communicate. how exactly do these things work? can some one explain to me please.thank you.

#### praondevou

Joined Jul 9, 2011
2,942
Hi everyone! The building I live in has doors which open only when i touch my " key fob " to a special box attached to the door. The key fob is a tiny oval grey device the size of a tootsie roll. My question is how does the key fob work? I broke open one of them and didnt see any battery. Nor did i see any antenna to communicate. how exactly do these things work? can some one explain to me please.thank you.
There are several technologies.
If it only works when you get very near the box it's most likely that inside the box there is e.g. a 125kHz "transmitter", normally a simple coil. The magnetic field decays very fast this is why it only works near the transmitting coil.

In the system I describe inside the key fob there is a resonant LC circuit adjusted to the transmitting frequency. The energy received from the magnetic field can be used for integrated circuitry. From there several approaches are possible. The key fob circuit could modulate the transmitted signal by absorbing energy from it, this modulation can be detected by the receiver which also sits in the "box". There is also a possibility that the 125kHz magnetic field is used to energize a transmitter inside the keyfob that transmits at a higher frequency, e.g. 433MHz.

The frequency of the transmitter determines the maximum distance to the keyfob to work. Since you have to put the keyfob very near the box I assume it's LF,i.e. 125kHz.

#### gerty

Joined Aug 30, 2007
1,302
And the reader has to be programmed to recognize your particular fob, in other words you cannot just go out and buy another just like it and expect it to work.

#### Nathan Hale

Joined Oct 28, 2011
159
but where is the power supply to the fob?? there is no battery inside when i broke open one of them.

#### praondevou

Joined Jul 9, 2011
2,942
but where is the power supply to the fob?? there is no battery inside when i broke open one of them.
It`s powered by the magnetic field of the transmitter. No need for a battery. Think of it as a transformer without a core.

Read section 5.3.3 (passive tags) of THIS document.