Thanks! I got it to work just fine. Now I would like to turn a LED on when light is present. Basically a reverse of the above project.

Here you go..... this works opposite of my first circuit....

My .02

 

Can I substitute a 2N4401?

 

Can I substitute a 2N4401?

Yes, 2N4401 should work just fine....

 

In the latest schematic posted by BMorse:

you can add the capability of adjusting the threshold of light.

Replace R2 with a 2.2k fixed resistor in series with a 10k to 100k potentiometer, wired as a rheostat by using just the center "wiper" terminal and one "end" terminal.

 

BMorse
Actually the pencil schematic will not work, I built it on a breadboard and it does nothing for what the op wants (It does not even work!!)

I disagree.

I built the pencil circuit and you can see that it works. Visit http://www.tech-service.net/joe/dark_detection.asp to see the pictures or view the video. Also note the current drawn from the 9V source.

My photocell measured about 13k and less when exposed to the light and about to 500k in the dark.

OP from post number 1
I am trying to do a very simple LED circuit for a nightlight, using a Photocell to turn it on at night and off in the daylight.

 

I too built the pencil schematic , and works fine with 6.5 vdc.

 

Adding the 10k potentiometer and 2.2k works fine. Is there a way to have the LED go to about 50% brightness when R3 is completely covered (dark)?

 

Adding the 10k potentiometer and 2.2k works fine. Is there a way to have the LED go to about 50% brightness when R3 is completely covered (dark)?

Change R1 with a pot also, so you can change the LED current and adjust brightness, you might want to put a fixed resistor (680 up to 1k) in series with the pot so when the pot is turned all the way to VCC, it doesn't burnout your LED....

 

I disagree.

I built the pencil circuit and you can see that it works. Visit http://www.tech-service.net/joe/dark_detection.asp to see the pictures or view the video. Also note the current drawn from the 9V source.

My photocell measured about 13k and less when exposed to the light and about to 500k in the dark.

So maybe I misquoted myself , The circuit WILL WORK if you have a specific photocell to use, I tried the circuit with 4 photocells and it will only work with one out of four, my circuit works with all 4.....

so my apologies, the circuit will work if you shop around long enough for the right photocell ....

I guess the important part is the op got his circuit working no matter which circuit he used.....

My .02

 

I guess I got lucky with the first photo resistor. Now I would like to modify Bmorse's
LED ON in full light schematic, but do not know how. Here is the request, FULL ambient light = FULL brightness LED. Night or dark room = half bright LED.

 

So maybe I misquoted myself , The circuit WILL WORK if you have a specific photocell to use, I tried the circuit with 4 photocells and it will only work with one out of four, my circuit works with all 4.....

It worked with the only photocell I had handy. The three the OP had listed, all would have worked in the pencil schematic. I don't know what your photocells measure in light or in dark.

Your choice of paralleling the LED with the transistor consumes alot of power.

You might want to check the power budget of your circuit with the LED energized and not energized.

On your post labeled #17 of this thread, why did you choose a 10k ohm resistor for R1? And on post 29, you changed the same functioning resistor to 1k. Neither of those resistors in that function allows 15 mA through the LED.

 

Here is the request, FULL ambient light = FULL brightness LED. Night or dark room = half bright LED.

Look into using a 555 timer pulse width using a photocell to accomplish your task.

Which of the three photocells you described in your original post have you decided on using?

 

I used a photo resistor from Radio Shack.
What about increasing resistance to dim LED?

 

You might want to check the power budget of your circuit with the LED energized and not energized.

On your post labeled #17 of this thread, why did you choose a 10k ohm resistor for R1? And on post 29, you changed the same functioning resistor to 1k. Neither of those resistors in that function allows 15 mA through the LED.

Power budget for LED night Light = Wall wart......

I used 10K in post #17, since the OP wanted to dim the LED a bit, and did not want full brightness..... I was just following through with what has been posted... Post 29, 1k, since op still didn't want full power but to make LED visible in "light", since that circuit was for "LED on with Light present"....

If the op wanted to use the full brightness we would have stuck with the 470ohm resistor (for 15mA through the LED).

 

that is a old circuit childrens play all the time with !
however the battery will die in 10 days ...they get bored and dont use it any more .

 

that is a old circuit childrens play all the time with !
however the battery will die in 10 days ...they get bored and dont use it any more .

What battery???

If your children are playing with a Mains powered Night light , you might want to do something about that before they get electrocuted!

 

What battery???

If your children are playing with a Mains powered Night light , you might want to do something about that before they get electrocuted!

Here's a quote from the OP:

I got the same parts he lists:

9 Volt Battery
Photo-Cell
NPN Transistor (2N 4401)
Super Bright White LED
100K ohms Resistor
470 ohms Resistor
9 Volt battery Snap

 

Here's a quote from the OP:

I got the same parts he lists:

9 Volt Battery
Photo-Cell
NPN Transistor (2N 4401)
Super Bright White LED
100K ohms Resistor
470 ohms Resistor
9 Volt battery Snap

Here is another quote from the OP:

Except I want to run this of of a Wall Wart

 

Here is another quote from the OP:

Ya got me!