Found in a 1965’s Electronic Design, this circuit operates with a voltage of 1.5 V. The relay can be of a reed relay or other type.
Using a 555, this configuration appears elsewhere on this site. This is from American...
This circuit from a 90s Electronics Now, uses a 4010 gate as a signal amplifier. The circuit...
The circuit shown in the figure has a more efficient action, with an attenuation of 12 dB per...