This circuit makes use of a low impedance microphone. The relay is according to the supply voltage which can be between 9 and 15 V. The operational gain is approximately 100.
This circuit was found in an Electronic Design from the 80's. It shows how to use a MOSFET driver with...
Found in Popular Electronics from the 90s, this generator uses integrated circuits from that time....
CMOS 4046 (PLL) is not very common. In this circuit it generates short pulses to excite a high...