This sensitive 5-band circuit was found in a September 1995 issue of Electronics Now magazine. The headphones, for greater sensitivity, should be high impedance.
Using a capacitor as a sensor, this circuit was found in the 1989 Radio Electronics Experimenter magazine....
Using bipolar, FETs and unijunction this long-lasting timer can reach 10 hours with good quality...
This circuit is found in an American documentation from the 80's. The circuit measures the power of...