This sensitive three-transistor circuit was found in Electronics Now magazine and is earphone-readable. The tuned range depends on the coil.
This first order Butterworth filter is tuned to frequencies around 1 kHz. The frequency is given...
This very low current circuit was found in the April 1995 Electronics Now magazine. The frequency...
This circuit was found in an American 1980s documentation. It makes use of an unusual MOSFET today...