This circuit was found in a Popular Electronics from the 90's. The output signal can be applied to a headphone or speaker. The microphone can be a piezoelectric tweeter capsule or other type.
This circuit was found in old documentation. The SCR can be any of the 106 family. Note the sensor...
This circuit was found in Ferranti's 1974 transistor manual. The transistors are from the period and...
Short duration pulses can be obtained with the relaxation oscillator that uses common transistors and is...