# Need to build a circuit that changes the output to 0V when signal exceeds range and then switch back to actual signal after a certain time

I need to build a circuit that changes the output for a certain time when my signal exceeds its range.

I have a kHz-bandwidth signal (incorporating DC) that is part of a control loop and sometimes it goes out of range (+/- 15V). When that happens, I want the output to switch to 0V for a certain amount of time (10 ms to 2 s) and then change back to the actual signal.

What I found so far is a Window Detector circuit that triggers when the signal exceeds the range and a 555 timer circuit which can keep the signal on high for a certain time. I do not, however, know how to switch between the actual signal and 0V when the output from the 555 timer is set to high.

Thanks for everyone's help, I am quite new to designing actual circuits.

• can you give us a rough idea of what "a certain amount of time" is (as in: 1 µs? 1000 s?) and what the bandwidth of your signal is? (50 Hz? 10 kHz? Audio? 30 MHz?) Jul 13, 2020 at 13:25
• Most of the time the signal is modulated to move the piezo, but over time it drifts and then hits the maximum range. You may be better off solving the original problem than try this ad hoc method. Can you describe the setup in detail with block and circuit diagrams ?
– AJN
Jul 13, 2020 at 13:56
• Seconding AJN; this sounds like a problem better solved at the root. If your signal is drifting, you should look into what's causing it to go out of range instead of just clamping it. It's probably a poorly-tuned control circuit, or just a sign that you need to add some anti-windup code into your controller. Jul 13, 2020 at 14:12
• @MarcusMüller That's what I'm guessing it is, anyway. Either straight DSP or a digital control system using one. Retuning the control circuit would also, in this case, be changing the code in the DSP. Jul 13, 2020 at 15:56
• Thanks for all the discussion. You are very right that we should actually fix the root problem which seems to be the windup of the PI Controller. We also realized that what we wanted to do by arbitrarily changing the output to some value might not even help. However, we have a physical, analog PI Controller that was self-built by someone in the past. I'll have to look into how anti-windup could be implemented in such a case. Jul 13, 2020 at 16:17