Lots of different electronic systems have now been developed for keyless entry. These include systems that require a coded electronic key, such as RF and infrared transmitters, RFIDs (Radio Frequency Identification Devices), keypads and swipe cards. There are also units that do not require a coded electronic key and these include fingerprint, face and iris recognition.

Regardless of format, electronic keys usually comprise a small keyfob-style transmitter and a receiver that goes with the door lock mechanism. The transmitter sends a string of data that is unique to each individual lock and this data must match the data stored in the receiver before the lock will be released. The concept is roughly similar to a standard metal key which has a pattern of peaks and valleys along its length. These peaks and valleys must match the tumblers within the lock in order for the lock to open.

With any type of lock, there is always a problem of security. Keys can easily be copied, while many conventional RF and infrared transmitters are far from tamper-proof. One technique is to use a special receiver to intercept and copy the transmitted code. Once copied, the signal can then be re-transmitted to the door lock to gain unauthorised entry.

In fact, this technique was commonly used by car thieves in carparks and proved very effective against early electronic locking systems. It could also be used to open automatic garage doors and gain access to buildings.

Modern transmitters now circumvent this problem by changing their code each time they send a signal. So if an unauthorised person captures the transmitted code, re-sending this code will not unlock the door. This is because the door lock is now expecting a new code based on an algorithm that both the transmitter and receiver have in common.

Fig.1: a PIC16F628A microcontroller forms the heart of the transmitter circuit. It contains all the software necessary to generate the rolling code and drives an infrared LED (LED1).

This code changing technique is commonly called a "rolling code", although it is sometimes also called "code hopping". It renders copying useless and thus provides a very high level of security. It is also virtually impossible to send a correct code without having a valid rolling code transmitter. This is because of the huge number of code variations possible.

Because of its security advantages, a rolling code transmitter forms the basis of the Rolling Code Keyless Entry System described here. In fact, the odds of picking a correct code at random for our rolling code transmitter are one in 1.4 trillion or one in 10¹².

If you want to know more about rolling code transmissions, refer to the separate panel elsewhere in this article.