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Siddharth,
I am glad that you figured how to calculate the resistors.
But a 7805 won't work with only 3V because it needs at least 6.5V. I suggest powering it with 6 cells which give 9V when new, and 6.6V when exhausted. Use a 9V torch bulb. This circuit is going to eat batteries quickly.

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siddharth, the statement in the previous post is not true. You need at least 6.5 volts to get a steady 5 volts out of a 7805, but there is no "low cut off voltage" where they will not work. You have a loss. If you supply 3 volts, you will get something like 2.5 volts on the output. This application is not common for a 5 volt regulator, but it will work. Also, since it is not a common voltage from the regulator output, you will not want to calculate your voltage divider for 5 volts.

MP

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  • 3 weeks later...

Siddharth,
You can't use a regulator as an on-off switch. It operates exactly the way that I said:


Siddharth,
Your original circuit will not do what is required ......:
2) With daylight, the lights will be bright. For the lights to be bright at night, swap the LDR and R1. (or use a PNP transistor as an emitter-follower, which you are doing).
3) With daylight, the lights will dim a bit but will not shut off, because the output voltage of the regulator cannot be less than its label.
Replacing the regulator with a power transistor will avoid these problems.


With the base of the transistor high, then the regulator is floating and therefore will be a piece of wire from its input (15V) to its output, as in my quote #2 above. "The lights will be bright" because the output voltage will be high, as you see.
With the base of your transistor low, then the output of the regulator will be 5V plus the base-emitter voltage of the transistor, as in my quote #3 above. "The lights will dim a bit but will not shut off" because the output voltage of the regulator will not be 0V, as you see.
Never mind what MP said, he must be using faulty translation software.
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I tried the one which was given in the datasheets (the one which gives low output when light goes up).

However i tried the circuit which uses an NPN transistor to the input of a 7805 such that the collector is the input of unregulated voltage, the emitter is connected to the input of 7805, and worked very well. The output voltage changed from 0V to 5V whenever the base voltage changed.

It would be nice if i could replace the transistor with a photo transistor but which one will be suitable.

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Siddharth,
A photo transistor, and even a photo-darlington transistor, do not have enough output current to power an incandescent light bulb.
The regulator in your circuit does not boost the current.

That is good that your emitter-follower/regulator circuit works well. If you provide an input voltage that is 0.7V more than is required by the light bulb, then you don't need the regulator. Just connect the light bulb to the emitter and to ground.

A cold lightbulb draws up to 10 times the current than when it is lighted, so select a transistor that is rated for that much current.

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Siddharth, with this change you will have a voltage drop at the NPN transistor and then another voltage drop at the regulator. You will only have the voltage of the base of the NPN available to the regulator (less vbe), then the voltage drop I mentioned in a previous post. I no longer see a purpose for the regulator unless it is a safety to keep the lamp from getting more than 5 volts. If this is the case, you could use a 5.1 volt zener at the base of the NPN and remove the regulator.

I have posted your circuit for better view.

MP

post-555-14279141627527_thumb.jpg

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Siddharth,
That would be cool to see the guts of the OPT101 through its clear case. Its built-in opamp doesn't have much output current but it will drive a booster transistor nicely.
Did you see the warning about soldering? They recommend BAKING it before soldering, to get rid of humidity that leaks into its clear plastic. They say that if you don't bake it then it may fail (heh, heh, maybe the steam that is produced from the temperature of soldering will cause it to blow-up!). Its data sheet is here:
http://www.qsl.net/wb9ajz/laser/data/opt101.pdf

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