# Circuit for producing logic high when input is oscillating

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%

• 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. Oct 31, 2022 at 1:36
• 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? Oct 31, 2022 at 2:03
• 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. Oct 31, 2022 at 4:15
• @JonathanS. yes you are correct. In my case though I either had an oscillating input or a GND signal. This is why it worked. Oct 31, 2022 at 4:19