This circuit uses a heated filament as a sensor. With the air current it changes resistance, and the circuit detects it. The circuit is from a 1969 EEE.
This circuit produces the hum of a mosquito, scaring the female of the species that are the ones...
This circuit is from an American publication from 1974. The circuit can be designed with PNP BC558...
This circuit was found in the Spanish magazine No longer in existence, Mundo Electrónico, in a...