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# Light Switch

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Will such a circuit work?

What should be the value of the resistances?

How do you calculate it?

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Not very effectively. The LDR will be fighting the trim pot with a parallel resistance. What are you trying to accomplish with this circuit? A variable power source dependent upon light?
The center leg of the 78XX series regulators are meant to be at ground potential for the rated voltage. If you lift them above the ground potential, then the voltage of the regulator is higher. For example, if you were to connect a diode between the middle leg and ground on a 5V regulator, you would be 0.7 V above ground potential and thus would have a 5.7V regulator.
I hope this helps.

MP

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I want to use the transistor as a switch. It should permit the conection to ground when light does not fall on the LDR. I do not want to use a relay hence i thought of such a circuit.Can any changes be made possible.

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It seems that you are looking for this reaction between the trimpot and LDR to simulate a comparator. In that case it might work well enough for your purpose. It would not be hard to breadboard and see. I do not know the specs on the LDR. You might be able to get rid of the trim pot and make the other resistor a trim pot for adjustment. Again, this would depend upon the specs of the LDR.
Keep in mind that a LDR does not perform a switching action. It will have an analog response to different amounts of lightness and darkness. Therefore a lot will depend upon how gradual the darkness comes to the circuit. If this becomes a problem, you could add a comparator to the base of the transistor for a specific on/off threshold.

MP

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Will any switching transistor(npn) do?

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Siddharth,
The diagram below is actually from the 78XX data sheet.
MP

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Siddharth,
1) What are you going to use this voltage-changing circuit for?
2) How many volts will feed the regulator?
3) How much output current when its output voltage is high?
4) What voltage output do you need when it is dark?
5) What voltage output when it is in bright light?
The transistor doesn't have to be a switching type since an LDR does not change resistance super-fast. A 2N2222 switching or 2N3904 audio will be fine.

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I found a Simular Circuit in this Datasheet on the last page.
The values should work with the L78xx.

http://www.linear.com/pdf/1038fa.pdf

LT1038.pdf

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I want to build a light switch as i had seen here on this site. But i do not want to use a relay as i have to operate small light bulbs which run on DC source.

I got this circuit from the datasheet of 78XX which you have.But had no idea of what value of resistor should i use.

1. A switch
2. 15-12 volts
3. Required value
4. Idealy zero
5. Output voltage provided by the regulator(15- 12 volts)

I have a transistor name SL100, will it work?

Any idea how to calculate the resistance value?

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Siddharth,
.
.
.
The transistor doesn't have to be a switching type since an LDR does not change resistance super-fast. A 2N2222 switching or 2N3904 audio will be fine.

Hate to pop anyone's bubble here, but the diagram from the data sheet is using PNP transistors. 2N2222 and 2N3904 are NPN. If you are going to change the transistor in the diagram, you should stay with PNP type. Also, PNP are better to use because a ground on the base will turn it on rather than a voltage.
PNP and NPN are not interchangeable.

MP

What

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I agree with you Ante,
A Schmitt trigger is better than a simple transistor since it will stay after switching, while a simple transistor may cause the lamps to flicker if a shadow falls on the LDR or a if cloud goes by.
We still don't know how much current is required. Recall that an incandescent light bulb draws 10-12 times more current when cold, so the transistor must handle the power-up surge and be driven hard.
The data sheet circuit certainly isn't suiteable nor practical for switching.

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He said he needs a switch not a light controled switch . The answer he gave for #1 is "a switch" That is what I gave. Here is a simple Light Switch.

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Audioguru,
The power surge is one of the reasons I want to use a Schmitt trigger if the light comes on slowly the transistor will be fried before the lamp starts to glow. If the transistor is driven directly to saturation it can cope much better with the current.

Ante ::)

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Thanks for all the comments,bursting of bubbles etc..
I want a light switch.
As per your suggestions i should use a schmitt trigger is much better.
I have found such a circuit below.
Is it Okay?
Anyway can still anybody tell me how do you calculate those resistance values?

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Ante,
Yes, let's get that transistor switching fast. The schmitt trigger, because of its positive feedback, will do that.

Siddharth,
That's a nice circuit that you have found but it can be improved:
1) The opamp is not a schmitt trigger because positive feedback is missing. Add a 100K ohm resistor between pins 3 and 6. That should give about 2V of hysteresis.
2) The expensive and failure-prone relay might be able to be eliminated if you tell us what your maximum load current will be.
3) The expensive "replacement series" ECG128 transistor can be replaced by a much cheaper one if we know what your maximum load current will be.
Where are those bubbles? What are you going to use this light switch for? Maybe the light heats a soap-solution that makes the bubbles but only at night?

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Thanks!

I my self have no idea of much load current will i need.You must have seen those small light bulbs used in toys or as small indication lamps (or a small torch bulb) instead of LEDs. I want to use those.

I want to make a simple night lamp and don't want to use a bulb which needs a mains supply and want to avoid the use of a relay.

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Why make a mountain out of a mole hill just try the easy ones I posted. You can get most of the parts out of an old Transistor Radio or anything that is junk. Free sounds better to me. Or you can even use a Photo Cell.

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siddharth, my comment on not wanting to pop anyone's bubble was only in reference to a post recomending a transistor which was not the same type as on the data sheet (NPN vs PNP).

You were on the right track to begin with. The one you posted and the data sheet. There are probably thousands of ways to do this and they will all work.

I really enjoy it when someone posts a circuit and asks for ideas regarding it. What really frustrates me is when several people jump in and try to talk them into doing it a different way with no additional information causing them to believe the circuit will not work. Why change your design because someone else does not do it that way? Then comes the confusion. By the way, in your posted schematic with NPN, you can use 2N3904. In the data sheet schematic with PNP you can use a 2N3906. These are not the only transistors that you can use, but they are very common.
I hope you decide on a circuit for your project soon.

MP

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Thanks MP. You removed me from the confusion. I will stick with my original idea only. I liked that circuit since it was the simplest of all.
Sometimes even i get confused with simple circuits.

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Siddharth,
Thanks for telling to us the application and all of your requirements. Your original circuit will not do what is required if you connect an unknown quantity of incandescent lights between the output of the regulator and ground:
1) The regulator will shut-down if you try to power too many lights because it has a maximum output current of only 1.5A.
2) With daylight, the lights will be bright. For the lights to be bright at night, swap the LDR and R1.
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.
4) Without having hysteresis, the lights may flicker during shadows or clouds.
Replacing the regulator with a power transistor will avoid these problems.
Confusing?

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I am going to use only one small torch bulb which runs on two heavy duty 1.5V cells. Talking about flickering due to a cloud coming in between; a LDR will still provide enough resistance since there is some amount of ambient light present.

I still have problem on the value of resistances i should use. Could you tell me the method how to do that?Is it such that i measure the resistance of LDR in light and dark and then calculate the resistances such that the transistor saturates or cuttsoff?

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siddharth, you are correct. Once you know the light and dark resistances of the LDR, you can look at it like it is a simple resistor and use the voltage divider formula. The reason I like to use PNP transistors in such an application is because they are turned ON when the base is at ground potential.

MP

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