Relays are electromechanical switches consisted of a coil and a number of contacts, you can consider the relay as a push button except that it is pushed electrically rather than pressing it by hand, the relays are activated by applying low voltage to their coils, but they are capable of driving high voltages through their contacts. Relays are suitable for driving high power electric equipment, such as lights, electric fans and air conditioning.


- 1 Uno board
- 1 USB data cable
- 1 Relay
- 1 LED
- 1 Resistor (220Ω)
- 1 Resistor (1KΩ)
- 1 Transistor (NPN)
- 1 Diode
- Several jumper wires
-1 Breadboard

Experimental Principle

A relay is an electronic control component in which its coil represents the control system and its contacts represents the controlled system. It is generally used in automatic control circuit. Actually, it is an "automatic switch" which uses low current to control high current. It plays a role of automatic regulation, security protection and circuit switch.

An electromagnetic relay is generally composed of an iron core, coil and armature, spring and contacts. There will be an electric current flowing through the coil as long as a certain voltage (determined by the datasheet of the relay) is supplied on both ends of the coil, this will change the state of the relay’s contacts. After the coil is powered off, the contacts will return to their original state. This change in contacts state achieves the purpose of circuit conduction and cut off. In general, normally open and normally closed contacts can be distinguished like this: when relay coil is not energized, static contact in disconnected state is called "normally open contact", static contact in connected state is called "normally closed contact". In this experiment, when the relay is energized, the LED will light up; when the relay is not energized, the LED will dim, that means that we are using the normally open contact.

Experimental Procedures

Step 1: Connect circuit as shown in Fig1. The corresponding schematic diagram is shown in Fig2. (Schematic is generated by EAGLE)
Step 2: Program the Arduino (download program from documents).
Step 3: Compile the program.
Step 4: Burn the program into the UNO board.

circuit connection

Fig1: the connection of the relay

eagle schematic

Fig2: the schematic

Now, if a high voltage is supplied, the relay will be activated and the LED will light up; if a low voltage is supplied, the relay will be deactivated and the LED will dim. In addition, you can hear ticking sound caused by opening normally closed contact and closing normally open contact.

 Experiment summary

Though this experiment is a little bit more complicated than previous ones, but it will get you another step closer to control circuits, this experiment represents a small control system that is used for coils, motors and inductive loads, so it is important to understand this experiment in order to be able building a control circuit for any other application you need.


1-Arduino progrme (code)
2-Eagle file