This sensitive 5-band circuit was found in a September 1995 issue of Electronics Now magazine. The headphones, for greater sensitivity, should be high impedance.
This circuit is from a 1967 Triplett manual. The circuit uses components of the time, the common...
The basic idea of this 1989 circuit is to trigger a relay for a while when a call signal occurs on...
This setting is valid for other transconductance operational amplifiers. The circuit is from...