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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. The first relays were used in long distance telegraph circuits as amplifiers: they repeated the signal coming in from one circuit and re-transmitted it on another circuit. Relays were used extensively in telephone exchanges and early computers to perform logical operations.

Hope it helpfull to you

Step 1: Parts Required

To do this, you'll need:

Basic Parts for wiring up Arduino

  1. one Pref. board
  2. four 5 volt DC Relay
  3. four leds for indicating purpose
  4. four 220 ohm resistor
  5. four diodes of 1N4007
  6. Four 2N2222 npn transistor
  7. four two-pin block connector
  8. some jumper wires

Step 2: Circuit Diagram

I will describe the circuit for one really, really have five legs 1 for NC normally-closed 1 for no normally open and a common and two legs for 5 volt DC supply so whenever you will five volt input in these two legs the relay will be activated, so for the polarity you have to add one diode like this but you can avoid this step after that you have to add a led with 220 ohm resistor so till now this is the normal relay circuit you can ignore led step if you don't want to get a notification life continuously. Now what you have to do is to add NPN transistor and connect the collector of NPN to ground and then connect the ground terminal to the ground 5Volt Dc supply now connect the one leg of block terminal with common and another leg with NO of really that's it and yes connect your base of NPN transistor with your arduino and use your relay switch copy this systematic as per your requirement

Step 3: Demo

I just programmed the arduino and attach with my homemade relay circuit board and connect all four appliances with the relay board. So whenever I turn the arduino the first relay is ON and its ON for 3 Seconds and after that it goes off and the second relay is on 3 sec similarly for third and fourth and after fourth all will be Off for 3 seconds and after that all four legs will be turned on at a time for 3 seconds as you can see in this in above video and it will never stop unless I cut the power supply of the arduino this video is just a demo video to show you guys how it works with arduino that's it for now hope you enjoyed my instructables and video and learn something new form it.

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Sir what will happen if I use 6V DC relay instead of 5v
<p>Nice instructable. One note, however, one should include a resistor to limit the current to the base of the transistor.</p>
<p>You can but no need of that because arduino is not giving that that much of output</p>
You are relying on devices to operate outside of their recommended parameters. May work for a while (or even forever) but it is not a good idea, especially when the solution is so simple (a few hundred Ohm resistor between the microcontroller and the base).
<p>A 1K or so resistor in the base circuit should be included otherwise you will greatly shorten the life of the transistor and/or Arduino </p>
<p>you are correct... you can add a resistor but I'm use this device since very long and it's working like a charm </p>
<p>very usefull, thanks</p>
I have been involved in electronics for 55 of my 65 years and never stumbled into why relays are called &quot;relays&quot;! Thanks! <br>Great Instructable... especially for those of us just beginning to explore Aurduino and Raspberry Pi computing.
<p>thanks sir.... it really means alot to me</p>

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Bio: I have some interest in electronic. you can find me at https://www.facebook.com/theTechWorks
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