Found in a Popular Electronics from the 90s, this circuit activates a recorder for automatic recording of telephone calls. The circuit was used with old cassette recorders of the time.
Using a 4046 PLL this circuit recognizes a 1.6 to 1.8 kHz tone by driving an output LED. The...
The LEDs, in addition to flashing if the transistor is good, also indicate the polarity of the...
Found in an EDN from the 1980s, this circuit uses a double 555. The circuit activates a relay that...