This circuit is from an American publication of the 80's. It makes use of components of the time. However, the circuit is the basis for a modern shield application since the input is TTL.
In the figure we have a classic Clapp oscillator configuration for a frequency of a few tens of...
This circuit takes advantage of the inductance of the phone in the resonant circuit. The circuit is...
In the figure we have the way to increase the voltage of a 78xx by adding a zener diode. The...