Using Relays — SIK Circuit 13

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

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.

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.