This circuit was obtained in an Electronics Now from the 90s. The circuit uses operational amplifiers from the time and the indicator is of the Sonalert type with internal oscillator.
This transmitter modulator amplifier for the 11-meter band (CB) was published in Radio Electronics...
This oscillator, which operates from 2 V, was found in the June 1993 issue of Radio Electronics magazine....
This amplifier for electrometer can work with currents of 0.1 pA in a band of 1 Hz. The circuit is...