Home Basic Electronics How to use a relay

How to use a relay

A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches.

The relay’s switch connections are usually labeled COM(POLE), NC and NO:

COM/POLE= Common, NC and NO always connect to this, it is the moving part of the switch.

NC = Normally Closed, COM/POLE is connected to this when the relay coil is not magnetized.

NO = Normally Open, COM/POLE is connected to this when the relay coil is MAGNETIZED and vice versa.

A relay shown in the picture is an electromagnetic or mechanical relay.

Fig. Relay and its symbol

There are 5 Pins in a relay. Two pins A and B are two ends of a coil that are kept inside the relay. The coil is wound on a small rod that gets magnetized whenever current passes through it.

COM/POLE is always connected to NC(Normally connected) pin. As current is passed through the coil A, B, the pole gets connected to NO(Normally Open) pin of the relay.

Here is an example,

First of all try the following circuit.

This is a dark sensor circuit.

Fig. Dark sensor using two transistors

Output of this circuit: When you block light falling on LDR, the circuit switches on the LED- D1.

Now, replace LED-D1 and R2- 330R with a relay and diode.

Reconfigure the circuit as shown in the figure below:

Note: In R3, you can keep any resistor from 330R to 4.7K, this resistor is for sensitivity of the dark sensor.

The following circuit also works as a dark sensor. When you block light falling on LDR, the relay gets activated and Pole of relay gets connected to NO pin that eventually gives power to LED- D1.

Fig. Dark sensor using two transistors and a relay.

Light sensor using relay and transistors

In this case, the configuration of relay has been changed. Here, NO (Normally open) terminal has been left open. In normal case, the D1-LED remains ON. When light falling on LDR is interrupted, pole of relay gets connected to NO terminal. Hence, NC (Normally connected) terminal does not get power and that switches the D1- LED off.

Fig. Light sensor using two transistors and a relay.

Connect to COM(pole) and NO if you want the switched circuit to be on when the relay coil is on.

Connect to COM(pole) and NC if you want the switched circuit to be on when the relay coil is off.

WORKING WITH 220V

WARNING: IF YOU ARE A NOVICE DO NOT PLAY WITH 220V AC. CALL AN EXPERIENCED PERSON FOR ASSISTANCE.

Fig. Dark sensor circuit for 220V powered lights.

A relay can be used to turn on lights working on 220V, AC. The AC powered light has to be connected to relay as shown in the picture above.

Fig. Connecting wires on relay

The following video shows a soldered/finished prototype.

PROTECTION DIODE FOR RELAY

Fig. Protection diode in the circuit

Transistors and ICs must be protected from the brief high voltage produced when a relay coil is switched off. The diagram shows how a signal diode (eg 1N4148 or 1N4001 or 1N4007) is connected ‘backwards’ across the relay coil to provide this protection.

Current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the relay coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil (and diode) so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to transistors and ICs.

GENERAL SPECIFICATION OF A RELAY

06VDC- means that the voltage across the relay coil has to be 6V-DC.

50/60Hz- The relay can work under 50/60Hz AC.

7A, 240VAC- The maximum AC current and AC voltage specification that can be passed through NC, NO and pole pins/terminals of relay.

One more example (update 19.3.2014)

real store logo
real store logo

05VDC- It means that you need 5V to activate the relay. In other words, it means that the voltage across the relay coil has to be 5V-DC.

10A 250VAC     10A  125VAC  – The maximum AC current and AC voltage specification that can be passed through NC, NO and pole pins/terminals of relay. Some countries have 220V AC power standard, so, it works in those countries also.

10A 30VDC   10A 28VDC- The maximum DC current and DC voltage specification that can be passed through NC, NO and pole pins/terminals of relay.

Tips:

- If you are using a 5-6V relay, use a 6V power supply.

- If you are using a 9V relay, use a 12V power supply.

I am Sagar Sapkota living in Espoo, Finland. I have done Master's degree in Electronics and Communications with major in Electronics Productization from the University of Turku, Finland. I love to share my electronics works with the world. You can buy my kits at buildcircuit.org. - Sagar Sapkota

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39 COMMENTS

  1. I have assembled this circuit considering Relay option to activate bulb.However,as soon as I connect power,relay gets connected as “ON” and bulb gets lit.I do not get any control from LDR. What could be the reason?
    Kindly suggest any troubleshooting option.

    Thanks

  2. Thank you, I built the low-voltage dark-sensing circuit and it works fine. Now coupling with a solar charger for 6VDC SLA, to drive four LED spotlight.s

    Very helpful circuits.

    Stephen Gard

  3. I think he used two transistors because of the orientation of the LDR the way it forms a voltage divider with R3. Hope I’m not wrong but Q1 there is an inverter.

  4. woo! this is seriously amazing sites. Thanks a lot sir, i am very thank full to you, may you live long , helping the other.

  5. Hmm it seems like your blog ate my first comment (it was extremely long) so
    I guess I’ll just sum it up what I submitted and say, I’m thoroughly
    enjoying your blog. I as well am an aspiring blog writer but I’m still new to everything. Do you have any helpful hints for inexperienced blog writers? I’d certainly appreciate it.

  6. Hi,
    Will this still work if you change the LDR for an Infrared Sensor? Also, i’m planning on using this for another project, turning on an LED Lamp (works on 3 AAA batteries), do i need to change anything else besides the LDR?

  7. please can you show us how the connections of a relay are made by making a video which connects the relay in the bread board according to the circuit diagram..

  8. Hi there!

    I am loving the tutorials on this site they are so professionally prepared, thanks! In the case of the dark sensor + relay, is it OK to use 12v as the source voltage for the dark sensor portion? Also, a typical 5 pin relay marked “24v”, will it work on 12v?

    Thanks

  9. sir i would like to know , whats the maximum input current rating that we can give to the poles A &B of the relay?
    how many hours it can serve continusoly if i used this to switch the bulbs?
    how it depends on the input current reting?
    will it depends on coil resistence??

  10. Dear Sir,
    We would like to know where can I buy this relay 6vdc to 240vac?
    or
    Could you provide me to get this relay?
    I need 50 unit of this product.
    many thanks,
    Best regards,
    Eg

    • I bought this relay 5 years ago, so I cannot remember from which seller did I buy. But, you can search on ebay.com for different kinds of relay. I am sure that you can find the needed one.

    • in using 12vDC relay to run a fan?? ; does power go to 85 & 86. fan leads go to 87 & 87A ?? and switch to open (run) goes to 30????

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