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

Components for this experiment are available at buildcircuit.net.

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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.

dark sensor

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.

dark sensor 2

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.

You can buy all the components required for this experiment at buildcircuit.net.

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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.


dark sensor 3

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.


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)

relay 2

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.


– 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.

Buy the components for all the experiments published on this page at buildcircuit.net.

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

    • 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.

  2. 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??

  3. 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?


  4. 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?

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

  7. 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.

  8. 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

  9. 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.


  10. I am using Din Rail Timer having relay voltage specifications : 5A @ 230V AC/ 24VDC / Resistive ….My question is can i give 70V DC as a coil voltage my current draw out will be not more than 1 A .

  11. I have a 12VDC fan with reversable polarity for pushing or pulling air, and I would like to use ( 2 ) lighted switches, one for each direction. My question is how do wire in the relay/s so both switches can’t be on at the same time ? the fan only draws 1/10th of an amp

  12. hi i need somo help.i have build a circuit using a 12v relay.but it fluctuaute that means it gives tik tik tik sound continously.what may be the problem?

  13. Thanks for this. I was able to build this and put it into my lighted model kit. Now when the lights go off in the room the model lights up.

  14. Hi i am building dark detector with ac bulb circuit..i just want to know minimum how much amps we need for this circuit

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