Lab 2 · Analog Inputs — Reading the Joystick¶

In this lab you connect the joystick to the Raspberry Pi through the MCP3008 chip and write a program that reads and prints the joystick's position in real time.

Part 1 · Setting up the hardware¶

The wiring diagram¶

Connect everything according to the wiring diagram. It is the single source of truth for where every wire goes.

HOTIS Joystick and Fan Wiring Diagram

Download the full PDF

Step 1 · Place the MCP3008¶

Put the MCP3008 chip on the breadboard so it straddles the center gap, with pins on both sides.

Check the chip's orientation

The MCP3008 has a small semi-circular divot on one end. Make sure it faces the direction shown in the diagram. Inserting it backwards will cause the chip to not work — and may damage it.

Step 2 · Attach the power supply¶

Place the power supply board on one edge of the breadboard.

Step 3 · Wire everything up¶

Follow the wiring diagram carefully. Colors of wires do not matter, but positions do. If the diagram shows a wire going from pin 28 on the Pi to the second pin from the left on the top of the MCP3008, yours must do the same.

What is a breadboard?

Think of the breadboard as an extension for the chip's pins — it holds all the wires in place so they do not fall off when the board moves. Each short row of holes is internally connected, so a wire plugged into any hole in a row connects to the chip pin in that same row. See Your toolkit for a full explanation.

Part 2 · Programming the Joystick¶

You are going to write a program that reads the X-axis position of the joystick and prints it to the terminal.

Step 1 · Open the file¶

In the terminal:

cd hotis-projects-starter
git reset --hard
geany joystick.c

git reset --hard

This discards any previous edits and restores the original file. See Resetting your work.

Step 2 · Read and print the joystick value¶

Find the comment in joystick.c that says // Here you'll want to read a value from the joystick and store it in a variable. Your code goes below that line.

You need two things: 1. Read the joystick value and store it in a variable. Which function reads an analog input? Which constant names the joystick X channel? → analogRead() · Pins & constants 2. Print the value so you can see it change. How do you print an integer variable? → Printing with printf

Think before you type

What type should the variable be? analogRead() always returns a value that is zero or positive — does that suggest anything about the type? → Variables & types

Step 3 · Build and run¶

make joystick
./joystick

Move the joystick. The number on screen should change as you move it. If it doesn't, check Troubleshooting.

Bonus · Why does Y always read 0?¶

You will notice the Y value never changes — it always prints 0. That is a clue, not a bug in your hardware.

Try it

Look at the wiring diagram. Where is the Y-axis wire from the joystick module plugged in on the Pi? What channel number does that correspond to? How would you read that channel? → analogRead()