Oscillation is the last of the four main applications of capacitors. An oscillating capacitor is used with an inductor to form a resonant circuit. Resonant circuits are used when an alternating frequency is needed to provide power to a system or to transmit information through the air. An example of a basic oscillation circuit is a remote keyless entry system. When the button is pressed to unlock the door, power is applied to the circuit and an oscillator (capacitor and inductor) is enabled. This generates an alternating frequency which is then fed to an antenna. The antenna radiates the signal through the air which is picked up by another antenna in the car, and the door is unlocked. The use of the oscillator is required to send a specific frequency to the car so that it can be decoded. Another use of an oscillator is for remote tire pressure monitoring. In this case, a radio frequency oscillator within the car is used to generate an RF signal that is used to power a circuit in the tire. Once the power is applied the tire pressure is measured and transmitted back to the car and read by the car’s computer system. If the tire pressure is low an alert is issued so that the tire can be evaluated. Both of these systems use the oscillating circuits as a means of transmitting information through the airwaves. There are many other systems which use oscillating circuits such as cellular phones which use the capacitors in the antenna circuits.