ESP32 Eight-Channel Relay Tester

Pan and zoom to explore the wiring. Remix the project to edit it in your own workspace.

1. 01

   Leave the relay screw terminals empty for the first test

   Do not connect lamps, motors, wall power, or any other load to the relay screw terminals yet. This first test only checks that each relay clicks and its small channel light turns on.

   • Tip: Place the relay module where you can see all eight small channel indicator lights.

   • ⚠ Do not put wall power on the relay terminals while learning this test — a wiring mistake can cause electric shock, fire, or damage.
2. 02

   Place the eight resistors and transistors

   Put the eight 1 kΩ resistors and eight 2N2222 transistors on a breadboard, leaving space between each channel. Keep each resistor with its matching transistor: R1 with Q1 through R8 with Q8. Before wiring, check the data sheet or package marking for your exact transistor’s flat-side pin order; different 2N2222 package styles can have different leg orders. Identify the base, collector, and emitter before applying power.

   • Tip: Mark the transistor channels 1 through 8 with tape so the matching wires do not get mixed up.

   • ⚠ Putting a transistor in with the wrong legs connected can prevent the relay from working and can stress the ESP32 output pin.
3. 03

   Wire the eight ESP32 control paths

   With the ESP32 unplugged, wire GPIO16 → R1 → Q1 base (control), GPIO17 → R2 → Q2 base (control), GPIO18 → R3 → Q3 base (control), GPIO19 → R4 → Q4 base (control), GPIO21 → R5 → Q5 base (control), GPIO22 → R6 → Q6 base (control), GPIO23 → R7 → Q7 base (control), and GPIO25 → R8 → Q8 base (control). Each resistor must be in series between its ESP32 pin and transistor base; it limits the small control current.

   • Tip: A resistor has no positive or negative end, so either leg can face the ESP32.

   • ⚠ Do not connect an ESP32 GPIO straight to a transistor base without its 1 kΩ resistor — the resistor protects the ESP32 pin.
4. 04

   Connect the transistor outputs to the relay inputs

   Connect Q1 collector → relay IN1 (signal), Q2 collector → IN2 (signal), Q3 collector → IN3 (signal), Q4 collector → IN4 (signal), Q5 collector → IN5 (signal), Q6 collector → IN6 (signal), Q7 collector → IN7 (signal), and Q8 collector → IN8 (signal). Connect every transistor emitter → ESP32 GND (ground). These transistors pull a relay input down to ground to turn its channel on.

   • Tip: Keep the channel numbers matched from resistor to transistor to relay input; this makes the click sequence easy to verify.

   • ⚠ Do not connect the relay’s 5 V VCC terminal to any ESP32 GPIO or to the ESP32 3V3 terminal — 5 V on an ESP32 signal pin can damage it.
5. 05

   Power the relay board from the external 5 V supply

   Connect external supply +5V → relay VCC (relay input-side power) and external supply +5V → relay JD-VCC (relay coil power). Connect external supply GND → relay GND (ground) and external supply GND → ESP32 GND (shared ground). If the module has a VCC-to-JD-VCC jumper fitted, remove it before making these two separate +5 V connections. The ESP32 remains powered from its USB cable.

   • Tip: Use a regulated 5 V supply rated for at least 2 A because all eight relays can be switched on together during the test.

   • ⚠ Make sure +5 V and GND are not swapped — swapped power can damage the relay board. Do not feed the external 5 V supply into the ESP32 3V3 pin.
6. 06

   Run the relay click test

   Plug the ESP32 into USB, then turn on the external 5 V relay supply. Press Schematik’s Deploy button. The program switches relay 1 through relay 8 on one at a time, then turns all eight on briefly before repeating.

   • Tip: You should hear one click at a time and see the matching relay indicator light. The serial output names the channel currently being tested.

   • ⚠ If a relay stays on unexpectedly, unplug the ESP32 USB cable and turn off the external 5 V supply before changing wires.

#include <Arduino.h>
// ESP32 eight-channel relay test through eight 2N2222 transistor switches.
// Each ESP32 HIGH passes through a 1 kΩ base resistor, turns its 2N2222 on,
// and pulls the matching active-low relay input to ground.


// Forward declarations
void setAllRelays(bool on);

const uint8_t RELAY_1_PIN = 16;
const uint8_t RELAY_2_PIN = 17;
const uint8_t RELAY_3_PIN = 18;
const uint8_t RELAY_4_PIN = 19;
const uint8_t RELAY_5_PIN = 21;
const uint8_t RELAY_6_PIN = 22;
const uint8_t RELAY_7_PIN = 23;
const uint8_t RELAY_8_PIN = 25;

const uint8_t relayPins[] = {
  RELAY_1_PIN, RELAY_2_PIN, RELAY_3_PIN, RELAY_4_PIN,
  RELAY_5_PIN, RELAY_6_PIN, RELAY_7_PIN, RELAY_8_PIN
};
const uint8_t RELAY_COUNT = sizeof(relayPins) / sizeof(relayPins[0]);
const unsigned long RELAY_ON_MS = 700;
const unsigned long GAP_MS = 300;
const unsigned long ALL_ON_MS = 1000;
const unsigned long ROUND_PAUSE_MS = 3000;

void setAllRelays(bool on) {
  for (uint8_t i = 0; i < RELAY_COUNT; i++) {
    digitalWrite(relayPins[i], on ? HIGH : LOW);
  }
}

void setup() {
  Serial.begin(115200);
  for (uint8_t i = 0; i < RELAY_COUNT; i++) {
    pinMode(relayPins[i], OUTPUT);
    digitalWrite(relayPins[i], LOW); // Transistor off: relay input stays high, relay off.
  }
  Serial.println("8-channel relay test ready. No mains load is required for this test.");
}

void loop() {
  for (uint8_t i = 0; i < RELAY_COUNT; i++) {
    Serial.printf("Testing relay %u\n", i + 1);
    digitalWrite(relayPins[i], HIGH);
    delay(RELAY_ON_MS);
    digitalWrite(relayPins[i], LOW);
    delay(GAP_MS);
  }

  Serial.println("Testing all relays together");
  setAllRelays(true);
  delay(ALL_ON_MS);
  setAllRelays(false);

  Serial.println("Round complete; all relays off.");
  delay(ROUND_PAUSE_MS);
}

“Deploy to device” opens this project in Schematik, where you can flash it to your board over USB.