Circuit Project Electronic

Typical remote-control systems and robotics applications use standard R/C servos, which often require a reversal of the direction of rotation. Since varying the input signal’s pulse width between 1 and 2 msec controls the servo’s output position, a circuit that adjusts the pulse width to cause direction reversal can often come in handy. Many such circuits exist that use relatively sophisticated servo-control ICs, but the implementation in Fig 1 uses a standard CMOS IC to produce a reliable design at low cost.

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http://www.edn.com/archives/1994/031794/06di6.htm