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Using Relays — SIK Circuit 13
Ed

නිර්මාතෘ

Ed

17. මාර්තු 2026FI
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Using Relays — SIK Circuit 13

Control a relay with a transistor to switch between two LEDs. Learn about electromechanical switching, SPDT relays, Normally Open vs Normally Closed contacts, and flyback protection.

උපදෙස්

1

Parts & Introduction

A relay is an electrically-operated mechanical switch. A small current energizes a coil that physically moves a contact arm, switching a separate circuit. Relays can control high-power devices (motors, lights, appliances) from low-power Arduino signals. This experiment uses an SPDT (Single Pole Double Throw) relay to alternate between two LEDs.

Parts Needed

  • 1x Arduino Uno + USB cable
  • 1x Breadboard
  • 1x SPDT Relay
  • 1x NPN Transistor (P2N2222A)
  • 1x Diode (1N4148)
  • 2x LEDs (different colors)
  • 2x 330Ω Resistors
  • 14x Jumper Wires

Materials for this step:

SparkFun Inventors Kit - V3.2SparkFun Inventors Kit - V3.21 kit
Arduino Uno R31 piece
Breadboard1 piece
SPDT Relay1 piece
NPN Transistor (P2N2222A)1 piece
Diode (1N4148)1 piece
5mm LED1 piece
330 Ohm Resistor330 Ohm Resistor2 pieces
Jumper WiresJumper Wires8 pieces

Tools needed:

Computer with Arduino IDE
2

Hardware Hookup

Wiring Instructions

Transistor + Relay Coil
  1. Connect transistor Base through 330Ω resistor to Arduino Digital Pin 2.
  2. Connect transistor Emitter to GND.
  3. Connect one side of the relay coil to transistor Collector.
  4. Connect other side of relay coil to 5V.
  5. Flyback Diode: Band (cathode) to 5V, anode to Collector.
Relay Contacts + LEDs
  1. Connect relay COMMON through a 330Ω resistor to 5V.
  2. Connect relay NC (Normally Closed) to LED 1 positive leg.
  3. Connect relay NO (Normally Open) to LED 2 positive leg.
  4. Connect both LED negative legs to GND.

Materials for this step:

SPDT Relay1 piece
NPN Transistor (P2N2222A)1 piece
Diode (1N4148)1 piece
5mm LED1 piece
330 Ohm Resistor330 Ohm Resistor2 pieces
Breadboard1 piece
Jumper WiresJumper Wires8 pieces
3

Arduino Code

Open the Arduino IDE and upload the following sketch to your Arduino board.

relay.inoarduino
/*
SparkFun Inventor's Kit
Example sketch 13 — RELAYS

Use a transistor to drive a relay, alternating two LEDs.

Hardware connections:
  Transistor BASE through 330 ohm resistor to digital pin 2
  Transistor EMITTER to GND
  Relay coil: one side to COLLECTOR, other to 5V
  Flyback diode: band to 5V, anode to COLLECTOR
  Relay COMMON through 330 ohm to 5V
  Relay NC to LED 1 positive, Relay NO to LED 2 positive
  Both LED negatives to GND

This code is completely free for any use.
*/

const int relayPin = 2;
const int timeDelay = 1000;

void setup()
{
  pinMode(relayPin, OUTPUT);
}

void loop()
{
  digitalWrite(relayPin, HIGH);  // Relay on (COM connects to NO)
  delay(timeDelay);

  digitalWrite(relayPin, LOW);   // Relay off (COM connects to NC)
  delay(timeDelay);
}

Materials for this step:

Arduino Uno R31 piece

Tools needed:

Computer with Arduino IDE
4

Test & Experiment

What You Should See

You should hear the relay clicking, and see the two LEDs alternating — one on, one off — switching every second.

Troubleshooting

  • LEDs not lighting: Check LED polarity — longer leg is positive.
  • No clicking sound: The transistor or coil circuit isn't working. Verify transistor orientation and coil connections.
  • Relay not making good contact: The SIK relay is designed for soldering, not breadboarding. Press it firmly to ensure all pins contact.
  • Confusion: Don't mix up the TMP36 temperature sensor with the transistor!

Experiments to Try

  • Replace the LEDs with a motor on the NO contact — use the relay as a motor on/off switch.
  • Use a button to control when the relay toggles.
  • Add variable delay to create morse code patterns.

ද්‍රව්‍ය

9

අවශ්‍ය මෙවලම්

1
  • 13
    Computer with Arduino IDE
Estimated Total
$181.00

Related blueprints

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Using a Flex Sensor — SIK Circuit 9electronics/electromech1 shared material
Driving a Motor — SIK Circuit 12electronics/electromech1 shared material
Driving a Servo Motor — SIK Circuit 8electronics/electromech1 shared material
Reading a Temperature Sensor — SIK Circuit 7electronics/electromech1 shared material
Flex Sensor Glove — Control 5 Servo Motors with Hand Gestureselectronics/electromech1 shared material
Using an LCD — SIK Circuit 15electronics1 shared material

CC0 පොදු වසම

මෙම බ්ලූප්‍රින්ට් CC0 යටතේ නිකුත් කර ඇත. ඔබට අවසර නොමැතිව පිටපත් කිරීම, වෙනස් කිරීම, බෙදා හැරීම සහ භාවිතා කිරීම කළ හැක.

බ්ලූප්‍රින්ට් හරහා නිෂ්පාදන මිලදී ගැනීමෙන් නිර්මාතෘට සහාය වන්න නිර්මාතෘ කොමිසම විකුණුම්කරුවන් විසින් නියම කළ, හෝ මෙම බ්ලූප්‍රින්ට්හි නව අනුවාදයක් සාදා ආදායම බෙදා ගැනීමට ඔබේ බ්ලූප්‍රින්ට්හි සම්බන්ධතාවයක් ලෙස ඇතුළත් කරන්න.

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