Low-Cost Window Comparator

Posted on Aug 20, 2012

This circuit outputs a TTL-compatible - pulse whenever the signal falls within the limits set by potentiometers

Low-Cost Window Comparator
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and can be varied to suit the application. Op amps, E, D, and C are used with the two potentiometers to supply reference voltages, derived from a 1-V precision source, for two other op amps (A and B) configured as voltage comparators. The input signal is taken to the negative inputs of both these comparators. C is wired as a noninverting summing amp, used to derive the higher reference voltage. Consequently, the acceptance window is set. Because the voltage across potentiometer P2 is 0.5 V, the window can be set between 0 and 0.5 V above the value chosen for the lower reference value. The lower value is set by PI. The outputs from the comparators are sent to the inputs of inverting Schmitt triggers F and G. Although these triggers operate from a 5-V supply, they have an extended input-voltage range and are capable of handling the comparator"s output voltage swings. The two monostables, J and K, are triggered on the rising edge of the Schmitt outputs. J and set and reset the bistable latch formed by two NOR gates. Latch output Q controls the reset of output monostable L, which can only be triggered if its input goes low, taken from the output of G, while its reset is high. From the timing diagram, when the signal input exceeds the lower reference level, the latch is set and Q goes high. When the higher reference level is exceeded, the latch is reset and Q goes low. If only the lower reference is exceeded, the latch will be set and L is triggered when G"s output goes low. When both levels are exceeded, the latch is set, then reset. When G"s output goes low, it won"t trigger the mono-stable because its reset is low. The circuit, as shown, was used for a bandwidth-limited, 500- to 5 000 Hz input signal with an amplitude range of ± 1 V. Modifications can easily be made to cover other frequency and amplitude ranges.

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