Interfacing Relay with PIC Microcontroller

A relay is an electromagnetic switch which is used to switch High Voltage/Current using Low power circuits. Relay isolates low power circuits from high power circuits. It is activated by energizing a coil wounded on a soft iron core. For detailed working of relay please visit this page. A relay should not be directly connected to a microcontroller, it needs a driving circuit.

A relay should not be connected directly to a microcontroller due to following reasons..

  • A microcontroller is not able to supply current required for the working of a relay. The maximum current that a PIC Microcontroller can source or sink is 25mA while a relay needs about 50 – 100mA current.
  • A relay is activated by energizing its coil. Microcontroller may stop working by the negative voltages produced in the relay due to its back emf.

Interfacing Relay with PIC Microcontroller using Transistor

A relay can be easily interfaced with microcontroller using a transistor as shown below. Transistor is wired as a switch which carries the current required for operation of the relay. When the pin RB7 of the PIC microcontroller goes high, the transistor BC547 turns On and current flows through the relay.  The diode D1 is used to protect transistor and the microcontroller from Back EMF generated in the relays coil.  Normally 1N4148 is preferred as it is a fast switching diode having a peak forward current of 450mA. This diode is also known as freewheeling diode.

Interfacing Relay with PIC using Transistor

Interfacing Relay with PIC using Transistor

Note: VDD and VSS of the pic microcontroller is not shown in the circuit diagram. VDD should be connected to +5V and VSS to GND.

MikroC Program

void main()
{
   TRISB.F7 = 0; //Makes RB7 a output pin

   do 
   {
      PORTB.F7 = 1;   //Turns ON relay
      Delay_ms(1000); // 1000 mS delay
      PORTB.F7 = 0;   //Turns OFF realy
      Delay_ms(1000); //1000mS delay
   }while(1);
}

Interfacing Relay with PIC Microcontroller using ULN2003

If you want to use more relays, using transistors will be difficult. In these cases you may use ULN2003 or ULN2803. These are monolithic IC s consists of High Voltage High Current Darlington transistor arrays. You can connect seven relays using ULN2003 and eight relays using ULN2803. When using these driver ICs we don’t need to connect freewheeling diode as they have built in clamp diodes.

Interfacing Relay with PIC Microcontroller using ULN2003

Interfacing Relay with PIC Microcontroller using ULN2003

Note: VDD, VSS of the pic microcontroller and GND of ULN2003 are not shown in the circuit diagram. VDD should be connected to +5V and VSS, GND to 0V (GND).

MikroC Program

void main()
{
  TRISD = 0x00;    //Make PORTD as output

  do
  {
     PORTD.F1 = 1;  //Turns ON relay 1
     PORTD.F2 = 1;  //Turns ON relay 2
     PORTD.F3 = 1;  //Turns ON relay 3
     PORTD.F4 = 1;  //Turns ON relay 4
     PORTD.F5 = 1;  //Turns ON relay 5 ...etc..

     Delay_ms(1000); // 1 second Delay

     PORTD.F1 = 0;  //Turns OFF relay 1
     PORTD.F2 = 0;  //Turns OFF relay 2
     PORTD.F3 = 0;  //Turns OFF relay 3
     PORTD.F4 = 0;  //Turns OFF relay 4
     PORTD.F5 = 0;  //Turns OFF relay 5 ...etc...

     Delay_ms(1000); //1 second Delay
  }while(1);
}



  • EE103

    I am trying to do a similar connection using a pic18f14k50 using mplab to program the pic in C. How similar is the code for that?

  • http://www.electrosome.com/ Ligo George

    Sorry, I haven’t yet used MPLAB… but note one thing…
    for pic 18f series…
    to set direction of ports use TRIS registers..
    to read data from ports use PORT registers..
    to write data to port use LAT registers..
    ..which is different form 16f series..

  • Khaba Elite Bulu

    Can the ULN2003 driver drive a contactor coil? I wanted to reduce the number of transistors required to drive a 24V or 12V high power contactor coil by using a driver IC, any better idea on that?

  • http://www.electrosome.com/ Ligo George

    Yes you can connect a coil with 24 or 12V, but you should consider the current ratings too… One channel of ULN2003 can handle 500mA current..

  • Mohammad Uzair Rasheed

    C program not working it gives an error as
    A #Device required before first line

  • http://www.electrosome.com/ Ligo George

    Which Compiler you are using?? Select the proper device first…(PIC 16F877A)
    use MikroC Pro Compiler

  • Zain Chaudhry

    em using CCS compiler

  • Ric Ryaan Ybanez

    , i have tried 6 relays with transisor in each relay, but when i let them click at the same time, only 1 will click. this is very helpful, ill try this ULN,, thnx to this. it givves me hope for my final year proj,, kudos!!

  • Bassant M. Mohamed

    i bigan a project making a simple robot bs i want to use the relay …i want to know how to connected to pic16f877a and how to write the code to make it work using microcontroller ( C program)

  • Tinku

    what happens if we use 9v relay instead of 12v???

  • http://www.electrosome.com/ Ligo George

    No problem.. just change the power also to 9V..

  • Girish

    hi i used this technique with 89v51rd2 controller using transistor to drive relay,but the problem i faced that mcu was frequently resetting whenever the relay switches,can any one suggest me for what reason it is happening…?

  • Tinku

    thanx. but what happens if we give 12v to a 9v relay?? will it work or it ll damage the relay??

  • http://www.electrosome.com/ Ligo George

    Try using separate power supply for relay and microcontroller or use proper filtering capacitors..

  • http://www.electrosome.com/ Ligo George

    It might work.. but it is may damage the relay ..