This circuit was found in a 1975 documentation. Current general-purpose transistors can be used, and the power is supplied with relatively high voltage. The gain is 30 dB.
This circuit activates an old conventional cassette recorder when the telephone line is answered and...
In the figure we have two possible configurations for multivibrators with relay. The idea comes...
The LED is optically coupled to the LDR1 sensor to provide negative feedback to the circuit and...