Circuit converts pulse width to voltage

-October 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. IC1A is a one-shot multivibrator with its time constant a function of R1, the FET's on-resistance, and C1. IC1B is a pulse-width comparator that compares the reference pulse from IC1A with the incoming pulse. Upon the rise of the incoming pulse, IC1A's Q output clocks high and drives IC1B's D input high. If IC1A times out before the input pulse falls, IC1A's Q output goes low, driving the D input of IC1B low. This action drives the of IC1B 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 IC1A to match the incoming pulse. The voltage across C2 indicates the incoming pulse width. (DI #2431).

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