You'll need a soldering iron and solder, as well as some wire.
We are building this project around the FeatherWing Doubler to connect the Feather M4 Express and Adalogger FeatherWing.
You could also use Feather Stacking Headers for a different form factor, to have the boards stacked on top of each other rather than next to each other. Decide what you'd like to do, then plan ahead accordingly!
In this example, we'll solder male headers onto the Feather M4 Express and FeatherWing, and female headers onto the FeatherWing Doubler.
Do the same for the Adalogger FeatherWing, attaching male headers.
Attach female headers to the FeatherWing Doubler. You can use one your soldered feathers to help keep the female headers straight while you solder.
We'll be playing the audio from the Feather M4 Express through this headphone jack. We'll solder the headphone jack and some other components to the FeatherWing Doubler, under where the Feather M4 Express will go. The Feather M4 Express should go into the right Feather slot, in case you'd like to attach a LiPo battery.
The headphone jack can fit under the Feather M4. I've moved the Feather M4 to the side so we can see where the headphone jack and other components go.
The middle three pins of the headphone will be connected to ground. You can connect them to the Doubler ground rail.
The outer pins of the headphone jack should be connected in series through a 100uF capacitor and a 100 ohm resistor to the Feather M4's A0 and A1 pins. Make sure the cathode sides of the capacitors are connected to the headphone jack. The A0 output (in green) will be the audio left channel and the A1 output (in blue) will be the audio right channel.
Here is a schematic of the circuit. Note that pin 1 of the stereo jack is the middle physical pin. Pins 3 and 4 are the next two inner physical pins. Pins 1, 3, and 4 are all electrically connected to ground. Note that they are not connected to pin 2.