This interesting circuit found in American documentation generates a busy signal, without you having to take the phone off the hook if you do not want to answer. The components used are common.
This code practice oscillator was found in an English publication from the 80's. The circuit can...
This crystal oscillator is indicated for the calibration of IF steps of common radio receivers. The...
In the figure we have the simple way of controlling a load or having a signal from an optical...