The 3.5 mm socket is usually provided to connect a headphone. This signal is a low level audio signal. When you receive Morse, the signal is probably an audible frequency carrier (more or less a sine waveform) modulated by a square waveform with a 100% depth (also known as ASK). And of course some noise added. You can convert this signal to a digital 0/1 signal in many different ways.

1) Using an envelope demodulator (a diode, a capacitor and a resistor), followed by an analog comparator (an opamp for example, though a simple transitor will do). This approach is cheap but noise sensitive, and you probably will have to adjust the comparator level by hand. Or maybe you can use the radio volume control for this purpose.

2) Using a tone detector chip (used in telephone gadgets) or a more elaborated PLL detector. This approach is much less noise sensitive, but more expensive.

If I had to design this converter I should begin looking at the received signal (from the 3.5 mm socket) using an oscilloscope, after I had dialled a Morse station. The more interesting features are: tone frequency, amplitude, modulating pulses lenghth (deeh-daah and spaces) and noise. Then, I should try the simplest circuit. If this circuit fails, go for a more complex solution.

Alfredo del Rio.