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 simple circuit was found in the August 1984 issue of Radio Electronics magazine. The circuit...
This simple two-transistor circuit used transistors from the 60s, having been found in a...
This circuit is from an old manual, but can be easily assembled as the integrated one is still...