This two-transistor circuit is from a December 1964 Radio Electronics. Modern transistors can be tested. The headphone must be high impedance or crystal with a 2k2 resistor in parallel.
The circuit is from a June 1995 Electronics Now. The relay is power-dependent and both R1 and R2...
This circuit for several frequencies was obtained in an American documentation from the 90s. The...
The circuit shown in the figure is suggested by AllegroMicro (www.allegromicro.com) and consists of an...