This alarm oscillator was found in the December 1974 issue of Radio Electronics magazine. The power transistor must be equipped with a good heat sink. The Frequency basically is given by C1.
A double port MOSFET is used in this circuit, which drives a 100 uA moving coil meter or close to it....
This is the receiver for the previous transmitter. The microphone is of the high impedance...
This 500 V line current monitor with the LTC6101 was obtained from Linear Technology's own 2005...