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.
This circuit is from an American technical documentation from the 70s, but it can be easily...
This circuit converts a range of input voltages at frequencies from 800 Hz to 3.7 kHz. This is a...
This circuit has a configuration that is widely known and explored on our website. The SCR can be...