Send data to an Arduino by using an audio cable

The simplest way would be to simply feed it through any logic IC which features Schmitt Triggering.

A Schmitt trigger has what is called input hysteresis.

This includes many of the 7400 series of logic ICs. There is a list Here, just Ctrl+F for "schmitt".

Alternatively, if you really want to do it yourself, it's pretty simple to achieve with an Op-Amp.

The way this circuit works is fairly simple.

• Take the above schematic, and imagine the "input" pin is at -V.
• Since the voltage at the "+" input is less then the voltage at the "-" input, the value of "Output" will be Low (in this case, approximately -V).
• Now, "Input" begins to rise. However, notice that there are two resistors. To cause the output to change to High, the voltage at the "+" input has to increase until i is greater then the voltage at the "-" input. This means that the value of input has to increase above the value of the negative input by an amount determined by the ratio of the resistor divider.
• Now, as soon as the "+" input is higher then the value of the "-" input, the value of output will go High (in this case, it will be approximately the value of +V).
• Since the output is also fed into the input through the resistor divider, once the output has gone positive, the threshold to cause the output to go negative changes as a function of the resistor divider.
• When the Voltage at input decreases, the same happens in reverse: Because the end of the voltage divider is now at +V, the value of input has to go below the value of the - input to cause the output to become low.

All together, this is called positive feedback (as you can see, the output is connected to the positive input. As a result, the transition threshold is affected by both the output state and the input state

The term for this, e.g. the output as both a function of the input and the previous output is Hysteresis.
When hysteresis is implemented in a circuit like this, it is called input hysteresis.

Apparently you have a audio signal with 3.3 Volt peak to peak amplitude and want to couple that into a "microphone" input of some other audio device. Microphone inputs are meant to take the very small signals produced by microphones. These are often 1 mV or less, with peaks maybe a few mV in normal operation, although this depends a lot on the microphone.

For starters, you probably want to attenuate your input signal by at least 1000 in voltage. That may still be a bit high, but most microphone inputs can probably handle that. You will have to experiment. Maybe you need a attenuation of 3000 or 5000 so that you set the volume control near the middle of its range when listening. A simple start would be:

That will attenuate by about 2000 in voltage. Note the capacitor to AC couple the signal, which removes the DC bias of the input signal.