HTTP requests from Arduino - Step-by-Step Execution

Concept Flow - HTTP requests from Arduino

Start Arduino

↓

Connect to WiFi

↓

Create HTTP Client

↓

Send HTTP Request

↓

Receive Response

↓

Process Response

↓

End or Repeat

Arduino connects to WiFi, sends an HTTP request, waits for response, then processes it.

Execution Sample

Arduino

void setup() {
  WiFi.begin("SSID", "password");
  while (WiFi.status() != WL_CONNECTED) {}
  HTTPClient http;
  http.begin("http://example.com");
  int code = http.GET();
  if (code > 0) {
    String payload = http.getString();
  }
  http.end();
}

void loop() {
  // put your main code here, to run repeatedly:
}

This code connects Arduino to WiFi, sends a GET request to example.com, and reads the response.

Execution Table

Step	Action	Condition/Status	Result/Output
1	Start WiFi connection	WiFi.status() != WL_CONNECTED	Connecting...
2	Wait for WiFi	WiFi.status() == WL_CONNECTED	Connected to WiFi
3	Create HTTPClient	N/A	HTTP client ready
4	Begin HTTP request	http.begin("http://example.com")	Connection opened
5	Send GET request	http.GET()	Response code received
6	Check response code	code > 0	Valid response received
7	Read response	http.getString()	Payload stored in variable
8	End HTTP connection	http.end()	Connection closed
9	Finish	N/A	Program ready for next action

💡 HTTP request cycle ends after closing connection with http.end()

Variable Tracker

Variable	Start	After Step 2	After Step 5	After Step 7	Final
WiFi.status()	WL_IDLE_STATUS	WL_CONNECTED	WL_CONNECTED	WL_CONNECTED	WL_CONNECTED
code	undefined	undefined	200	200	200
payload	empty	empty	empty	"<html>...</html>"	"<html>...</html>"

Key Moments - 3 Insights

Why do we wait in a loop until WiFi.status() == WL_CONNECTED?

Because the Arduino needs to be connected to WiFi before sending HTTP requests. See execution_table step 2 where it waits until connected.

What does the code variable represent after http.GET()?

It holds the HTTP response code (like 200 for success). Check execution_table step 5 and 6 for this.

Why do we call http.end() at the end?

To close the HTTP connection and free resources. This is shown in execution_table step 8.

Visual Quiz - 3 Questions

Look at the execution_table at step 5, what does http.GET() do?

A Connects to WiFi network

B Reads the response payload

C Sends the HTTP GET request and returns response code

D Closes the HTTP connection

💡 Hint

Check the 'Action' and 'Result/Output' columns at step 5 in execution_table

At which step does the Arduino confirm it is connected to WiFi?

A Step 2

B Step 4

C Step 1

D Step 7

💡 Hint

Look at the 'Condition/Status' column for WiFi.status() in execution_table

If the response code is 0 or negative, what happens?

A HTTP connection is not started

B Response is invalid, payload is not read

C Payload is read normally

D WiFi disconnects

💡 Hint

See execution_table step 6 where code > 0 is checked before reading payload

Concept Snapshot

Arduino HTTP Requests:
1. Connect to WiFi with WiFi.begin()
2. Wait until WiFi.status() == WL_CONNECTED
3. Create HTTPClient and call http.begin(url)
4. Send request with http.GET()
5. Check response code > 0
6. Read response with http.getString()
7. Close connection with http.end()

Full Transcript

This example shows how Arduino makes HTTP requests. First, it connects to WiFi and waits until connected. Then it creates an HTTP client and begins a connection to a URL. It sends a GET request and waits for the response code. If the response code is positive, it reads the response payload as a string. Finally, it closes the HTTP connection. Variables like WiFi.status(), code, and payload change during these steps. This process repeats as needed to communicate with web servers.

Practice

1. What is the main purpose of using HTTPClient in Arduino sketches?

easy

A. To control digital pins

B. To connect to WiFi networks

C. To read analog sensor values

D. To send and receive HTTP requests easily

Solution

1. Step 1: Understand the role of HTTPClient

   The HTTPClient library is designed to simplify sending HTTP requests like GET and POST from Arduino.

2. Step 2: Differentiate from other libraries

   Connecting to WiFi is done by WiFiClient, not HTTPClient. Reading sensors or controlling pins is unrelated.

3. Final Answer:

   To send and receive HTTP requests easily -> Option D

4. Quick Check:

   HTTPClient = HTTP requests [OK]

Hint: HTTPClient handles web requests, not WiFi or sensors [OK]

Common Mistakes:

• Confusing HTTPClient with WiFi connection library
• Thinking HTTPClient reads sensors
• Assuming HTTPClient controls pins

2. Which of the following is the correct way to start a GET request using HTTPClient in Arduino?

easy

A. http.connect("http://example.com");

B. http.startGET("http://example.com");

C. http.begin("http://example.com");

D. http.open("http://example.com");

Solution

1. Step 1: Recall HTTPClient method for starting requests

   The correct method to initialize a request is begin() with the URL as argument.

2. Step 2: Check other options

   Methods like connect(), startGET(), or open() do not exist in HTTPClient library.

3. Final Answer:

   http.begin("http://example.com"); -> Option C

4. Quick Check:

   Use begin() to start HTTP request [OK]

Hint: Use begin() to start HTTP requests in Arduino [OK]

Common Mistakes:

• Using non-existent methods like connect() or open()
• Confusing begin() with WiFi connection functions
• Forgetting to pass the URL string

3. What will the following Arduino code print if the HTTP GET request to http://example.com/data returns status 200 and body "OK"?

HTTPClient http;
http.begin("http://example.com/data");
int httpCode = http.GET();
if (httpCode > 0) {
  String payload = http.getString();
  Serial.println(httpCode);
  Serial.println(payload);
} else {
  Serial.println("Error");
}
http.end();

medium

A. 200
OK

B. Error

C. 404
Not Found

D. 0

Solution

1. Step 1: Analyze the HTTP GET response code

   The code checks if httpCode > 0. Since the server returns 200, this condition is true.

2. Step 2: Print status code and response body

   The code prints the status code (200) and then the response body ("OK") on separate lines.

3. Final Answer:

   200 OK -> Option A

4. Quick Check:

   200 status means success, prints code and body [OK]

Hint: Status > 0 means success; print code and body [OK]

Common Mistakes:

• Assuming error when status is 200
• Confusing status code with response body
• Not calling http.end() to close connection

4. Identify the error in this Arduino code snippet for sending a POST request:

HTTPClient http;
http.begin("http://example.com/api");
http.addHeader("Content-Type", "application/json");
int httpCode = http.POST("{\"value\": 42}");
if (httpCode == 200) {
  Serial.println("Success");
}
http.end();

medium

A. Incorrect header name; should be "ContentType"

B. Missing WiFi connection before HTTPClient usage

C. POST method requires no argument

D. http.end() should be called before POST

Solution

1. Step 1: Check prerequisites for HTTP requests

   Before using HTTPClient, Arduino must be connected to WiFi. This code snippet does not show WiFi connection setup.

2. Step 2: Validate headers and method usage

   Header name "Content-Type" is correct, POST accepts a string argument, and http.end() is correctly called after POST.

3. Final Answer:

   Missing WiFi connection before HTTPClient usage -> Option B

4. Quick Check:

   WiFi must connect before HTTPClient [OK]

Hint: Always connect WiFi before HTTP requests [OK]

Common Mistakes:

• Skipping WiFi connection setup
• Using wrong header names
• Calling http.end() too early

5. You want to send sensor data as JSON to a server using Arduino's HTTPClient. Which sequence correctly sends a POST request with JSON data and handles the response?

hard

A. Connect WiFi -> http.begin(url) -> http.addHeader("Content-Type", "application/json") -> http.POST(jsonData) -> check httpCode -> http.end()

B. http.begin(url) -> http.POST(jsonData) -> http.addHeader("Content-Type", "application/json") -> check httpCode -> http.end()

C. Connect WiFi -> http.POST(jsonData) -> http.begin(url) -> http.addHeader("Content-Type", "application/json") -> check httpCode -> http.end()

D. Connect WiFi -> http.addHeader("Content-Type", "application/json") -> http.begin(url) -> http.POST(jsonData) -> check httpCode -> http.end()

Solution

1. Step 1: Establish WiFi connection first

   Arduino must connect to WiFi before making HTTP requests.

2. Step 2: Follow correct HTTPClient method order

   Start with http.begin(url), then set headers, then send POST with data, check response code, and finally call http.end().

3. Final Answer:

   Connect WiFi -> http.begin(url) -> http.addHeader("Content-Type", "application/json") -> http.POST(jsonData) -> check httpCode -> http.end() -> Option A

4. Quick Check:

   WiFi -> begin() -> headers -> POST -> end() [OK]

Hint: Connect WiFi first, then begin(), addHeader(), POST, end() [OK]

Common Mistakes:

• Calling POST before begin()
• Adding headers after POST
• Skipping WiFi connection