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## Simple Automatic Street Light System

A super simple light activated switch circuit is explained here. The circuit can be used for automatically switching ON and switching OFF a particular lamp or group of lamps in response to the varying ambient light levels. The unit once built can be used for switching OFF a lamp when dawn breaks and switching it ON when dusk sets in.

The circuit can be used as an automatic street light controller system or a simple light activated switch.

Let’s try to understand the functioning of this useful yet simple automatic street light circuit and how it is so simple to construct:

Referring to the proposed automatic street light circuit figure we can see a very simple configuration consisting of just a couple transistors and a relay, which forms the basic control part of the circuit.

Of course we cannot forget about the LDR which is the prime sensing component of the circuit.

The transistors are basically arranged such that they both complement each other oppositely, meaning when the left hand side transistor conducts, the right hand side transistor switches OFF and vice versa.

The left hand side transistor T1 is rigged as a voltage comparator using a resistive network. The resistor at the upper arm is the LDR and the lower arm resistor is the preset which is used to set the threshold values or levels.  T2 is arranged as an inverter, and inverts the response received from T1.

Initially, assuming the light level is less, the LDR sustains a high resistance level across it, which does not allow enough current to reach the base of the transistor T1. This allows the potential level at the collector to saturate T2 and consequently the relay remains activated in this condition.

When the light level increases and becomes sufficiently large on the LDR, its resistance level falls, this allows more current to pass through it which eventually reaches the base of T1.

The transistor T1 conducts, pulling its collector potential to ground. This inhibits the conduction of the transistor T2, switching OFF its collector load relay and the connected lamp.

The power supply is a standard transformer, bridge, capacitor network, which supplies a clean DC to the circuit for executing the proposed actions.

The whole circuit can be built over a small piece of vero board and the entire assembly along with the power supply may be housed inside a sturdy little plastic box.

The LDR must be placed outside the box, meaning its sensing surface should be exposed toward the ambient area from where the light level is required to be sensed.

Care should be taken that the light from the lamps does not in any way reach the LDR, which may result in false switching and oscillations.
Parts List for the above simple automatic street light circuit system

R1, R2, R3 = 2K2,
VR1 = 10K preset,
C1 = 100uF/25V,
C2 = 10uF/25V,
D1 ---- D6 = 1N4007
T1, T2 = BC547,
Relay = 12 volt, 400 Ohm, SPDT,
LDR = any type with 10K to 47K resistance at ambient light.
Transformer = 0-12V, 200mA

•  Stumble

1. hi ..what is the cost of this project?

1. May be \$2 or Rs100/-

2. Hi.
Actually i need your Contact number. Can you give your number? Or message me to this num. i'll call you. 9600582084

3. Hi,

Please feel free to contact me through my email ID

4. Can the circuit be built without a control switch?

1. Don't use any software, instead do it in your mind, or even better if you can make it practically.
I have already explained the functioning in the article.

4. Sir, want to add ldr switch for 6v emergency light. plz suggest simplest way to replace manual switch with ldr

1. What type of LDR switch? A day/night switch or flashlight operated toggle switch???

5. Day night to automatically off in day for 6v emergency light.

1. One such circuit has been discussed in the following article, third circuit from bottom:

6. dear sir , i am going to make this circuit. instead of outside light i am going use LED near by the LDR. this will operate the relay ?

1. Yes, the LDR and the circuit will respond to all kinds of visible light...

7. ok sir, i will give u feedback. thank u

1. sir , i did the circuit.please tell me setting method and where i can place a led to know relay status.

2. allow light to fall on the LDR, adjust VR1 such that the relay just switches OFF.

Now if you cover the LDR with your hand, the relay should switch ON back.

You can connect an LED across the relay coil with a 1K series resistor.

Do not allow the light from the lamp connected with the relay contacts to fall on the LDR.

3. thank you sir,

* when i switch on the device the relay off condition.

* if i cover with my hand on the LDR the relay switch on.

* after remove my hand from the LDR ,the relay is not off.( if i touch the R3 leg the relay switch off)

please tell me the circuit is right or wrong .

4. The difference between the light and the darkness falling on the LDR should be large, only then it will work properly.

meaning either the light should be more when the LDR is uncovered, or the darkness should be more when its covered.

5. ok sir . i understand. iam using big size LDR only. when i switch on the circuit the realy is in N/C mode. then i cover the LDR the relay goes to N/O mode. after remove my hand from the LDR the relay is not comes to N/C mode. i am using 250ohm relay . please help me solve ..

6. The darkness level should be large, means it must be very dark compared to the light that you are using on the LDR...

relay rating is OK

7. why not to use IC555

9. sir,can you please tell me how to connect an LDR between two npn transistors

10. explain your requirement in detail.

11. i am not get the 400ohm relay. what can i do to work it? how can i use 200ohm relay?

1. you may use a 200 ohm relay in place of the shown relay.

2. but my 200ohm relay dose not stop, relay always NC mode.

3. have you connected everything as shown in the diagram??

12. yes sir, i use Transformer 0-12V, 300mA C1 = 100uF/63V,C2 = 10uF/63V

1. What about the LDR, what type have you used?

2. remove C2 initially.....

13. Please sir help relay work

14. Help me please do not work used for really

15. Sir,

My name is raigon jose and i am from kerala

And i made 1 solar street light with 12 VOLT 100 WATTS panel & 42 ah SMF battery & Lights 1 watt Edisson LED 20 Numbers

( 3 V x 4 ) A pair of 4 Led Series -------- 12 v ( 5 Rows) 20 WATTS Total

I have a problem faced from automatic ldr switching Circuit
LDR SWITCH CIRCUIT Installed in light Dom
LDR is installed inside the light cover
When evening 5 'O Clock the Light will be switch on & morning after 7.30 am will be switch off

In that circuit when we adjust that preset ,i think it will switch on only after the dark will come to ldr

Give me one solution for to adjust time of switching on and off lights..................

16. Sir,

My name is raigon jose and i am from kerala

And i made 1 solar street light with 12 VOLT 100 WATTS panel & 42 ah SMF battery & Lights 1 watt Edisson LED 20 Numbers

( 3 V x 4 ) A pair of 4 Led Series -------- 12 v ( 5 Rows) 20 WATTS Total

I have a problem faced from automatic ldr switching Circuit
LDR SWITCH CIRCUIT Installed in light Dom
LDR is installed inside the light cover
When evening 5 'O Clock the Light will be switch on & morning after 7.30 am will be switch off

In that circuit when we adjust that preset ,i think it will switch on only after the dark will come to ldr

Give me one solution for to adjust time of switching on and off lights..................

1. Hello Raigon,

Make the first circuit given in this linl:

R1,R2,R3 = 10K, 1/4 watt

P1 = 100K pot.

Also connect a 10 K resistor across pin#6 and pin#3 of the IC.

And alos connect a 1K resistor across base/emitter of the transistor

transistor will be BC547

relay 12v/spdt

17. hi sir, This is naresh.
I want to save power. for that I need a switch works automatically. I have a problem with my cooler, having water pump separately. Now I want to set a switch to stop/start that pump automatically.

Aim is---> after working 10min, pump will switched OFF for 20min then again it will Switched ON for 10min and so on.............

how can I make this in my home.

g.naresh968@gmail.com

1. Hi Naresh,

make the following circuit:

it needs just the following modification: the in4148 diode should be connected to pin3 and not pin15 as given in the diagram

18. Hi sir, i'm a new comer in this forum but i have been reading your articles based on electronics circuit and it's impressive, although I'm industrial electrical student and i tried hard to make electronic my hobby. Sir please i need your help,i made these circuit but it is not working and i used necessary components listed only that i used 500mA transformer but the same voltage rating. When i ON the switch the coil of relay energized and bulb light up,when i covered the ldr varied the resistor the bulb remain the same no change occurred, so sir i need some correction.

1. Hi Onwuzo,

I have removed the diode from the emitter of T1, do it in your circuit also, it will surely work now.

19. Hi swagatam, i removed the emmiter diode as you said but there was no effect the bulb still the same when i cover ldr. Please help sir tell me what i will do for the circuit to function well. Thanks

1. Hi Onwuzo,

the circuit should work now.

cover the LDR completely and adjust the preset so that the relay just activates. This will set up the circuit.

20. sir i have got it, it is now working thanks for your help and immediate responsed. Please sir can i get one with transformerless that can handle 500w fluorescent tube. Awaiting for your response

1. you can replace the transformer power supply network with the circuit shown in the following article:

21. sir how much maximum power can this particular circuit handle i mean bulb wattage.

1. it will depend on the relay contact ratings, you may upgrade it to any desired level.

22. R1,R2,R3 HOW MANY WATT?

23. CAN i use battery in this circuit ? How ?

24. HOW MANY WATT?

25. HOW CAN I USE BATTERY IN THIS Circuit

1. this circuit is for AC application.

26. HOW WATT r1 r2 r3 ?

27. Hello sir, i had built the circuit and it worked fine only that am facing this problem now and want you to help me solve it. One day the circuit stopped working, when i troubleshoot i didn't saw any faults and later found out it was low voltage drop which fall 140v instead of 220v so when i knocked on the relay it make contact and light start illuminating. I have changed relay to 9v relay the same thing happened, and now we are having problem of low current always. so my question is there any way to do amendment so that it will function under circumstances of low voltage and normal voltage. Another question is that when dark start falling on the ldr relay start cracking noise which last about a minutes and 30seconds before it stopped and light will be steady vice versa. My question is there a way you should help me do this so that relay will pick one time when conducting instead of cracking noise please help.

1. replace the relay with a BT136 or similar triac.

do the following mods:

remove relay.
connect collector of T3 to positive directly.
connect emitter of T3 to ground via a 1K resistor.
connect triac extreme right lead to emitter via a 220 ohms resistor
connect extreme left lead of triac to ground
connect center lead to load and load to mains one point and mains other point to circuit ground.

2. Hello ONWUZO, I built this circuit yesterday, I was happy it worked but as you have mentioned it makes a cracking noise for 1.5 minutes. That was your relay chattering due to the biasing voltage at the base of T1. My remedy is to add a 10k 1/4w resistor from the wiper of the pot VR1 to the base of T1. This will give C2 more time to hold the charge. If chatter persist add another 10uF/25V parallel to C2. It should work perfectly. Have fun experimenting!!!

3. ...or simply connect a capacitor across the relay coil that will instantly stop all possibility of chattering.

4. I am trying to use BT136 and Transformerless-power supply like

But confused here " connect center lead to load and load to mains one point and mains other point to circuit ground."

What do you mean by "circuit ground" negative side of C2 ( if yes then how a 40 watt filament AC B22 base Indian Bulb could run) or Phase or Neutral of Mains of Transformer less power supply.

5. circuit ground is the line which is common to the triac right lead and the circuit negative. This line also gets connected with one of the mains inputs (preferably to neutral).

The other mains input (preferably phase) goes to the load wire as explained in your above comment.

28. sir am confusing in your descriptions u mention when i remove relay.
connect collector of T3 to positive
directly.
connect emitter of T3 to ground via a
1K resistor.
connect triac extreme right lead to emitter via a 220 ohms resistor
connect extreme left lead of triac to
ground
to mains one point and mains other
point to circuit ground. Sir am lost here first where is T3 in the circuit 2nd if i remove relay in place of bt136triac. I know the terminal pinout of triac bt136 which is terminal1, terminal2(tab), and gate3 so use it like this to describe to me.

1. Hi Onwuzo,

Please click the diagram to enlarge, T3 is the relay transistor...i know it should have been written as T2 but in the diagram it's indicated as T3.

center is termina2
gate is exterme right.

29. where t1 base gone?

1. to center pin of the preset

30. Thanks sir for your explanation i appreciate your kindness. What value of cap do i add across relay coil.

1. You can use a 220uF/25V cap.

31. What is D1------D6=1N4007

33. Sir i want to make smart street light system..in this with the help of sensors the intensity of light inc. And decreases...plzz help

1. Harsimran you can try the following design:

34. Sir,
what do you mean of (across relay coil)? does it mean (in parallel with the relay coil)?

1. yes I meant parallel to the coil

35. thank you Sir

36. Which pin of relay connect to supply and transitor?

1. The relay coil terminals will connect with those points

37. Whow to connect realys pin to suppy and other components. Please explain in detail i have dast demo day after tomorrow.

38. Sir, How to connect VR1 ?

1. the center of the preset will go to T1 base, the other pins across R1 and ground.

39. 220 ohm caught fire!!!

I have tried the following method to avoid Chattering of Relay at afternoon using BT136

""replace the relay with a BT136 or similar triac.

do the following mods:

remove relay.
connect collector of T3 to positive directly.
connect emitter of T3 to ground via a 1K resistor.
connect triac extreme right lead to emitter via a 220 ohms resistor
connect extreme left lead of triac to ground ( I have tired but not work then Connected to Phase)
connect center lead to load and load to mains one point and mains other point to circuit ground.( as well as Neutral)
circuit ground is the line which is common to the triac right lead and the circuit negative and the circuit negative (Left is already connected to negative as you told earlier) . This line also gets connected with one of the mains inputs (preferably to neutral).

Since Center pin of Triac is already connected to neural as well as negative, Left Pin of triac again if connect to negative or ground or neural circuit not completed so I connected Left pin of Triac to Phase and middle pin to neural or negative via load of a 40 watt bulb, I used a transformer less power-supply using 474k capacitor. But lamp lighten but within a second 220 ohm resistor caught fire I immediately turned off switch.

It will be great help if you post a diagram using BT136 and transformer less power-supply.

1. do not use bridge rectifier for the transformerless power supply, you can take the help of this article diagram:

40. Beni
Sir! thankyou very much for your webesite.The AC is changed to DC.so which type of lamp may feet to it now to get enough light?

1. thanks Beni, the lamp can be seen connected with the AC mains via the relay contacts, so the lamp will be an AC lamp

2. sorry,i asked you before watching it good.thanks again

41. HI SWAGATAM how can i add like more 5 LAMPS 20W each to the circuit

1. Hi Davis, yes you can accommodate more number of lamps as long as it's below the relay's maximum current handling capacity.

42. HI SWAGATAM the circuit works fine when i used transformer power supply and used 1 watt for resistors but when i replaced the transformer with the cheap transformerless supply as you said then the relay could activate but bulb could not light yet am planning to use transformeless supply to be able to connect 10 bulbs 15w each rated 0.09A since my relay can support up to 7A with 240V . help me what is the problem with this supply to be able light the bulbs, thanks

1. Hi Davis, 15 watt LED cannot be operated with a transformerless power supply, so I would suggest you to continue with your transformer based power supply.

43. Hi Swag,
I am using a triac with diac to control 4w lamp. 120vac feeds lamp. Lamp to 100k variable resistor to diac as well as LDR to common. The circuit works fine. When I add a step down transformer 120vac to 12vdc in parallel with lamp circuit still works a so does transformer.
If I remove lamp however and only have transformer, circuit does not operate properly no matter value of VR. Let me know how to send circuit diagram if it helps. THX

1. Hi Pete, it could be because the triac /diac design which you have incorporated is very basic and is not optimized for handling inductive loads, therefor it might not be able to switch a transformer, but is working well with a resistive load such as a bulb...

or may be its because the transformer being an inductor load offers very high resistance initially, which stalls the triac operation, and connecting a bulb in parallel solves this issue...

2. you cn send it to