A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays. Relays are used where it is necessary to control a circuit by a separate low-power signal, or where several circuits must be controlled by one signal.

In this lesson, we will show you how the 1-Channel Relay Module works and how to use it with the Osoyoo Uno board to control high voltage devices.



  • Osoyoo UNO Board (Fully compatible with Arduino UNO rev.3) x 1
  • 1-Channel Relay Module x 1
  • Breadboard x 1
  • Jumpers
  • USB Cable x 1
  • PC x 1


  • Arduino IDE (version 1.6.4+)

About 1-Channel Relay Module

What is a Relay?

A relay is an electrically operated device. It has a control system and (also called input circuit or input contactor) and controlled system (also called output circuit or output cont actor). It is frequently used in automatic control circuit. To put it simply, it is an automatic switch to controlling a high-current circuit with a low-current signal.


The advantages of a relay lie in its lower inertia of the moving, stability, long-term reliability and small volume. It is widely adopted in devices of power protection, automation technology, sport, remote control, reconnaissance and communication, as well as in devices of electromechanics and power electronics. Generally speaking, a relay contains an induction part which can reflect input variable like current, voltage, power, resistance, frequency, temperature, pressure, speed and light etc. It also contains an actuator module (output) which can energize or de-energize the connection of controlled circuit. There is an intermediary part between input part and output part that is used to coupling and isolate input current, as well as actuate the output. When the rated value of input (voltage, current and temperature etc.) is above the critical value, the controlled output circuit of relay will be energized or de-energized.

NB: input into a relay can be divided into two categories: electrical quantities (including current, voltage, frequency, power etc.) and non- electrical quantities(including temperature, pressure, speed, etc.)


  • The features of 1-Channel Relay module are as follow:
  • Good in safety. In power system and high voltage system, the lower current can control the higher one.
  •  1-channel high voltage system output, meeting the needs of single channel control
  • Wide range of controllable voltage.
  •  Being able to control high load current, which can reach 240V, 10A
  •  With a normally-open (NO) contact and a normally-closed (NC) contacts

Pins Out


It has a 1×3 (2.54mm pitch) pin header for connecting power (5V and 0V), and for controlling the relay. The pins are marked on the PCB:

  • GND – Connect 0V to this pin.
  • SIG  – Controls this relay, active Low! Relay will turn on when this input goes below about 2.0V
  • VCC – Connect 5V to this pin. Is used to power the opto couplers


The 1 channel relay module could be considered like a series switches: 1 normally Open (NO), 1 normally closed (NC) and 1 common Pins (COM).

  • COM- Common pin
  • NC- Normally Closed, in which case NC is connected with COM when INT1 is set low and disconnected when INT1 is high
  • NO- Normally Open, in which case NO is disconnected with COM1 when INT1 is set low and connected when INT1 is high

How relay works?

The working of a relay can be better understood by explaining the following diagram given below.

There are 5 parts in every relay:

1. Electromagnet – It consists of an iron core wounded by coil of wires. When electricity is passed through, it becomes magnetic. Therefore, it is called electromagnet.

2. Armature – The movable magnetic strip is known as armature. When current flows through them, the coil is it energized thus producing a magnetic field which is used to make or break the normally open (N/O) or normally close (N/C) points. And the armature can be moved with direct current (DC) as well as alternating current (AC).

3. Spring – When no currents flow through the coil on the electromagnet, the spring pulls the armature away so the circuit cannot be completed.

4. Set of electrical contacts – There are two contact points:

.Normally open – connected when the relay is activated, and disconnected when it is inactive.

.Normally close – not connected when the relay is activated, and connected when it is inactive.

5. Molded frame – Relays are covered with plastic for protection.


The diagram shows an inner section diagram of a relay. An iron core is surrounded by a control coil. As shown, the power source is given to the electromagnet through a control switch and through contacts to the load. When current starts flowing through the control coil, the electromagnet starts energizing and thus intensifies the magnetic field. Thus the upper contact arm starts to be attracted to the lower fixed arm and thus closes the contacts causing a short circuit for the power to the load. On the other hand, if the relay was already de-energized when the contacts were closed, then the contact move oppositely and make an open circuit.

As soon as the coil current is off, the movable armature will be returned by a force back to its initial position. This force will be almost equal to half the strength of the magnetic force. This force is mainly provided by two factors. They are the spring and also gravity.

Relays are mainly made for two basic operations. One is low voltage application and the other is high voltage. For low voltage applications, more preference will be given to reduce the noise of the whole circuit. For high voltage applications, they are mainly designed to reduce a phenomenon called arcing.

Relay Applications

Relays are used to protect the electrical system and to minimize the damage to the equipment connected in the system due to over currents/voltages. The relay is used for the purpose of protection of the equipment connected with it.

These are used to control the high voltage circuit with low voltage signal in applications audio amplifiers and some types of modems.

These are used to control a high current circuit by a low current signal in the applications like starter solenoid in automobile. These can detect and isolate the faults that occurred in power transmission and distribution system. Typical application areas of the relays include

  • Lighting control systems
  • Telecommunication
  • Industrial process controllers
  • Traffic control
  • Motor drives control
  • Protection systems of electrical power system
  • Computer interfaces
  • Automotive
  • Home appliances


  • The maximum DC load is 10A, the maximum DC load voltage is 30V the maximum AC load is 10A, the maximum AC load voltage is 250V.
  • Why is it overheating when the relay is under use?  If the load voltage or current is rather high, it will give off extra heat, it is all right when the temperature is confined within 65℃. You are advised to choose another one that can support larger power if the temperature is upper 65℃.
  • The relay will discharge spark or magnetic field during the process of mechanical opening and closing. In order to prevent it from being interfered, you’d better keep it away from other MCU or chips.

High Voltage Warning

Before we continue with this lesson, I will warn you here that we will use High Voltage which if incorrectly or improperly used could result in serious injuries or death. So be very caution of what you are doing.


Arduino Control Lamp


In this example, we will show how to use the Osoyoo UNO board to control a relay to turn on/off the lamp.


Build the circuit as below digram:


Code Program

After above operations are completed, connect the Arduino board to your computer using the USB cable. The green power LED (labelled PWR) should go on.Open the Arduino IDE and choose corresponding board type and port type for you project. Then load up the following sketch onto your Arduino.

const int relayPin = 2; //the base of the transistor attach to
String comdata = "";
int lastLength = 0;

void setup()
  pinMode(relayPin, OUTPUT);  //initialize the LED as output
  Serial.begin(9600);  // start serial port at 9600 bps:
  while (! Serial);
  Serial.println("Please input your command to control this Lamp:");  //print message on serial monitor

void loop()
 //read string from serial monitor
  if(Serial.available()>0)    // check if data has been sent from the computer
    comdata = "";
    while (Serial.available() > 0)  
      comdata += char(;
    if(comdata == "on")
      digitalWrite(relayPin, HIGH);//turn the lamp on
    else if(comdata == "off")
      digitalWrite(relayPin, LOW);//turn the lamp off
      Serial.println("Please input correct command !");
      delay (20000);

Running Result

A few seconds after the upload finishes, open the Serial Monitor, input “on”, you will see the lamp will be turned on; input “off”, you will see the lamp will be turned off.

Leave a Reply

WordPress spam blocked by CleanTalk.
Bombax Theme designed by itx