Design Ideas: March 17, 1994

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.

Q₁ functions as an input buffer, which allows correct control even if the input is not logic-level compatible with the CMOS chip. At the beginning of the active-high normal servo pulse, the output of Q₁ goes low, triggering timer IC_1A, which the circuit sets for 3 msec. This action forces the clear line of timer IC_1B high, getting this second timer ready to accept a trigger pulse. At the end of the normal servo pulse, Q₁ goes low, timer IC_1B'Swhich is configured as a latch-triggers, and its output remains high until IC_1A times out. Since IC_1B's output doesn't go high until the original input pulse goes low, the output of IC_1B is the difference between the input and IC_1A's 3-msec timer. Thus, as the input signal increases in width, the output decreases, and the circuit essentially reverses the direction of the servo-control pulse. D₁ and C₁ filter battery noise caused by the servo system and ensure that the servo-pulse reverser does not introduce any jitter into the system.