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The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

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  #1  
Old 04-04-2011, 07:34 AM
panzee panzee is offline
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Default Multiple led with ldr and batteries powered.

I'm a newbie on electronics.I would like to build a project which light up around 30 LED(preferable white led) upon light sensor activation and powered up by batteries. Can anyone give any advice for how much complicated does it need to be done.
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Old 04-04-2011, 08:15 AM
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Bill_Marsden Bill_Marsden is offline
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It isn't that complicated, but you do have some reading ahead of you. I have a tutorial for LEDs, you need to read the 1st chapter and the 1st half of the second...

LEDs, 555s, Flashers, and Light Chasers

Combine it with this LDR circuit...



What is your battery voltage? 30 LEDs is a lot, and as a general rule of thumb the more voltage the more efficient the LEDs run.
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Old 04-04-2011, 10:27 AM
panzee panzee is offline
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thanks for advise,
the idea is i have this headlight using 10 unit led powered by 3 units of AA (1.5v) batteries....so i just taught if it possible to light up more led with just using a PP3 (9 v) batteries and combine it with function of LDR. Should the LEDs best connected in parallel or in siries.
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Old 04-04-2011, 10:49 AM
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If you are talking a small 9V battery it will last around an hour, maybe less. LEDs are efficient, but the batteries aren't. Plus, a 9V battery very quickly drops to 7V. With a lot of circuitry this doesn't matter, with LEDs it does.

Figure it like this. Each white LED drops around 3.6VDC. If you have a deep 9V battery you can power two white LEDs in a chain (total voltage 7.2VDC). For 30 LEDs that means 15 chains each pulling 0.02 amps, for a total of 0.3 amps. For a small battery this is a lot of current, 1/3 of an amp!

If you had 12V you could have 3 LEDs in a chain (total voltage 10.8VDC). This means you would have 10 chains each pulling 0.02A, for a total of 0.2 amps. Better, but not great.

If you had 24V you could have 6 LEDs in a chain, 21.6VDC, and 5 chains. Total current is 0.1 amps.

See where this is going. 30 LEDs is not an insignificant number. If you have a large battery it can power them a long time, but small batteries will not cut it.

One last example, 4.5VDC can only power 1 LED. If you have 10 LEDs that is 0.2 amps, again, not good for the battery.

LEDs always require a resistor. While we have been talking the voltage needed to power them up it is current that lights them up, you need 20ma (0.02 amps), but you have to exceed their dropping voltage, which is a constant. The resistor is not optional.
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Old 04-06-2011, 08:56 AM
panzee panzee is offline
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i'm not sure about the schematic i try to create,whether it will work or not.
please give opinion on it,thanks.



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Old 04-06-2011, 12:26 PM
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Bill_Marsden Bill_Marsden is offline
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Very good!. The basics are there, it may do what you want it to do first time.

On the chain with a single LED, you might as well add a second LED, it will not take any more current, and the battery can't see the difference, but you would get more light, and LEDs are cheap (it is batteries that are expensive).

I think you can get rid of the 10KΩ connected to the base.

A side note about schematics, it is a really good idea to label parts (example, R1, R2, etc.). This allows a second person to refer to it by name instead of a general description, which may be misunderstood.

All About circuit has a hosting site for pictures, as well as a blog location. My blog is at:

Bill's Index

It has a pointer to my albums, which are all schematics (some are lemons). Go figure.
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Old 04-06-2011, 02:24 PM
wayneh wayneh is offline
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Quote:
Originally Posted by panzee View Post
I'm a newbie on electronics.I would like to build a project which light up around 30 LED(preferable white led) upon light sensor activation and powered up by batteries. Can anyone give any advice for how much complicated does it need to be done.
You said you would like to build it. Is there a reason to not just BUY one? What you're describing is exactly provided by a string of solar LED xmas lights. I've picked up some strings for free at garage sales. You didn't mention solar recharging, so I guess you wouldn't need that extra function.
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Old 04-15-2011, 02:47 AM
panzee panzee is offline
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the idea for building it because,it has be used indoors as a room warning light(do not enter,warning,etc...) when people has enter the room *noted that the room is seldom use. I don't want to involved direct electrical current from the house and easily manage. Hopefully i could build as many LED as battery could hold and last. So far i manage to work with 10 series of LED which the light is not sufficient. If i would like build a 30 units of LED which could draw current around 200 mA, what type of transistor should i use?
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Old 04-15-2011, 07:39 AM
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Bill_Marsden Bill_Marsden is offline
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Almost any type of transistor will work, as long as you are careful to make sure it is either on or off, not between. A 2N2222A (a popular transistor with local hobbiests and the American military) is rated up to 600ma, which would work OK for your application. When it is only partly on it will get pretty hot however, which could shorten its life some.

What part of the world are you? The reason we label our profiles so it is displayed in the upper right hand corner, it helps recommend parts local to your area.

Are you using the schematic you displayed?

Quote:
Originally Posted by wayneh View Post
You said you would like to build it. Is there a reason to not just BUY one?
I think I've pointed this out before, but the reason to do it yourself is to learn. Either way it is not expensive, so why not do it yourself?
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General info:
If you have a question, please start a thread/topic. I do not provide gratis assistance via PM nor E-mail, as that would violate the intent of this Board, which is sharing knowledge ... and deprives you of other knowledgeable input. Thanks for the verbage Wookie.
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panzee (04-15-2011)
  #10  
Old 04-15-2011, 08:34 AM
panzee panzee is offline
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Thanks for your advise,it really help so much....i confuse how to pick out the type of transistor suitable for the circuit.Afraid for it getting damage. Last time i try to leave out the circuit (the one with 10 unit of LED) to work and it hold at least around six hours until it sort of burn out. First i thought it was the battery run out. later,after changing it to a new one the circuit still doesn't run. I'm not sure whether it is the transistor or the preset resistor are damage,because the LED still working.

btw,i'm from malaysia. i have been use the schematic and build it like you recommend and it really work well.

next, i will try with 30 unit led and see how it would last.
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