PWM is most commonly used in radio control equipment today, where transmitter controls change the width (duration) of the pulse for that channel between 920 μs and 2120 μs, 1520 μs being the center (neutral) position. The pulse is repeated in a frame of between 10 and 30 milliseconds in length. Off-the-shelf servos respond directly to servo control pulse trains of this type using integrated decoder circuits, and in response they actuate a rotating arm or lever on the top of the servo. An electric motor and reduction gearbox is used to drive the output arm and a variable component such as a resistor " potentiometer " or tuning capacitor. The variable capacitor or resistor produces an error signal voltage proportional to the output position which is then compared with the position commanded by the input pulse and the motor is driven until a match is obtained. The pulse trains representing the whole set of channels is easily decoded into separate channels at the receiver using very simple circuits such as a Johnson counter. The relative simplicity of this system allows receivers to be small and light, and has been widely used since the early 1970s. Usually a single-chip 4017 decade counter is used inside the receiver to decode the transmitted multiplexed PPM signal to the individual "RC PWM" signals sent to each RC servo. Often a Signetics NE544 IC or a functionally equivalent chip is used inside the housing of low-cost RC servos as the motor controller —it decodes that servo control pulse train to a position, and drives the motor to that position.