Light Detector, No Microprocessors, Just Simple Electronics :)


Introduction: Light Detector, No Microprocessors, Just Simple Electronics :)

About: I will try my best to provide cheap and easy intructables!

I will show you two circuits, one turns an LED off when theres no light, and the other turns an LED on when theres no light.

for the first one you will need:
-R1(LDR) 10K
-R2 (1.2K) colour code: brown, red, red.
-R3(10 ohms) colour code: brown black black.
-T1 almost any low voltage NPN transistor will do.
-L1 LED i used a red one.
-jump wires
-2 1.5 volt bateries and pack for them

for the second one you will need:
-R1 (LDR) 10K
-R4 (10K) colour code: brown, black, orange.
-R3(100 0hms) colour code: brown, black, black.
-T1 any low voltage NPN transistor
-L1 LED i used a red one.
-jump wires
-2 1.5 volt bateries and pack for them

it cost me under 1€ to make this, everything was salvadged from old radios, etc...
exept the LDR, but you can easiy get one from those light switch thingies at the dollar shop, or from the internet very cheap... add some more stuff if you get it from the net though... you dont wanna pay 10 cents for the part and 8 dollars for shipping =|

Step 1: Turn LED Off When Theres No Light

the 1.2 K resistor goes in the middle pin of the transistor and to - on battery
dont forget the round part of the transistor is facing you!


Step 2: Turn LED on Whe Theres No Light

it's basically the same circuit but the LDR now goes to - and the R4(10K resistor) goes to plus.
all you have to do is this small change and the whole purpose of the circuit changes!

dont forget the round part of the transistor is facing you!


if you have any doubts/problems plz tell me.

Step 3: Apendix

Now i'm going to try to explain what some parts do.
on the first circuit the 1.2K resistor controls the sensitivity, go ahead change it! if it's 10K you will need total darkness to turn the LED off, so change it betwen 1K and 10K less than 1K will be way sensitive and wont work, and over 10K you will need something darker than total darkness, wich i dont think exists, maybe outer space :P

the second circuit the 10K resistor also controls the sensitivity, if you put 150K you will need total darkness to turn the LED on, so go ahed change it aswell! but only betwen 1K and 150K.

If it doesnt work:
check the polarities of the LED, and see if the transistor is the right way in, if the LED doesnt go on, on the first circuit, dont be sad, that circuit is tuned so it works in my room, just grab a potenciometer and tune it the way you want to, then with a multimeter read the resistance of the pot, and put the closest resistor you can find to that resuslt, thats how i got 1.2K and 10K

best of luck!

Now a video of what you should expect:



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    62 Discussions

    Please tell me.can i make a touch sensor circuit without using any microprocessor ?if yes how?

    could I connect a motor on that two wires on the right? please answer...........

    3 replies

    yes you could, although it has to be a small motor, very small, like a pager motor, i actually did that lol heres a pic of the bot:

    i only have one motor yet though xD

    but yea if its a small motor, its fine, if you want to use bigger rmotor, you'll have to put another transistor

    or you could use a smal 5volt reed relay to power a bigger(like over 5 volt) motor

    i used a 2n2222 transistor and it works great.

    cool project

    i used a 2n 2904 transistor.. in this case the first circuit works for the purpose of second and vice a versa why so??

    for the transistor i used a 2n2222. works great

    i did exactly the same thing but it doesn't work

    In the first circuit...
     Turn LED off when there's no light!

     If i use 5VDC for the power supply do i need to change the resistor on the base of the transistor if so, at what value. Also the led resistor needs to change to compensate for the leds current. BUT i have that covered.

    Does any other components need to change at 5VDC. This will be tested in a circuit to detect, very bright light.

    Reply would be appreciated. 

    2 replies

    if you wanna detect very bright light, then i think you have to get a smaller value resistor at the base of the transistor, also you need a 220 ohm resistor to not burn the led, instead of the 10 ohm, i have there.

    try using a potenciometer, to find the resistance you need for the light you want to detect.

    OK. BUT. I  Already changed the LED resistor for the led at 5v.
     as stated, Got this covered..  Thanks for pointing that out.

    So you're saying to use a potentiometer at the base of the transistor for base current to find the resistance i need for the light i need to detect.

    How will that deterrmine the resistance of the light I'm detecting.

    And what vulue would the pot need to be 1k, 5k. 20k. or Higher.

    So the LDR dose not need to be replaced. i just need too lower the base current.
    Reply Thanks.


    Hello Armando96: I'm new in electronic. I follow your instructions but led always is turn on. Can you please tell me what is exactly values that you uses with transitor. You mention that uses any NPN transistor but led allways is turn on. Please let me know. Thank you.

    1 reply

    ok, i know for sure that a BC546 works, try using this one, if it still doesnt work, try changing the light drastically, say from a dark room, to puting lamp very close to the ldr.