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.
Used with telephones at the time, this circuit was found in Popular Electronics from the 90s. The...
The presented circuit is suggested by Texas Instruments (www.ti.com) in its Application Report...
One touch of the X1 sensor and the capacitor C1 charges, causing the power transistor to drive by...