This circuit was found in an Electronics Now from the 90's. The integrated regulator circuit and the power transistor must be equipped with heat radiators.
This timer with 1 second accuracy was obtained in a 1973 documentation. The Buzzer is a low-power type...
We find this circuit in a Popular Electronics of the 90s. The circuit is passive and the coils...
The basic idea of this circuit is to indicate the tolerance of a capacitor by reference to a...