This circuit was found in a publication by Edelman from 1924. The power comes from the battery and the listening is through a high-impedance headphone.
This circuit from a Popular Electronics from the 90's can generate signals from 0.03 Hz to 3 kHz...
The presented circuit provides an output voltage proportional to the input current, according to...
The 4528 is a dual-retriggerable monostable that needs only the timing network formed by R and C...