Dark Sensor ( Light Sensor / Automatic Street Light )




Posted in TechnologyElectronics

Introduction: Dark Sensor ( Light Sensor / Automatic Street Light )

Here is my new simple Electronics project about  Automatic Street Light Control System or Dark Sensor.

it is a simple and powerful concept , which uses transistor ( BC 547 NPN) as a switch to switch ON and OFF the street light system automatically .
It automatically switches ON lights when the sunlight goes below the visible region of our eyes. ( e.g in evening after Sunset ).

it automatically switches OFF lights when Sunlight fall on it ( e.g in morning ) , by using a sensor called LDR (Light Dependent Resistor) which senses the light just like our eyes.

Needs no manual operation for switching ON and OFF...

Step 1: Things You Need...


* 1    BC547 transistor
* 1   220 k resistor
* 1   330 ohms resistor
* 1   LED (any colour )
* 1   9v battery
* 1   battery clip
* 1   LDR ( light dependent resistor  )

Step 2: AND...

*  Connecting wires
*  soldering rod and wire
*  wire stripper
*   Cutter

Step 3: Circuit Diagram

solder the transistor on the board and solder one wire of the LDR to the emitter and other to the base  , solder the negative wire of the LED to the base of the transistor with a 330 ohms resistor and also solder 220k resistor with the one side of the 330 ohmsresistor .

Step 4: Finsih... Uuuhuuuu... :p

The simple circuit is ready for testing now . Connect 9v battery terminals to the circuit as show in fig and see the output . As you block light falling on Light dependent resistor (LDR), the LED glows .

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2 Questions

how can i make darkness sensor using only 9v battery,LDR,BC547 transistor

If instead of using 220 k resistance i use 22k only will there be any problem...actually my ckt is not wrkng??


hi ! please help me , i want to connect 10 or more LED, asap thanks

Plz send the pic for wiring as it may clear all the doubt, I'm very New to electronics and the diagram is very Confusing.

The Circuit works as Light sensor not Darkness. In dark, the LED dims, in pitch dark it turns off and in sunlight, LED glows bright

Is it ok to use 6 volts

is that okey if i will use 6v insted of 9volts

Could the circuit be modified to work the other way round, so that it switches ON in sunlight and OFF in dark? I suspect its v. simple, but so am I!

1 reply

Yeah just dont use the transistor

I want to connect 10 white LEDs with 9 volt.. suggestion please guys

Hey can someone help me plz?

can someone tell me what the 2 resistors specifically

How may bulbs can be connected in this circuit. Can you explain me ways to connect about 6 bulbs (total Approx 9 volts- 1.5 Volt Each). Also upto how much volt led i can connect in this circuit

How do you make sure that when the light turns itself on, it doesn't then sense the light and turn itself back off?

2 replies

you can add a capacitor of 100uf at the base of the transistor to delay the time of biase raise time, so if a 5 second of light accidentally hit the LDR it will not effect imediatly, rathar it will take about 8 seconds.

yeah , it is true..
in this case should keep some distance in between LED and LDR .

The circuit can run 2 LED's for over a month with just 2 AA batteries. I use a similar one to light up my wall clock with 2 bright warm white flat top LED's.

2 replies

Hi please help me with this? - im a total newbie trying to learn how to dabble in the "dark arts" :-) Cant get my circuit to work and dont know whats wrong. Here are the various parts i have used.


Bc547; Tip127; 2n3904 - could this be the problem? The tip127 is the only PNP Transistor i can see according to the data sheet...

3x Different types of Leds. Red:1.2vF White :3.2vF,

3x differnet types of light detectors

- Photo Transistor 940nm

-Photo Sensor 940nm


Battery i am using is a 1.0ah 4.1v battery (4.1v is actual tested voltage)

2x POTS, 1x 1k, and another 10k pot.

Doing above on a breadboard

The problem is, The both LEDS stay on when connected, they just get brighter with more light, it seems my circuit is more of a light detector that switches LEDS on when it detects light rather than the opposite way around?

Ive tried switching the transistors, switching the leds, switching the POTS, and some other resistors i had (not sure of spec right now) but still cant get it right?

what other info can i add for you to assist?

Thanks, Andrew.

what exactly is your goal here, !!,, draw it for me and give me all details , and i may help you.

in the current situation, only low voltage control, but with a SPDT relay ( 220v AC - 30 amp- 24v DC )you can put its coil in the collector way, then you can control a bigger and high voltage lights