Raspberry Pi - Blinking an LED with Rust

Prerequisites

• Raspberry Pi (I am using Raspberry Pi 4 Model B)
• LED (8mm, 3.5V, 30mA)
• Resistor (220Ω)
• Breadboard
• Jumper wires (2 MF)

We assume that you have already installed Rust on your Raspberry Pi. If not, you can follow the instructions here.

GPIO Pins

GPIO stands for General Purpose Input/Output. Raspberry Pi has 40 GPIO pins. These pins are used to connect external devices to the Raspberry Pi. These pins can be used as either input or output pins. We will be using these pins to connect our LED to the Raspberry Pi.

Image Source - Raspberry Pi Documentation

The numbers in the above image are the Physical Pin Numbers. Also known as the Board Pin Numbers. The numbers in the image below are the BCM Pin Numbers. Also known as the GPIO Pin Numbers. These numbers are used in the code to refer to the GPIO pins.

Image Source - Raspberry Pi Documentation

The Circuit

The LED that I am using has the following specifications:

• Forward Voltage: 3.5V
• Forward Current: 30mA (0.03A)

One can tap into 3.3V or 5V power provided by the Raspberry Pi pins. Since, the LED requires 3.5V, we will be using the 5V pin. Also, we need a resistor to limit the current flowing through the LED. We can use the following formula to calculate the value of the resistor:

R = (V - Vf) / I

where,
R = Resistor value in Ohms
V = Voltage of the power source
Vf = Forward Voltage of the LED
I = Forward Current of the LED

The value of the resistor in Ohms will be:

R = (5 - 3.5) / 0.03
R = 50Ω

So, we need at least a 50 ohm resistor. I’ll choose 220 ohms. The higher the resistor value you use, the dimmer the LED.

Here is the circuit diagram we will be referring:

Circuit Diagram for LED

We will be using the BCM Pin 23 (Physical Pin 16) to power the circuit. The power flows through a 220Ω resistor and then through the LED. Note that the longer leg of the LED is connected to the resistor, while the shorter leg of the LED is connected to the ground pin 6 of the Raspberry Pi.

The Code

It's time to write the code that will make our LED blink. Create a new Rust project named led-blinking:

cargo new led-blinking

Add the following dependency to the Cargo.toml file:

[dependencies]
rppal = "0.14.1"

The rppal (Raspberry Pi Peripheral Access Library) crate provides access to the Raspberry Pi's GPIO, I2C, PWM, SPI and UART peripherals. You can read more about it here.

Now, update the src/main.rs file with the following code:

use std::thread;
use std::time::Duration;

use rppal::gpio::Gpio;
use rppal::system::DeviceInfo;

// GPIO uses BCM pin numbering. BCM GPIO 23 is tied to physical pin 16.
const GPIO_LED: u8 = 23;

fn main() {
    println!("Blinking an LED on a {}.", DeviceInfo::new()?.model());

    let gpio = Gpio::new().unwrap();
    let mut pin = gpio.get(GPIO_LED).unwrap().into_output();

    loop {
        // Blink the LED by setting the pin's logic level high for a second.
        pin.set_high();
        thread::sleep(Duration::from_millis(1000));

        pin.set_low();
        thread::sleep(Duration::from_millis(1000));
    }
}

Let’s understand the code. First, we import the thread and time modules from the standard library. Then, we import the Gpio struct from the rppal::gpio module. The Gpio struct provides access to the GPIO pins. Then, we import the DeviceInfo struct from the rppal::system module. The DeviceInfo struct provides information about the Raspberry Pi device in use.

Next, we create a new instance of the Gpio struct. Then, we get the pin 23 using the get method. The get method returns a Result<OutputPin, Error>. We use the unwrap method to get the OutputPin struct. Then, we convert the OutputPin struct into an output pin using the into_output method.

Finally, we enter an infinite loop. In each iteration of the loop, we set the pin to high using the set_high method. Then, we sleep for a second using the sleep method. Then, we set the pin to low using the set_low method and sleep for a second again.

Build and Run

Now, let’s build the project with cargo build:

led-blinking on master [?] via 🦀 v1.55.0
➜ cargo build
   Compiling libc v0.2.101
   Compiling rppal v0.12.0
   Compiling led-blinking v0.1.0 (/home/gaurav/Projects/led-blinking)
    Finished dev [unoptimized + debuginfo] target(s) in 2.25s

Then, let’s run the project with cargo run:

led-blinking on master [?] via 🦀 v1.55.0
➜ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s
     Running `target/debug/led-blinking`

You should see the LED blinking. If you want to stop the program, press Ctrl + C.

Glowing LED

Conclusion

In this blog post, we learned how to blink an LED with Rust on Raspberry Pi. We also learned how to calculate the value of the resistor required to limit the current flowing through the LED. If you have any questions or suggestions, feel free to drop a comment. A few useful links are given below:

• Raspberry Pi Documentation
• RPPAL
• RPPAL Documentation

Thanks for reading, see you in the next one!