Circuit converts pulse width to voltage

Ron Hegler, Picker International, Cleveland, OH -- EDN, 10/28/1999

The circuit in Figure 1 stems from a radio-controlled modeling application, which requires a voltage proportional to the width of the incoming servo pulses. The circuit is optimized for a positive-going pulse width of 1 to 2 msec, repeating at intervals of approximately 17 msec. The output produces a voltage of 0.95V for a 1-msec pulse to 2.25V for a 2-msec pulse. The circuit operates similarly to a PLL, but it locks onto the pulse width, rather than to the frequency, of the incoming signal. IC_1A is a one-shot multivibrator with its time constant a function of R₁, the FET's on-resistance, and C₁. IC_1B is a pulse-width comparator that compares the reference pulse from IC_1A with the incoming pulse. Upon the rise of the incoming pulse, IC_1A's Q output clocks high and drives IC_1B's D input high. If IC_1A times out before the input pulse falls, IC_1A's Q output goes low, driving the D input of IC_1B low. This action drives the of IC_1B high when the input pulse falls. The output connects to the FET through D₁ and R₂. C₂ filters the output to adjust the time constant of one-shot IC_1A to match the incoming pulse. The voltage across C₂ indicates the incoming pulse width. (DI #2431).