With the flight controller and radio receiver both attached to the quadcopter, we can now connect them together electrically.
Now, I am going to take a moment to explain the connections on the radio receiver, then, in the next section, I will explain the connections on the KK2.1, and finally I will explain how to attach the two.
About the Radio Receiver Connections
So, if you take a look at the radio receiver, you will notice that the connections come in sets of three pins. The label on the top of the receiver shows which rows belong to which channel. Each channel corresponds to a different action on the radio transmitter. For example, channel one receives commands when you move the right stick left and right, channel two receives commands when you move the right stick up and down, et cetera
Each of the three pins for each channel carries a different electrical connection. There is a tiny little legend underneath the label for channel one that shows which pin carries which signal, but this legend is difficult to see, so let me put it in words: the outside row of pins is ground, the middle row of pins is power, and the inner row of pins is signal.
About the KK2.1 Connections
Now that you understand the connections on the radio receiver, direct your attention to the flight control board. Looking at the board from the bottom (button side) you will find the connections for the radio receiver to the left of the screen. There are five rows of three pins. Just like on the radio receiver, each row of three pins corresponds to a different radio transmitter channel.
The channels on the KK2.1 board are a bit more difficult to understand than the ones on the radio receiver though because, if you take a look at the bottom of the board (you might need to peek through a slot in the mounting plate), you will notice that the rows are labeled for airplanes, not for quadcopters. Starting with the top row of pins, the order goes aileron, elevator, throttle, rudder, auxiliary. These labels correspond to the various control surfaces (flaps) used to control airplanes in flight. So we will need to form a mental map of the way these airplane controls correspond to quadcopter controls.
Just like airplanes have four control channels, so too do quadcopters, but the terminology used for quadcopter control is a bit different. The four control channels are roll, pitch, yaw, and throttle. To translate the KK2.1 receiver connections from airplane terminology to quadcopter terminology, we just need to substitute the airplane-related words for quadcopter-related words:
aileron → roll
elevator → pitch
rudder → yaw
throttle → throttle (which means altitude for quadcopters)
This terminology is actually really important. You should make sure to memorize the relationship between airplane controls and quadcopter controls because throughout the rest of this tutorial I will often be using these terms interchangeably; and this is the same for many other sites. It will make your life much easier if you do not have to come back here to look at the above chart every time you encounter one of these words.
The auxiliary connection is the same for airplanes and quadcopters since it is just used to control other accessories like lights or the auto-leveling feature.
Connecting the Radio Receiver to the Flight Controller
Armed with your new knowledge of the radio receiver and flight controller pin layouts, we can now connect the two parts together. Before we begin, there is just one more piece of information to consider: on the radio receiver, all of the ground and power pins (the outside and middle pins) are connected together. Therefore, we only need to connect one ground pin and one power pin to the KK2.1 flight control board. The practical upshot of this is that we will only need three servo leads to complete all the electrical connections.
First, plug one of the servo leads across the three pins on the radio receiver that correspond to channel one. Orient the plug so that the brown wire is on the outside. Then, plug the other end of the servo lead into the aileron plug on the KK2.1 board, which is the top one. This plug should be oriented with the brown wire on the outside pin. On my quadcopter, I stuck the wire underneath the flight control board just to prevent the wire from hanging out in open air.
Now for the next wire. Since we’ve already connected a ground and power wire from the KK2.1 board to the radio receiver, we will only need a signal connection for the remaining three signals. Conveniently, the servo leads happen to have three wires. So, connect the second servo lead going down the row of inside pins (signal pins) with the yellow wire plugged into channel two, the red wire plugged into channel three, and the brown wire plugged into channel four. Then, plug the other end of the wire into the pins closest to the screen on the KK2.1. The yellow wire should be closest to the aileron plug.
There is one last connection to make, the connection for the auxiliary channel, which we will use to turn on and off the self-leveling feature of the KK2.1 via the left switch on the radio transmitter. More on that later, for now connect another servo wire, this time across the entire fifth row of columns on the radio receiver. Connect the other end across the fifth row of pins on the KK2.1.