LED's

stereo_steve

Just a quick question about LEDs. I'm designing a bridge for a constrution studies project for the leaving. I've made a nice fancy
bridge but I want to light it up. I don't want big fat bulbs hanging from underneath it. I was thinking about using LED's. But I don't want to use the "normal" ones. I was thinking about using the small ones that you would find say in a mobile phone. Just wondering what that type of LED is called? Is it surface mount? Also Where could I get that type? Peats?

Would that type of LED work off a circuit board? I'd just be solering them to a wire and connecting them to a battery pack.

Thanks for help I get

PPC

You'd need ultra bright led's if ya want a good bit of light.
Maplin have some lovely high intensity blue leds.
You can put them on circit board and use them like that but theres no point really.
Get your self the amount of led's you need and a toggle switch to turn them on and off and yours set.
You'd get normal green ones really cheap, but the high intensity look alot better and blue or white leds will cost ya an arm and a leg.

stereo_steve

Thanks! I was hoping for green ones. About how much would eight hi-intensity green ones cost?

PPC

http://www.maplin.co.uk/products/product.php?cartid=0203062217541453603&modulecode=&moduleno=2058&manufacturer=Kingbright

quite cheap

llatsni

I saw the ultra-bright mobile phone type ones on farnell:

http://international1.farnell.com/New/new-frame.jhtml

they look pretty sweet, expensive though.

or at least they were, but theyre cheap now?????

daveirl

This post has been deleted.

tribble

Yup - LED's are basically diodes
nearly open circuts in one direction
without some resistors in series they will absolutly fry (with a popping kinda sound)
Get somebody with a Leaving in Physics to do the circut for ya

stereo_steve

What I was planning on doing was having 8 LEDs connected together in a series with an LDR as well. I want them to all work off a 3 volt battery supply which is recharged by a solar panel. Does anyone know what size resistor and what voltage LEDs I should get for the circuit to work? Thanks

daveirl

This post has been deleted.

stereo_steve

Thanks for the help Dave. I made a basic circuit drawing in ascii in word and I was going to email it to you when i realised the board doesn't let you send attachments! Here it is below: Thanks

Argh! I just saw it won't let me upload *.doc files! So i renamed it *.bmp. You'll have to rename it back to *.doc.

Giblet

http://www.boards.ie/members/giblet/schematic.jpg

Using 3-5v DC

Use a larger resistor (330 - 640ohm) if you are using 9vdc battery.

Inspector Gadget

Two things that might be relevant here:

1) You'll need a diode (preferably Schottky, for their low voltage drop, or failing that, "regular", rather than light-emitting, because of the voltages involved) in series with the solar cell to stop the batteries discharging into it - that would be BAD for the solar cell.

2) You just can't run 8 LEDs in series off 3v, as the voltage drops across the diodes would match the supply voltage, leading to the diodes getting nothing You have to run them in parallel (connect all the anodes together and all the cathodes together), rather than serial (anode-to-cathode with a return wire) like you'd do fairy lights. That way, the diodes only drop the voltage by one diode's worth rather than eight, if you get my meaning.

Gadget

stereo_steve

thanks Gadget. I'm actually going into town tomorrow to buy all the gear!

The solar cell I was planning on using is a special recharging one, that I saw at Maplin

Would this be ok to use?

The reason I have to connect the Leds in series is due to space restraints. I just don't have the room. Could 6 be run off the battery? If not four?

stereo_steve

Ok I finally went into town to get the stuff today after moaning at all of you for help ( thanks by the way). I went into Maplin and a guy behind the counter helped me. I've decided they are far superior to Peats, muh more helpful and know what they are talking about. Only problem now is when its dark the LED's go dim and when its bright they go bright. But I want it the other way around. Can Anyone see what is going wrong?

Also the guy in Maplin said that I wouldn't a resistor between the battery and the LED's. Is this right?

Heres the circuit below;




Thanks

Gavin

Maybe stick a capacitor in there to store energy whilst the light is high.. Don't know how big it would need to be though.

Gav

daveirl

This post has been deleted.

daveirl

This post has been deleted.

stereo_steve

Thanks a million Dave. Thats a big help! I was going insane yesterday trying to get it to work!

Inspector Gadget

Daveirl has basically hit the nail on the head as regards the light-controlled switch - nicely done, sir. The only thing I'd suggest is that you replace the two vertical positioned resistors with a variable resistor (something in the region of 2.3MOhm - Maplin have a 2.2MOhm device - UH10L (STG 20p) - that should do the trick. The the middle leg to the Base (B) of the transistor as shown in the diagram, and the other two legs to the connect to where the 300k and 2M resistors in the diagram go - it doesn't matter which way round. Before you decide to apply power, use a multimeter to adjust the resistance between the middle pin and the unmarked pin of the transistor - the Emitter (E) - to about about 2MOhm. Then fiddle with the variable resistor (stick in a screwdriver and adjust it - it's a dimmer switch) so that it switches on and off at the brightness level YOU want. just check that the transistor and/or LEDs aren't getting warm while you're twiddling.

As regards the solar panel, well, it doesn't say that it's diode-protected on the Maplin page, so I'd stick in a 20p diode (say, a 1N5821 - Maplin Code=JA49D) in series with the cell just in case. It's cheaper to build in a diode than replace an STG£8 solar cell... Also, check that the diode is in the right way before you fit the solar cell - connect the diode across the battery, and there'll be a spark from the battery if it's the wrong way round (assuming the battery's charged).

Also, it's not a matter of "that's a bad idea" as regards the LEDs, it's a "that won't work - ever". When you run your supply through the first diode, it'll drop from 2.2v by about 0.6v to about 1.6v. As this goes through the second diode, it'll drop by a further 0.6v to 1.0v. Two diodes later and your entire supply voltage is dropped across the diodes, and because the later diodes aren't conducting because they're not getting any voltage, none of the LEDs will light. This is laws-of-semiconductor-physics stuff, that can't be gotten around. Wire them in parallel - have two "rails" and solder each cathode to one rail and each anode to the other, and you'll be sucking diesel.

Happy hunting...
Gadget
P.S.> You are aware that under best-case conditions it'll take that solar cell well over a week to recharge that battery???

daveirl

This post has been deleted.

Inspector Gadget

Erm, stereo_steve, the solar cell you linked to is 3V/80mA (i.e. 240mW) whereas the solar cell in your diagram is 12V/125mA (i.e. 1.5W)?

The 12v/125mA solar cell will be quite large (in the region of 20x20cm, if I have done my calculations correctly) and cells of that price are more than a little pricey...

Gadget

stereo_steve

Yeah your right! They normally cost about €40 but they were on special offer for €15. It was the only one in stock! I'll reuse it for other things though. The only thing is 'll have to change the circuit Dave made around slightly. Would you know what value resistor I should put between the solar cell and the two 1.2V nicad batteries? And then what variable resistor I should get instead? Thanks

Inspector Gadget

Steve:

I'm not sure if you picked up on this earlier on, but, just in case, in the clearest language I can, I'll re-state: YOU MUST PROTECT THE SOLAR CELL WITH A DIODE. A RESISTOR IS NOT GOOD ENOUGH. Driving over an amp through a solar cell (which is what will happen if you connect the solar cell in parallel with charged batteries without diode protection) is more than enough to burn it out in seconds; if you've only got one, this really is something you want to sort out.

As regards putting a resistor between the batteries and the cell, unless it's to drop the voltage, I'm really not sure why it's there; if it is for a voltage drop, the diode voltage drop (of ~0.4V for the Schotty diode I mentioned in a previous post in this thread, or 0.6V for a regular silicon one - just make sure whatever one you choose can take 3A or so at the requisite voltage so that it won't break down on you) will drop the solar cell's 3V to an acceptable voltage to charge the Ni-Cads (2.4-2.6v).

I've already mentioned what value variable resistor you should choose - somewhere in the 2.3 Megohm range (2.2's should be common as they're a standard value) - actually, I've even given you a part number, if you look back over the thread; however, you should also put a fixed-value resistor in series with the LED pack to act as a current limiter. Something in the 100kOhm range should be okay - all it needs to do, really, is to stop the batteries discharging through the diodes (and the variable resistor for that matter) at full whack, as they'll all pop.

Gadget

stereo_steve

I have the diode. I was wondering what sort of resistor to use for the voltage drop to charge the LEDs. But its fine now. Thanks for all the help gadget and Dave.

Inspector Gadget

Woops! I said this:

Something in the 100kOhm range should be okay

...it would appear that a stray letter (a "k") appeared in there. Oops. Something like this, perhaps, is better:

Something in the 100 Ohm range should be okay

...sorry...

Happy lighting...
Gadget