really really really simple problem

[pewtahpewtah]

hi, ive been struggling for days (maybe two weeks on and off) about how to simply make a circuit including and LED and a light sensitive resistor that turns the LED on when its dark and the LED off when its light out. I went out and bought transistors, resistors, LDRs LEDS batteries, wire, all kinds of things. Im a little uncertain on this darlington pair stuff, but i basically understand the circuit diagram i need. I think my main problem is a very basic lack of understanding of current, voltage, resistence, ohms, etc. i just cant seem to match up the parts i have and make them work. somehow that damn LED never goes out!! can anyone help me out? this is probably super easy. thanks,

blake

 

[pewtahpewtah]

specifics, sorry i forget them a second ago.

this is what i have:

9 volt battery.
12 volt rated LED
two NTE128 transistors from NTE electronics. (https://www.nteinc.com/specs/100to199/NTE128.html)
many resistors,
a few LDRs
a 10k ohm variable resistor
some wire
a breadboard (the electronics guy reccomended it)

if anyone can solve the equations necessary with different parts, ill go buy different parts. like i said, i have no idea what im doing. i just need something very simple, and relatively inexpensive.

im lookign for something to turn off an LED when the LDR is in daylight and on the LED when it is shaded.

 

[Pommie]

OK, throw everything away except the LDR and battery.
Go buy a bog standard LED.
Connect battery, LDR and LED in series.
If it don't work, reverse the LED.

HTH

Mike.
P.S. if the LED gets very bright and then stops working, you shone too bright a light on the LDR. If this happens, buy new LED and a 500ohm resistor and repeat the above but include the resistor is series with the rest.

 

[Pommie]

Woops, got it the wrong way around.

The way you want it to work,

Connect battery, 1000ohm resistor and LDR in series. Connect LED across LDR (one wire either side). Again, if it don't work reverse (swap leads) the LED.

HTH

Mike.
P.S. if it don't work, you need a brighter light.

 

[zachtheterrible]

or do this.

for the variable transistor, try 100k for starters.

n make sure those transistors u have are NPN. i think they are, but make sure.

 

[pewtahpewtah]

success

thanks zach for helping me on this one. you are the first person who has been able to help me with this simple problem. (not that everyone else didnt know what they were doing, but maybe im just extremely incompetent.) this circuit even turns the light on when its dark and off when its light, not vice versa, like all the others! so a big thanks. ive spent close to three weeks on this. now i can get on with life.

blake

 

[zachtheterrible]

i remember wen i figured that out, i was soooooo happy! look in the projects section under a thread called "drive anybody insane, gauranteed!" i implemented this circuit into another circuit, its really cool.

 

[Wrighty]

Im a little uncertain on this darlington pair stuff, but i basically understand the circuit diagram i need.
blake

All a darlington pair consists of is two transisters configured to produce massive current gains, this you do not need for powering a LED mate.