ESP32-DevKitC development board, ESP32-WROOM-32D, 38P

The ESP32-DevKitC development board with the ESP32-WROOM-32D module is itself the central microcontroller of the project, not a module that gets added alongside another board, so this tutorial uses this very board and shows how to quickly check the WiFi radio built into the module, before moving on to projects that actually connect to a network or to cloud services.

The platform used in the tutorial is the ESP32-DevKitC board with ESP32-WROOM-32D, programmed from the Arduino IDE using Espressif's ESP32 board package. The test does not use any external sensor or module, you only need the board and a USB Type-C cable to connect to the computer.

How it works:

The ESP32-WROOM-32D module integrates an 802.11 b/g/n WiFi radio that can work both as a station connected to an existing network and as its own access point. In this test, the WiFi.scanNetworks() function switches the radio into station mode and sends probe requests on the channels of the 2.4 - 2.5 GHz band, measuring for each network found the received signal strength RSSI, expressed in dBm, a negative value where numbers closer to 0 mean a stronger signal, for example -40 dBm is a very good signal while -85 dBm is close to the limit of reception.

The scan started with WiFi.scanNetworks() is blocking, meaning the program waits until it finishes before continuing, with a duration ranging from a few hundred milliseconds to a bit over a second, depending on the number of networks in the area and the channels covered.

On the datasheet of the ESP32-WROOM-32D module the average consumption is given as approximately 80 mA, however during moments of radio transmission, which happen at the start of each WiFi scan, current peaks on the order of 300 mA can occur, which is why the manufacturer recommends a power supply capable of delivering at least 500 mA. If the USB port or cable used has high internal resistance, these current peaks can cause a brief voltage drop on the 3V3 pin, enough to trigger a board reset through the brownout circuit, especially on weaker USB 2.0 ports or on hubs without their own power supply.

What you need:

• 1 x ESP32-DevKitC development board, ESP32-WROOM-32D, 38P
• 1 x USB cable with display, PD, QC4.0, Type-C to Type-C, 100W
• 1 x Set of 120 electrolytic capacitors, 0.22-470uF

Wiring:

For this test no connection to any GPIO pin is needed, the board only uses the WiFi radio built into the ESP32-WROOM-32D module, and the only connection required is the USB Type-C cable between the board and the computer, used both to power the board at 5 VDC and for programming and for displaying results in the serial monitor. If the USB port used cannot deliver at least 500 mA or the cable has high resistance, add between the 3V3 and GND pins on the board an electrolytic capacitor of 100 - 470 uF, placed as close as possible to the module, to buffer the current peaks that occur during scanning.

The code:

The program switches the WiFi radio into station mode, starts a blocking scan of the surrounding networks every five seconds, and displays on the serial port the SSID name and the RSSI signal strength for each network found.

// ESP32-DevKitC (ESP32-WROOM-32D) - WiFi network scan test
// Board: ESP32-DevKitC with ESP32-WROOM-32D (select "ESP32 Dev Module" in Arduino IDE)
// Wiring: none required on GPIO pins, this test only uses the onboard WiFi radio of the ESP32-WROOM-32D module
// Power and programming through the onboard USB Type-C connector
// Use a USB port or charger able to supply at least 500 mA, WiFi TX current spikes can reset the board on weak supplies
// No external library required, WiFi.h is included with the ESP32 Arduino core
#include <WiFi.h>
void setup() {
  Serial.begin(115200);        // Start serial communication at 115200 baud
  delay(1000);                 // Give the serial monitor time to connect
  WiFi.mode(WIFI_STA);         // Set WiFi to station mode for scanning
  WiFi.disconnect();           // Make sure WiFi is not connected to any network
  delay(100);
  Serial.println("ESP32-DevKitC WiFi scan test starting...");
}
void loop() {
  Serial.println("Scanning for WiFi networks...");
  int networksFound = WiFi.scanNetworks(); // Perform a blocking WiFi scan
  if (networksFound == 0) {
    Serial.println("No networks found.");
  } else {
    Serial.print(networksFound);
    Serial.println(" networks found:");
    for (int i = 0; i < networksFound; i++) {
      // Print SSID and signal strength (RSSI) for each network found
      Serial.print(i + 1);
      Serial.print(": ");
      Serial.print(WiFi.SSID(i));
      Serial.print(" (RSSI: ");
      Serial.print(WiFi.RSSI(i));
      Serial.println(" dBm)");
    }
  }
  Serial.println("");
  delay(5000); // Wait five seconds before scanning again, avoids repeated back-to-back current spikes
}

How the code works:

In the setup function, serial communication is initialized at 115200 baud with Serial.begin, then WiFi.mode(WIFI_STA) sets the radio into station mode, needed for scanning, and WiFi.disconnect() makes sure the board does not stay connected to a previous network before starting the test.

In the loop function, WiFi.scanNetworks() starts the scan and returns in networksFound the number of networks found. If networksFound is 0 a message is displayed saying no network was found, otherwise a for loop goes through each network found and displays, for each one, the name with WiFi.SSID(i) and the signal strength with WiFi.RSSI(i), expressed in dBm.

At the end of each cycle, delay(5000) halts execution for five seconds before repeating the scan, both to avoid unnecessarily loading the processor and the serial port and to avoid repeated current peaks at very short intervals.

Result:

In the Serial Monitor, set to a speed of 115200 baud, you will see every five seconds the list of WiFi networks found nearby, along with the name and signal strength of each one.

ESP32-DevKitC WiFi scan test starting...
Scanning for WiFi networks...
4 networks found:
1: HomeNetwork_24G (RSSI: -42 dBm)
2: Office_WiFi (RSSI: -58 dBm)
3: NeighborAP (RSSI: -71 dBm)
4: IoT_Guest (RSSI: -80 dBm)
Scanning for WiFi networks...
3 networks found:
1: HomeNetwork_24G (RSSI: -44 dBm)
2: Office_WiFi (RSSI: -61 dBm)
3: IoT_Guest (RSSI: -79 dBm)

Common issues:

The board resets or restarts right during WiFi scanning, the cause usually being a USB port or cable that cannot sustain the current peak on the order of 300 mA required by the radio transmission, the fix being to connect to a USB 3.0 port capable of at least 2 A or to a separate USB charger, using a good quality USB Type-C data cable and, if the problem persists, adding an electrolytic capacitor of 100 - 470 uF between the 3V3 and GND pins near the module.

If nothing appears on the serial monitor, check that the speed selected in the Serial Monitor is 115200 baud and that the COM port selected is the one for the ESP32-DevKitC board.

If the message displayed is always No networks found even though your phone sees networks nearby, check that the network in question operates on the 2.4 GHz band, because the ESP32-WROOM-32D module cannot scan networks on the 5 GHz band.

If a known network does not show up in the list even though the signal is good, check whether the router has SSID broadcasting disabled, in which case the hidden network does not appear in a standard scan.

If the board is not recognized by the computer when connected, install the driver for the CP2102 bridge from the chip manufacturer's website and try another USB Type-C cable that supports data transfer, not just power.

If uploading the program does not start, the board is not in Boot mode, disconnect the cable, hold down the BOOT button, reconnect the cable and retry uploading from the Arduino IDE.

Limitations and tips:

The ESP32-WROOM-32D module only scans and connects on the 2.4 GHz band, it has no support for 5 GHz. The WiFi.scanNetworks() function is blocking and occupies the processor for the entire duration of the scan, so in a more complex project the scanning must be placed carefully with regard to other real-time tasks. For stable operation, the power supply connected to the USB Type-C port or to the 5V pins must be able to deliver at least 500 mA at peak, not just on average. Starting from this test, the board can be extended for actually connecting to a WiFi network as a station, for creating its own access point, or for projects that send data to cloud services through MQTT or HTTP.