Circuit that lights LED when LDR goes dark ?

[petemoore]

Circuit that lights LED when LDR goes dark ?
  I was gonna play around with and search for some more when I decided to ask...any links to a schematic / circuit that lights LED when LDR goes dark ?

[sundgist]

I remember seeing a schematic in one of the Forrest Mims notebooks. Don't have them to hand at the minute but I'm sure someone with a copy can confirm. I can post at some point once I find it.

[PRR]

For no good reason at all, our wall-mount hair-dryer lights two green LEDs when the room gets dark.

So (of course) it is possible. A little LOGICal thinking might find a path.

[Tony Forestiere]

http://www.dutchforce.com/~eforum/index.php?s=4ed6354c68b0b49b6ba713963a74f3ed&showtopic=25239

[petemoore]

  2 transistors for 2 LED's ?
  Seems that 2 LED's -->l--->l--- from 1rst transistor emitter to ground works.

[Hides-His-Eyes]

I would just set up an LDR/resistor voltage divider into a schmidt trigger.

[petemoore]

I would just set up an LDR/resistor voltage divider into a schmidt trigger.
  I bet, but I've never introduced myself.. 
  So...why the Schmidtt ? [what I'm askin' myself, I'll take a looksee sometime.
  The transistor does a smooth scale of dimmer = brighter.

[Gurner]

One comparator.

+ve input leg of the comparator goes to the lug of a pot (VCC on one side of the pot, Gnd on the other)

-ve input leg of the comparator goes to your LDR (other leg of your LDR goes to ground) ....another resistor connects from the comparator -ve pin to VCC (forming a potential divider in conjunction with the LDR)

Output of the comparator drives the LED.

As the light changes, the LDR resistance changes, the voltage at the -ve pin changes, once it goes above/below the voltage level set by your pot on the comparator's +ve pin, the comparator output changes state (low to high etc)

Set the pot for your required light level  'trigger point'

A shcmitt trigger arrangement can be brought into play to stop the LED flickering on/off as the light level is in or around the threshold trigger point. (google comparators & hysterisis)

[~arph]

Depending on your application. If you need to light an LED when the ambient light dims, you don't need an LDR! you can do it with a bi-color led.

[defaced]

I'd go to the store and buy an autolighting LED night light and disassemble it.  I paid 2 bucks for one this weekend. 

[~arph]

^^^

That is half that price   

[deadastronaut]

 

here ya go pete...nice and simple and small..

http://www.aronnelson.com/gallery/main.php/v/chickpea/DARKLIGHTLDR.jpg.html

[Hides-His-Eyes]

I would just set up an LDR/resistor voltage divider into a schmidt trigger.
  I bet, but I've never introduced myself.. 
  So...why the Schmidtt ? [what I'm askin' myself, I'll take a looksee sometime.
  The transistor does a smooth scale of dimmer = brighter.

Schmidt trigger = turns on at one level, turns off at another.

So doesn't turn on until it gets pretty dark, but stays on until it gets much lighter again.

[petemoore]

  These are all viable solutions.
 Pick one intuitively or compare all comparitively to suit the application.
 Hafta say the 'one each' to drive 2 LED's via LDR, R Transistar...works just find.
 Base controls Emitter current up in see-saw fashion as LDR gets dark.
 Resistor on collector sets the current range, smaller resistor powers up 2 LED's.
 Doesn't look like it could be much simpler or much more apt to be able to be doing what I want, 1 active, 2 resistors, for C and B, the photocell and the
LED.
 
 I want a gradual increase in lumens [from LED] to [not] follow closely..the gradual decrease in lumens [of LDR]...to mis-state in attempts to state basic idea...
  The LDR tends to be nothing/fairly fast sweep as shadow encroaches, having it never go near 1k seems like a pretty good idea as does peripheral pickup area having less impeded 'view' so a light/shadow situation doesn't make the cell light/dark as my foot moves the shadow 3/8''...across the cell.
  Plenty of tweeks to figure...so far the LED/LDR light/dark show looks close to if not workable as is. 

[petemoore]

  what's with the strike throughs this week...I dunno !
  I tried double strikin' but can't figure to rid the post of strikes...enough !
  Hopefully you can read it..or instruct how to de-strike-through.
  Maybe what I'm doing this week that's causing it...

[petemoore]

  Thanks spaceman !
   This is the one I chose:
    http://www.aronnelson.com/gallery/main.php/v/chickpea/DARKLIGHTLDR.jpg.html
  It works quite well.

[deadastronaut]

your very welcome.

yeah its great ive used it on a few wah type/pot control projects...

[amptramp]

You can buy LED solar outdoor path lights that charge up a Ni-Cd battery in daylight and switch the LED on in the dark.  These have been around long enough to be garage sale items.  They have an LDR that is separate form the solar array, although the design could be done with just the array providing the ambient light information.

[Gurner]

You can buy LED solar outdoor path lights that charge up a Ni-Cd battery in daylight and switch the LED on in the dark.  

Won't people mistake your floor rig as a particularly well lit path then walk over it?

[petemoore]

Quote from: amptramp on Today at 05:27:18 PM
You can buy LED solar outdoor path lights that charge up a Ni-Cd battery in daylight and switch the LED on in the dark.
Checklist:
  LED/LDR/Solar Cell, Battery...and related circuit.
  Other than the charge storage components, not a bad item and parts collection, thinkin' it'd be a good thing to just fix..
  I gave it only a quick vulture-eye, but it looked like the juicy parts were cast inside a hard, clear, weatherproof shell, and the most interesting components might well be ruined if accessed at all.