Introduction: Dark Sensor With LDR, Transistor and a LED

You wan't do a dark sensor with a LDR, one transistor and one LED and you don't know how to do it? In this small tutorial I will explain to you how you can do that even if you know only the basics of electronic.

Parts you need:

1 Breadboard;
1 Power source of 12v;
1 Transistor (in this case we use the BC547);
1 Resistor of 1KOhm;
1 Resistor of 680KOhm;
1 LED (in this case blue);
1 LDR:
Some condutor cables to connect the parts.

Step 1: Connecting Parts in the Breadboard

In the above image we can see the circuit we will construct. It's a simple electronic circuit and if you know how to do something on a breadboard, you should be able to do that dark sensor.

Step 2: Connect Everything

Just do the circuit like this and all should be working fine.

Remember, the LDR resistance depends on the light you have on the room or where you are constructing that. If you have light, the LDR resistance is low so, the LED should be turned off. When you obstruct the light that focus on the LDR, the LED should light up. 

If for any reason your LED is always lighted up, try to chance the transistor. Switch the Emissor and the Collector. If the problem persist, increase the resistor you have in the base.

Step 3: All Should Be Working

Connect the power source you have to the breadboard. The positive pole in the red line and the negative in the blue line. 

Try your circuit and see what happens.

Here's the video of my circuit working:



If you have any questions feel free to ask in the coments. If you think I should improve something, let me know. It's my first guide so it's normal I made some mistakes.

Kind regards,
Ricardo Silva

Comments

author
LeeD40 made it!(author)2016-04-21

Hi, How can I use a power supply of only 5v?

author
Alex+Kabz made it!(author)2017-03-17

you can use ordinary batteries

author
AvinashC15 made it!(author)2016-07-21

why only bc547 type of transistor is used why cant we try with anyother one?

author
WoldieK made it!(author)2016-04-28

use npn transister what you want it makes you job ,like 2n23904,2n2222,bc547,bc548,bc550,mbs8090,2n2369 ,etc

to check use protuse software

author
WoldieK made it!(author)2016-04-28

no more 680k is used use only 680k===10-50k;1k,or less than that

author
skumar231 made it!(author)2016-04-19

arduino with ldr

author
WyrdPrincess made it!(author)2016-04-03

Hello. I just wanna ask, what are the use of every component in this circuit. Like why it has to be a 1k and 680k resistor? I need this for my finals. Thank you.

author
mariateresa.fumia made it!(author)2015-04-10

i made a very very basic circuit with just a led, a battery and e ldr but the leds turns on when ther's light and turns off in the dark? what could i possibly done wrong?

author
EdwinS27 made it!(author)2016-03-25

interchange the LDR and resistor

author
peoplespk made it!(author)2015-12-03

Is this can be used as study lamp or light house with some modification , basically I want to know how it can be modified for some other purpose, can you suggest some ? I do not have a good idea of all these things, just want learn and play, I am not from a electrical background .

author
EdwinS27 made it!(author)2016-03-25

use a relay

author
trinitty made it!(author)2016-03-04

It works very nice. Though I'd like to know if there is any way to switch the LED on and off suddenly and not just gradually, depending the amount of light. Hope it makes sense.

10x

author
SOHAM96 made it!(author)2016-02-16

what's the use of transistor in this actually???

author
Manoj+SuryaK made it!(author)2016-02-29

it acts like a switch (as in like a mechanical switch).

author
SuchibrataM made it!(author)2015-07-25

can we use laser light instead of normal light

author
JibinL made it!(author)2016-02-08

Yes

author
AngeloA17 made it!(author)2016-02-05

Uhgh. Can someone help me? Instead of LED, i want to use a motor(that dynamo thing.) Is it possible? Can u show me how?

author
PragatiK made it!(author)2016-01-28

how can i make this circuit more sensitive? please reply fast as much as u can becoz i have to give my model on 30 jan

author
DanishA6 made it!(author)2016-01-12

The circuit works but the led is extremely dim.

author
M+AliN made it!(author)2016-01-02

I have made my project but the thing i wanna add is i want to switch my leds upon the intensity of light

i.e if there is low light two leds would on or if there is high then 4

how it is possible ?

author
rshacker86 made it!(author)2015-10-20

Good work... can you please give its working...means how it works

author
chihiro_tange made it!(author)2015-12-19

The transistor is like a switch here. When it's dark, the LDR will have a very high resistance, so current will flow through R2, through the base and emitter of the transistor, thus "turning on" the transistor. Because of that current can flow through R1, the LED, the transistor and in to its collector and out through the emitter. That's why the LED will light.

When it's light, the LDR's resistance drops (that's how it works). The current destined for the base of the transistor will be diverted throught the LDR, preventing the transistor from turning on thus the LED will not light.

I hope that's not too difficult to understand :).

author
WalidM made it!(author)2015-03-30

I wnt to use dc motor instead of led . wht hv I need to do ?

author
RohanT4 made it!(author)2015-11-30

You'll need to connect the Collector of the Transistor to a of motor driver circuit. That has an independent power supply.

author
tao6258 made it!(author)2015-11-22

My motor is YAMAHA not a SUZUKI

author
NanasoS made it!(author)2015-03-13

In case of Breadboard we used General purpose board.

What is difference between

1 Resistor of 1KOhm;
1 Resistor of 680KOhm;

author
GabrielK5 made it!(author)2015-06-09

680 ohm is less than 1k ohm. R2 with ldr are in series so they gather and form total rezistanc eof roughly 680 ohm. The thing is that the less the rezistance the more current will flow. So when light is up on ldr its rezistance is low so R2 + R of Ldr will be 680 ohm , less than 1 k phm which means that current will flow to R2 and not R1

author
Electrify made it!(author)2015-09-05

hey that's not 680, its 680k

author
lilypadhoper made it!(author)2013-09-26

I have a solar powered waterfall pump that I want to shut off when no power is coming from the solar panel. How could I hook this up to make that work?

author
IshanFdo1 made it!(author)2015-09-02

But it will automatically turn off if there is no current from solar panel right ?

author
ShubhamP11 made it!(author)2015-07-27

What are the applications of this device ?

Please answer my question , I am making this for my project so I want the applications.

author
nandu.subash.3 made it!(author)2015-02-05

Can i use BC 548 instead of BC 547

author
WalidM made it!(author)2015-05-13

Bc 548 and BC547 are almost same

author
DexterE1 made it!(author)2015-05-08

author
SiddharthK made it!(author)2015-04-17

Can we use 2 resistors of resistance 1 Kilo ohms ???

author
NanasoS made it!(author)2015-03-13

In case of Breadboard we used General purpose board.

What is difference between

1 Resistor of 1KOhm;
1 Resistor of 680KOhm;

author
Imetomi made it!(author)2015-03-29

1st resistor is 1 ohm and 1k resistor is 1000 ohm. k means kilo, (1000)

author
theallanmachakanja1 made it!(author)2015-02-17

Hi
What's the purpose of the transistor on the circuit and how does it function

author
pirah.sikandar.7 made it!(author)2015-01-30

LED blow in dark and light both'....please help me out

author
pirah.sikandar.7 made it!(author)2015-01-30

LED blow in dark and light both'....please help me out

author
RiyaJ made it!(author)2015-01-22

can you give an explanation for this as i need to present this in my project

thank you

author
mthesmokemachine made it!(author)2015-01-16

Hi, how can I make a relay with the above circuit which can power a LED strip of 12v and 1A ? Please let me know if its possible. Thank you.

author
pirah.sikandar.7 made it!(author)2015-01-08

is it working on transducer principle? m confused.. plz help as soon as possible
thanks

author
bjoshi3 made it!(author)2014-12-19

ricardo silva u r a great and truth man , i am Bj from India , i was searching for this projects from a month and didn't got the perfect results and i lost hope that how will i pass in my school exams without the project . i was in depression i left the project thinking that i can't do this , my teachers refused to help me ......... but today just before a day of examinations i tried with Instructables and got success ............. thank ya man ......... obliged really .......... i did this with a 224ohm resistor with a 9v power supply

IMG_20141219_164728.jpgIMG_20141219_164843.jpgIMG_20141219_164855.jpg
author
normelb made it!(author)2014-12-10

if i use 12v led or relay what resistor value i need to use??? Thanks in advance

author
harisankar12111999 made it!(author)2014-11-29

itz a nice project

temp_-855955968.jpg
author
Rohan_Xperiment made it!(author)2014-11-28

Thanx...its working great. made a security project with ur system :D :)

LDR.jpg
author
nitishdash2010 made it!(author)2014-11-01

Guys, I only have a 3.7v battery. what are the resistor values that i need to change with??

author
HariniS made it!(author)2014-09-29

how does this circuit work? can u explain pls?

also can i use this for my cbse project?

author
davkjw made it!(author)2014-09-23

Working absolutely fine. Thank you! :)

About This Instructable

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Bio: 21 years old electronics student who loves all the things about electricity.
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