Remote Keyless Entry Systems

Basic Description

Remote keyless entry systems are found on virtually all cars currently sold in the U.S. These systems allow the holder of the key (or key fob) to unlock the driver's door, all doors and/or the trunk of a car at the push of a button as long as the vehicle is within about 50 feet. The key transmits an encoded RF signal to the main computer, which actuates the door locks. Some remote keyless systems are also capable of performing other functions such as arming or disarming an alarm, or even starting the car's engine.

In order to prevent unauthorized users from detecting and reusing the signals transmitted by the key, a hopping/rolling code is used. The controller chip and the key share an algorithms that generates a 40-bit code. When the key is pressed the transmitter in the key transmits the 40 bit code along with the function code. The function code tells the controller what action to take (e.g. unlock the driver's door). Every time the transmitter button is pushed a new 40-bit code is generated using a Pseudo-Random Number Generator. Same generator is used in the receiver to generate the same 40-bit code. This way the transmitter and the receiver remain synchronized. Since the transmitter's signal may not be picked up by the receiver every time the button is pushed, the receiver also calculates the next 256 codes that the transmitter is expected to transmit and accepts any of them. The transmitter and receiver re-synchronize themselves every time a code is accepted successfully. Since there are roughly a trillion possible codes, the odds of a randomly selected code being one of the 256 acceptable codes are extremely small.

Relatively new to the market is the "bi-directional" fob. This allows for vehicle to send status information back to the user. Some models use a small screen on the fob to inform the user of fluid levels, tire pressure, widows up/down/broken, door open/close, diagnostic data, and lock status.

The most advanced remote keyless entry systems are typically named Advanced Keys or Smart Keys. These keys have passive functions in addition to their active functions performed by pressing a button. A vehicle with a smart key can sense when the key is within a certain range of the vehicle (typically 1.5 meters) and then will unlock the driver car door or wait until the driver touches a specific section of the door (see Prius YouTube link below). These keys typically also include passive ignition capabilities. Once the driver is sitting in the driver seat and pushes an specific ignition button, the car will turn on only when the smart key is within range. This is all done without the driver ever using a mechanical key. It should also be noted that the active functions on these smart key remotes typically operate at larger distances than their traditional counterparts. Most smart keys can operate at distances of 200 meters.

Sensors

Radio frequency receivers

Actuators

Door locks, engine ignition

Data Communications

RF communications typically at 315 MHz (United States and Japan) or 434/868 MHz (Europe)

Manufacturers

Continental, Delphi, Denso, Valeo

For More Information

[1] Remote Keyless System, Wikipedia

[2] Saturn Astra Remote Keyless Entry, YouTube Video, 2008.

[3] 2010 Toyota Prius: Smart Key, YouTube Video, 2009.

[4] Testing Electromagnetic Compatibility of Remote Keyless Entry Modules in Automotive Applications, Agilent Application Note (pdf)

[5] How Remote Entry Works, HowStuffWorks.com

[6] Requirements of Remote Keyless Entry (RKE) Systems, Maxim Application Note 3395, Feb. 16, 2005.