Hello, Blink!

If you are familiar with Arduino chances are you have followed through their version of the hello world tutorial. The code shows the simplest thing you can do with an Arduino to see physical output, a blinking light.

This is the NodeMCU equivalent. Note that we will not be using an external LED but the devkit's built in instead.

NOTE: We learned how to load a script into the NodeMCU devkit in a previous tutorial, check this [introduction][intro] if you have not set your Mac environment.

Source Code

First, the source code:

local pin = 4
local lighton = 0

gpio.mode(pin, gpio.OUTPUT)

-- time is in milliseconds, so 2 seconds.
tmr.alarm(1, 2000, 1, function()
    if lighton == 0 then
        lighton = 1
        gpio.write(pin, gpio.HIGH)
    else
        lighton = 0
        gpio.write(pin, gpio.LOW)
    end
end)

Create file and upload

Create a file in your code directory named blink.lua and type the source code.

To upload the file to the board we will issue the following commands in a terminal window. cd to the code directory and execute:

$ esp file write blink.lua

In the hello-world tutorial we saw that we could use the esp file execute subcommand to run our program. Under the hood, esp is using Lua's dofile command. You can think of dofile as a kind of primitive operation to run chunks of Lua code.

To execute the blink.lua file we will use the esp run command, which sends a string- hopefully Lua code- to the NodeMCU board and executes it.

Will send send a Lua command to the board:

$ esp run 'dofile("blink.lua")'

GPIO

As far as the code goes is fairly simple but it highlights some important concepts like interacting with peripherals using the gpio module.

General-purpose input/output (GPIO) is a generic pin on an integrated circuit whose behavior—including whether it is an input or output pin—is controllable by the user at run time.

By making connections to those pins (they are also known as header pins) the Raspberry Pi can interface to the electronic world which consists (amongst other things) of sensors and indicators and actuators.

You can set the value high or low.

there is a desire to be able to use a computer to control (also known as to ‘output’) to electronic circuits, and to gain useful information (aka obtain ‘input’) from circuits

3.3V logic levels means that the NodeMCU will interpret anything very close to zero volts as a logic ‘0’ and anything higher than around 2V as a logic ‘1’. Inputs beyond 3.3V will damage the board.

The first thing we do is to initialize pin 4 as an output pin:

gpio.mode(pin, gpio.OUTPUT)

We then create a 2 second loop interval that will execute the passed in callback function.

tmr.alarm(1, 2000, 1, function()

Next we check the LED current state. If it's off we supply voltage to pin 4:

gpio.write(pin, gpio.HIGH)

Else we take pin 4 back to 0 volts, turning the LED off.

gpio.write(pin, gpio.LOW)