Keyless entry lets a driver unlock a vehicle without inserting a metal key into the door. A small key fob sends a coded radio signal to a receiver in the car when a button is pressed or when the handle is touched in a passive system. The car checks whether the signal is valid before it powers the door lock actuator.
This matters because it combines electronics, radio waves, security codes, and mechanical motion in one everyday technology.
Inside the door, a control module receives information from the antenna or receiver and decides whether to unlock the latch. If the code matches what the vehicle expects, the module sends current through a small electric motor or solenoid that moves the lock linkage. Modern systems use rolling codes or encrypted challenge-response messages so the same unlock signal cannot simply be copied and reused.
The range is usually short to reduce interference, save battery power, and limit unauthorized access.
Key Facts
- Key fobs commonly use radio frequencies such as 315 MHz or 433.92 MHz, depending on the vehicle and region.
- Radio wave speed in air is about v = 3.0 x 10^8 m/s, so a signal crosses 30 m in about 1.0 x 10^-7 s.
- Wavelength is found with lambda = v / f, where f is the radio frequency.
- A valid unlock command usually contains an ID code, a command such as unlock, and a changing security code.
- The door lock actuator converts electrical energy into mechanical motion to move the latch or lock rod.
- Passive keyless entry often uses low-frequency wake-up signals from the car and higher-frequency replies from the fob.
Vocabulary
- Key fob
- A small wireless remote that sends coded signals to a vehicle to control locks and other functions.
- Receiver
- An electronic circuit in the vehicle that detects radio signals from the key fob and passes them to the control module.
- Rolling code
- A security method in which the transmitted code changes each time the fob is used.
- Actuator
- A device that converts an electrical signal into physical motion, such as moving a door lock.
- Antenna
- A conductor designed to transmit or receive electromagnetic waves.
Common Mistakes to Avoid
- Thinking the fob sends a mechanical key shape through the air. The fob actually sends coded electromagnetic signals that the car's electronics interpret.
- Assuming any signal at the right frequency will unlock the car. The receiver also checks digital codes, timing, and security rules before allowing the actuator to move.
- Forgetting that the actuator is a mechanical part. The radio signal does not directly move the lock, it triggers electronics that power a motor or solenoid.
- Treating passive keyless entry and button remote entry as identical. Passive systems can detect a nearby authorized fob and often use a two-way exchange rather than only a one-way button command.
Practice Questions
- 1 A key fob transmits at 315 MHz. Using v = 3.0 x 10^8 m/s, calculate the wavelength of the radio wave.
- 2 A fob signal travels 20 m from a driver to a car. Using v = 3.0 x 10^8 m/s, calculate the travel time of the signal in seconds.
- 3 Explain why a modern keyless entry system uses rolling codes instead of sending the same unlock code every time.