Picture of How To Make A Dark Sensor
2013-04-11 17.43.04.jpg
Hey guys ,
In this project I am going to show you how to make a dark sensor using a single transistor and some few resistors in just 3 simple steps .
here is my video this is my old dark sensor


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Step 1: Things You Need

Picture of Things You Need
1) 1x BC547 transistor 
2) 1x 220 k resistor
3) 1x 330 ohms resistor
4) 1- small perf board
5) 1x led (any colour )
6) wires
7) 9v battery 
8) battery clip
9)ldr(light dependent resistor )
1) soldering rod and wire 
2) wire stripper 

Step 2: Getting started

Picture of Getting started
solder the transistor on the perf 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 ohms resistor
savdd5 months ago

hey is there a way to make it so that when i hover my hand over it, it will stay on then when i hover over it again, it will turn off. thx

Pranavthegreat (author)  savdd5 months ago

I don't think so you can with just a transitor , you might need a microcontroller and PIR sensor

aaamir26 months ago

i have a problem.
i have followed the steps
but instead of Dark its working as a Light Detector any idea why Thanks

Pranavthegreat (author)  aaamir26 months ago
just change reverse the transistor because using this circuit you can make both light as well as dark sensor
lukeD1 year ago
hello, nice ible!
is there a way to control how much light there is needed to light up the led? i think it would be possible with other resistors, wich one would that be?
I want it to light up when it is only half dark.
Pranavthegreat (author)  lukeD1 year ago
Replace any other resistor instead of 100 k or use a 100k pot so that u can change sensivity of the sensor
am i able to use it on 12v dc for a smd LED ??? IF SO TEll me about the things needed.
Pranavthegreat (author)  muhammad-zka1 year ago
I think so u cant as the the transistor will nt be able to bear that much current
I have a question because i am kinda dummie.i cant understand how this circle works, imean the led should light when there is light in the room and not the opposite.i have followed yous instructions and it really worked but i need an explanation i think. ;/
actually using the same circuit u can make a light sensor too you just have to change the place of the resistors , instead of the 220k ohms resistor use LDR and replace LDR with 220k ohms resistor and then you can light up the led when there is light , if still dont understand just try the project on the breadboard.
if i use 3 leds (1.5v each) and 4.5v power . do i need any resistors?or the resistors have a different purpose?
There is no need of any other resistor as you are already using 330ohms resistor which is for the led , I hope this will help you
ToXiCATOM1 year ago
Can i use H SC945P transistor?
Pranavthegreat (author)  ToXiCATOM1 year ago
you can try using it but i prefer you should use BC547 as it is suitable for this circuit you can also use BC546, BC548, BC549, BC550
hi thanks for sharing.
but i want to know that how can i control my room light with a dark sensor
hey rohit thank you for your comment and I am also trying to control my room lights with a dark sensor and I have searched a lot ,you can use a relay to control the 220W bulb or simply google it