This circuit is from a 1968 publication. Here we have several types of resistive sensors with individual adjustments triggering a single SCR. Power depends on the type of sound device to be triggered.
We found this circuit in the Radio Electronics magazine of January 1986. The frequency is given by the...
This circuit has a type of bias called "virtual earth" providing good gain, given by R2. The...
The figure shows how to convert a rectangular signal into a triangular signal using an integrator. The...