B21126E)
This circuit was taken from an old Popular Electronics. The circuit can have its frequency changed from an external signal. The center frequency is given by C1.
In fact, this circuit found in a 1960 transistor manual from Sylvania, simply turns on a lamp when...
This circuit converts pulse frequency to current, which is indicated on an analog instrument. The...
This circuit causes an LED to blink when night falls. Sensitivity is set at R1 and the blinking...