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 shield can be controlled by positive or negative voltages of the sensor acting as a...
This circuit is from a 1980s Signetics documentation. Equivalent operational amplifiers may be...
This circuit was found in an Electronics World of 1969. Transistors are general-purpose types. It...