2N2222 NPN Transistor: Switch a Small LED with ESP32

Before you start

Board and software

Classic ESP32-WROOM-32/32E DevKit with the listed GPIOs exposed and unused. Arduino IDE 2.x; esp32 by Espressif Systems 3.3.2 (review baseline, not a latest-release claim). Select the documented board or ESP32 Dev Module for a generic WROOM board. Other families need their own pin map. Serial Monitor: 115200 baud.

Supported hardware

Reference part: onsemi P2N2222A in TO-92, not every transistor sold as 2N2222 or PN2222. Its numbered leads are 1 collector, 2 base, 3 emitter. Read the manufacturer package view before wiring; the approved illustration does not establish manufacturer or pin order.

Prerequisites and parts

Complete IDE setup and Blink. Documented transistor, red indicator LED, 470 ohm LED resistor, 2.2 kOhm base resistor, 100 kOhm base-emitter pull-down and a breadboard. This example uses the DevKit 3.3 V rail for a few-mA LED load only.

Libraries

No extra library. Arduino-ESP32 3.3.2 GPIO and millis APIs.

Expected result

The external red LED switches ON for 500 ms and OFF for 1000 ms; Serial prints the commanded state, not a measured result.

Verification status

Documentation and source review only. Not compiled or tested on hardware. The approved product-style illustration identifies a family, not a verified physical pin layout or manufacturer-authenticated board. Follow the logical diagram and documentation for your actual part.

Overview

02Technical Specifications

Arduino library: Arduino-ESP32 3.3.2 (no additional library)

03Pinout

• P2N2222A pin3 emitter Return ESP32 GND Verify numbered package view.
• P2N2222A pin2 base Control GPIO25 through 2.2 kOhm Series base resistor required.
• 100 kOhm resistor Pull-down Base to emitter Keeps transistor off with input floating.
• ESP32 3.3 V Load supply 470 ohm → LED anode Anode to resistor; no bare LED across supply.
• LED cathode Load return P2N2222A pin1 collector Collector switches LED current to emitter.

04Wiring Diagram

GPIO25 → 2.2 kOhm → base; emitter/GND common; 100 kOhm base-emitter. 3.3 V → 470 ohm → red LED anode; cathode → collector.

Open wiring diagram at full size (new tab)

1. 1

   Unplug USB. Identify the onsemi part and its numbered leads from the package drawing.

2. 2

   Wire the LED and both base resistors; inspect for rail shorts.

3. 3

   Upload, then open Serial Monitor at 115200; compare LED behavior with the commands.

Wiring and matching Arduino code

Small LED low-side switch

GPIO25 → 2.2 kOhm → base; emitter/GND common; 100 kOhm base-emitter. 3.3 V → 470 ohm → red LED anode; cathode → collector.

#include <Arduino.h>
constexpr uint8_t OUTPUT_PIN = 25;
bool active = false;
uint32_t changedAt = 0;
void setup() {
  Serial.begin(115200);
  pinMode(OUTPUT_PIN, OUTPUT);
  digitalWrite(OUTPUT_PIN, LOW);
  Serial.println("Starts OFF; repeating on/off demonstration");
}
void loop() {
  const uint32_t now = millis();
  const uint32_t interval = active ? 500 : 1000;
  if (now - changedAt >= interval) {
    changedAt = now;
    active = !active;
    digitalWrite(OUTPUT_PIN, active ? HIGH : LOW);
    Serial.println(active ? "ON" : "OFF");
  }
}

The sketch commands GPIO25, not an onboard LED. The pull-down provides hardware bias before setup. HIGH is ON in this exact NPN circuit.

How it works

Technical references

• onsemi P2N2222A datasheet — Reference lead numbering, saturation test conditions and ratings.
• Arduino-ESP32 3.3.2 — Pinned GPIO and timing baseline; not compiled.
• Arduino-ESP32 3.3.2 GPIO implementation — Initialize the GPIO bus with pinMode before digitalWrite; hardware pull-down provides reset bias.