Light detector, no microprocessors, just simple electronics :)

Picture of light detector, no microprocessors, just simple electronics :)
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 =|
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Step 1: Turn LED off when theres no light

Picture of 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

Picture of 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.
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Please tell me.can i make a touch sensor circuit without using any microprocessor ?if yes how?
Thank you for this, sometimes you just need your mind jogged.
rohanbansal2 years ago
it works very nice.good images and explanations.thanks!!!
McBlaise5 years ago
could I connect a motor on that two wires on the right? please answer...........
amando96 (author)  McBlaise5 years ago
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
yeah it worked. i add a BC547 NPN and it worked using a regular motor!!
or you could use a smal 5volt reed relay to power a bigger(like over 5 volt) motor
robot13984 years ago
i used a 2n2222 transistor and it works great.

cool project
imakethings4 years ago
i used a 2n 2904 transistor.. in this case the first circuit works for the purpose of second and vice a versa why so??
imakethings4 years ago
hey thank you very much you have helped me a lot...!!
ARJOON4 years ago
for the transistor i used a 2n2222. works great
souichi4 years ago
i did exactly the same thing but it doesn't work
akhile19965 years ago
 where's the LDR

akhile19965 years ago
can i add a 3 volt buzzer

cybertron5 years ago

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. 

amando96 (author)  cybertron5 years ago
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.


DRM_MX5 years ago
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.
amando96 (author)  DRM_MX5 years ago
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.
sharlston5 years ago
i thought at first it sayed a lie dtector
It did'nt
yes i know but i mistaked it for "lie detector" not "light detector"
mhkabir5 years ago
could the circuit run on 12v and light 30 - 40 LEDs in paralell???????
amando96 (author)  mhkabir5 years ago
umm i thinkt he transistor would burn, i'm not used to working on higher voltages just the regulated 5v for most IC's... i think you can try, but i'm pretty sure the transistor would burn, maybe a MOSFET?
Which MOSFET??Any ideas?
amando96 (author)  mhkabir5 years ago
any that can withstand 12v i guess, IRF520?
Thanks. I'll try.
amando96 (author)  amando965 years ago
i know how you could do that, you'll need two power sources though, the 3v for this to work, and then the 12 volts, the plus connected to source on the mosftet, the leds connected to drain, and gate connected to where the LED is. has to work. i did the same but with a NE555 for a speed controller
joasan185 years ago
hey bro i did it but it did not work can you put pics for step by step like you did in the other one? please thank you god bless u
amando96 (author)  joasan185 years ago
all you have to do is: the resistor and the ldr are connected to the middle pin of the transistor right? the LDR goes to +3v(positive) and the resistor goes to 0v(negative), just switch order, so the LDR goes to 0v(negative) and the ressitor goes to +3v(positive) should work if the rest is right
i got it solve. thank you for your help all i did was i changed the rsistence of resistor that goes in the middle pin. thank you
comodore5 years ago
Nice Project! I really like it! I have 2 question! 1. Can you give me some examples of Low Voltage Transistors...a couple so I know witch one to buy... 2. Where does the potenciometer, variable resistor located? Where should I connect it? Thank you!
amando96 (author)  comodore5 years ago
about the transistors, i am not sure wich i used here, it had the letters erased, i know a C945 will work, coz i have a few The pot goes from the base of the transistor to either positive, or negative, depending on the curcuit. if the LDR is on positive, the pot will be on the negative, and vice versa. just use the middle pin of the pot, and one of the side pins. the pot will control when the led will go on/off, you can also try diferent resistors. sorry for the late reply, i didnt get an email for this reply :-s
imakethings5 years ago
very good project for beginners
imakethings5 years ago
recreate this wont work with the second type of circuit
ReCreate5 years ago
This Could Be made Much Simpler,The Photo Transistor In series to the Led and Resistor,Done. Though This Is Documented Well and All.
batista05 years ago
nice Instructable... I want to adapt it to another project, it works with different voltages or i have to change resistances?? please answer
amando96 (author)  batista05 years ago
you'll have to change the LED resistor, or it will burn... as for the other resistors, tiz fine... but what voltage? coz i think you'll have to do something with the transistor...
I have two 9v batteries, so 18v...
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