Previously, there was only one type of intercom: touch it and the door opens. Or it doesn't open if the keychain is from the wrong house. In this article - how it all works inside, how much data is stored there and why these key fobs do not need batteries.

Intercoms and access codes

What we call an intercom works like this:

      1) there is a strong magnet that keeps the door closed;

      2) there is a panel where you can put the key;

       3) at this moment, the intercom receives a code from the key and compares it with the codes stored in its memory;

If this code is not in the intercom memory, the door does not open, but if it is, it opens.

It turns out that the only thing you need from such a key is to transmit the right sequence of bits, which already exists in the intercom memory.

                         Contact keys

This is the most popular and the oldest key from the intercom - their production began in the 90's and made until now. From the outside it looks like a battery:

There's no battery inside - just a chip on a small circuit board.

One of the pins of the chip is connected to the top of the key and the other to the side.

The chip stores a certain sequence of bytes.

In general, it works like this:

The key is placed against the intercom and the side touches a special bezel.

A current of about 3-5 volts starts to flow through the bottom and side of the key.

This current enters the microcircuit and it starts to work

During operation the chip controls the resistance in the circuit. It gives pulses of current at the output, like opening and closing a faucet.

The intercom measures the current it receives from the key. It "hears" the pulses.

From these changes, the intercom makes a sequence of zeros and ones.

It compares this sequence with the ones it has in its memory, and if it matches, it opens the door.

It sounds cumbersome, but in fact the whole cycle takes about half a second.

However, in today's world, everything is improving and the intercom is no exception. The old intercom has been replaced with emergency intercom