# Multiple led with ldr and batteries powered.

Discussion in 'The Projects Forum' started by panzee, Apr 4, 2011.

1. ### panzee Thread Starter Member

Apr 4, 2011
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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.

2. ### Wendy Moderator

Mar 24, 2008
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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.

3. ### panzee Thread Starter Member

Apr 4, 2011
35
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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.

4. ### Wendy Moderator

Mar 24, 2008
20,772
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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.

5. ### panzee Thread Starter Member

Apr 4, 2011
35
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i'm not sure about the schematic i try to create,whether it will work or not.

6. ### Wendy Moderator

Mar 24, 2008
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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.

7. ### wayneh Expert

Sep 9, 2010
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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.

8. ### panzee Thread Starter Member

Apr 4, 2011
35
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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?

9. ### Wendy Moderator

Mar 24, 2008
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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?

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?

panzee likes this.
10. ### panzee Thread Starter Member

Apr 4, 2011
35
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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.

11. ### Wendy Moderator

Mar 24, 2008
20,772
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So what if the transistor smokes? That is the price of education. True, no one is happy when it happens, but it does happen (last time for me was several months ago).

I recommended a part, but I asked where you were located. I don't need the exact location, but parts in Britain tend to be slightly different than parts in the USA. Funny thing is, the transistors are pretty interchangeable even though they may be different part numbers.

I recommended a transistor, but didn't get an answer on your location. You name a location people will likely come back with a local retailer and part number.

The 555 would help with the transistor getting hot. This is because when it is wired like a Schmitt Trigger the transistor will be on or off, not in between. That in between state is where damage will occur. The two most efficient states for any transistor, be it a bipolar or a MOSFET, is completely on or completely off.

12. ### SgtWookie Expert

Jul 17, 2007
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Bill, this was in Panzee's last reply:

13. ### Wendy Moderator

Mar 24, 2008
20,772
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Opps, missed it. Sorry about that.

14. ### panzee Thread Starter Member

Apr 4, 2011
35
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would it be right after connected it as schmitt trigger and then goes into astable circuit to give the blinking effect?

15. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
The English didn't come out quite right there. Do you want it to blink?

16. ### panzee Thread Starter Member

Apr 4, 2011
35
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yes...sorry for for my grammar.

17. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Well, I've never tried it, but you could use this oscillator, and turn the unit off and on via the reset line.

555 Hysteretic Oscillator

Anything over 0.7V will turn the oscillator on.

Like I said, never done it quite that way. Want a hypothetical schematic?

You could use 2 X 555 chips (or a 556). This design would be a lot more certain to work.

When I get a chance I'll draw something up on the 2X chipset.

18. ### Audioguru New Member

Dec 20, 2007
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The BC108 transistor is obsolete and was replaced by the BC548 many years ago. The max allowed current for a BC108 was 200mA and the max allowed current for a BC548 is only 100mA.

With 15 strings of two LEDs the total current is 300mA so a stronger transistor is needed.

19. ### wayneh Expert

Sep 9, 2010
12,374
3,225
If the whole point is the learning and the satisfaction, then DIY is great fun. As only a means to an end, not so much. Especially if that same end is cheaply available already.

So I'm with you, but I also sense that a lot of posters here launch into a project to build something assuming that it will be quick, easy and cheap, and that they cannot simply go buy the solution, when in fact they can. And they get frustrated when they discover (learn) how hard and time-consuming a project can be if all you want is the finished result, the destination and not the journey.

Another reason I often ask posters about their motives is to simply understand their needs relative to something tangible. In other words, what are they looking for that the commercial product doesn't offer?

20. ### panzee Thread Starter Member

Apr 4, 2011
35
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I thought about using those,but don't really understand how it should be design. Still learning the basic concept of the timer itself. Also, i would like to reduce the number of led being used to 4 units with supply battery of 6 v hope it will simplify the circuit and prolonged the battery life.