Light Sensing Circuit

you need a variable pot to adjust resistance...You know you can buy the flickering candle led's for $2, with manual switch..or 12 for $9...

Amazon tea light set

please engineer

try to use the operational amplifier technique,,,it is very sensitive and can operate by any light intensity,,,try and if you failed we can operate together

,,,,,,,,,,,,,,thanks,,,,,,,,,,,,,,,,ahmed,,,,,,,,,,,,,,,,,,,,,,,

Redfred has given you a great answer.
Start with more volts, say a 9v old battery. You can then design the circuit properly. You also want a resistor in series with the LED to limit it's current, between the LED and -ve rail will work well, as it will help the transistor to start turning off.
I strongly advise against adding op amps and stuff, it just complicates it.
Keep it simple at first.
Del

Thanks redfred and del, and everyone else for that matter. The resistor did the trick. I am, however, curious as to how to use the cadmium sulfide photoresistors in a circuit. I tried using it in place of the phototransistor and the LED goes off at dark and on at light. Is there any way to get it to work in the same fashion except going on at dark? I am not sure it will make much difference cost wise but am just curious as to how they would work. (my electronics courses were still in vaccuum tubes, they were just starting to introduce transistors but did not go into much detail, so I am very much green in modern electronics. Thanks all very much.

The circuit with really great sensitivity can be found in a light sensitive night light. The product is very inexpensive and available in most hardware stores.

Byron, the problem is that I need to design it for my customer. Initially he called for simply an LED for his device, now he wanted it to go on at night. I can find hundreds of devices out there that turn off at light and on at dark, and likely I could copy a circuit that way. I have the circuit and all I needed was to increase the sensitivity which I accomplished thanks to this forum using a higher value resistor and it now works great. I still am interested just as curiosity for the photoresistor and tried switching the resitor and the photoresistor with no luck but think it is simply the size of the resistors that is the problem. I have a circuit that works as I want now so all I need to do is create a small enough package and it is done. thanks.

Like the man said earlier just replace the 1k with 22k or 27k.

If you want a real on/off trip, put another 2N3904 in darlington, and put a potentiometer say 0-47k in series with the base resistor until you find the threshold that you want.

engineertony, If you had read my reply you would note that I did replace the resistor and took care of the problem. My comment here was simply that I can very well go to hardware store and find night lights, that is not what I was after. I have been able to make the circuit as needed and it works great.

Glad I could help. Depending on how many you will be making, you might consider using a field effect transistor instead of a bipolar transistor. You'll get a sharper ON/OFF transition and possibly cheaper components.

Thanks, I will look at the FET idea. I am not sure how much cheaper they would be will check mouser. the transistor I used I got for about 14 cents (cost more for shipping) and I think there is a substantial discount for high volume, not sure about digikey. I will look into it though.

The price break difference of switching to a MOSFET would likely come at the 1000's quantity. I hope that you have to consider this redesign.

Sorry didn't see your success post.