This sensitive circuit was found in the June 1995 Electronics Now magazine. R1 can be from 10k to 1M depending on the desired sensitivity. Power is supplied by the relay.
We found this 4W transmitter in a Mullard transistor manual from 1960. The circuit is powered by...
The response curve of this circuit has a maximum boost of around 6 kHz. The vertical scale is in...
Using the 555 this circuit was found in a Popular Electronics from the 90's. The circuit directly...