I want to make a circuit which produces a logic high signal upon receiving an oscillating input voltage. In the past I have gotten away using an RC delay signal. When my input signal wasn't oscillating it was GND and when it was oscillating the RC filter would give me a voltage above GND depending on the duty cycle of the signal.

The last point is where I ran into trouble with the RC delay. If the duty cycle of the signal was 50% then the RC filter would give me high enough a voltage to trigger my logic gates. However, with a 10% duty cycle the RC filter's voltage will be too close to ground and the logic gates will remain untriggered.

How can I fix this in such a way that I will produce a true signal if I see my oscillating signal? It is typically 50-400 Hz, with a duty cycle of 10-90%

old circuit

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    \$\begingroup\$ Note that your old circuit will also think that something is oscillating if you simply apply a DC voltage to its input. It's not actually detecting oscillations. \$\endgroup\$ Oct 31, 2022 at 1:36
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    \$\begingroup\$ If the oscillation stops, what voltage will be present on the input? 0 V? same as Vcc? A high impedance and no set voltage? How quickly do you need the output to respond when the oscillation is applied or removed? \$\endgroup\$
    – The Photon
    Oct 31, 2022 at 2:03
  • \$\begingroup\$ If the oscillation stops we will see a GND signal. The response would ideally be quick but should at least be faster than 10 seconds between receiving an oscillating input and outputting a true value. \$\endgroup\$
    – Feynman137
    Oct 31, 2022 at 4:15
  • \$\begingroup\$ @JonathanS. yes you are correct. In my case though I either had an oscillating input or a GND signal. This is why it worked. \$\endgroup\$
    – Feynman137
    Oct 31, 2022 at 4:19

2 Answers 2


You want a retriggerable monostable multivibrator. "Multivibrator"* means that it's a square pulse generator. "Monostable" means that it just pops off once (as opposed to an astable multivibrator, which generates a train of pulses). When you give a start pulse to a monostable multivibrator, you get back a pulse of a length defined by the circuit (usually a resistor and a cap). "Retriggerable" means that if the thing is in the middle of giving a pulse and it gets another input pulse, that just stretches the output pulse by the design pulse time.

Set the on-time according to the frequency of your incoming pulse train, and the output should be constant on for a pulse train in.

* Wikipedia says that the name "multivibrator" comes from the fact that it's got a square wave output, and thus generates multiple harmonics. Hmm.


A monostable multivibrator will work, but a rectifier and comparator might be simpler/cheaper:


simulate this circuit – Schematic created using CircuitLab

Admittedly, we're talking about a difference of a few pennies here.


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