Annoyingly, no. 2 son has left a school project to the very very last minute, and I have to sort things out! On a Sunday afternoon, I asks ya!

His 'invention' has to include 32 (yes, 32) LEDs for illumination.
Progress so far: we have a bag of 50 LEDs from ebay ready for installation, and the carpentry for said gadget completed. We have several 9V batteries and quantity of wire and plenty of solder for the soldering iron!
On the research side, I found this web site HERE (http://www.kpsec.freeuk.com/components/led.htm) which gives useful info.
Now for the question: half way down the website page is the paragraph on wiring LEDs in series saying that one 9V battery can power 3 lamps plus a 220 ohm resistor. We have everything except the resistors, so we wonder if adding a 4th LED in place of the resistor will provide the appropriate circuit resistance.

Thoughts and suggestions appreciated.


Edited to add that we have RED lamps which according to the above website require 2 volts. No info at all provided with the bag of lamps.

No it won't. The resistor limits the current through the LEDs.

You could get away with a smaller resistor and the extra LED, but with no resistor and an extra LED, it will depend on the real battery voltage, the battery internal resistance, the battery capacity and how much current the LEDs can stand.

Of course, another question is how long is it supposed to work for?

But I think you are in trouble.....

I see you are a bit too far away to come and get some resistors from me...

You really need the spec for the LEDs, the voltage required to illuminate them can range from less than a volt to 5 volts. Also remember the higher the current through them the more light they will give out but the shorter will be their life and also your battery life. In addition one that fails could go short circuit causing a domino effect with the rest in series. If you will be running this off a dry battery then the battery output will vary from 9 volts new to about 6 volts when flat.

If you can get no info on the LEDs, play safe and connect, say, 8 in series and see what the output is like, then reduce the number one at a time until you get an acceptable level. As you have some extras you needn't worry too much about blowing up one or two. You will probably find you need about 5 or 6 for a decent output, depends on the LEDs. Also you will find that as you connect more strings in parallel they go a bit dimmer due to the internal resistance of the battery.

If your ideal setup is not 32 (e.g 8 strings of 4 or 4 of 8), but, say 35 (7 strings of 5) then it is probably easier to conceal the odd extras rather than try to find a resistor to replace then - i.e. put them on the PCB or whatever rather than in the display.

LEDs generally take about 20mA or less, so if you have them in for example 5 strings, your total current draw will maybe be 100mA and you can check if your battery life will be acceptable remembering that it's output varies as it ages.

Best of luck with it
UFO

and I have to sort things out!

No you don't. Tell him to sort it out himself and take a nap/do some gardening/go to the pub.

If you do it, next thing you know he will be at university and wanting lots more (financial ) help.

I thought homework was for the pupils to do, not their parents. Shouldn't your son be dealing with this and learning about electronics (and prior planning) in the process. Perhaps the more valuable life lesson here would be for him to bomb the project, take the consequences and learn not to make the same mistake again. What is the benfit of you doing the work? Are you hoping for a gold star or something?

In fairness to no. 2 son, he already put in a big effort into the construction side of things, the dovetail joints being of pretty good standard for a youngster! The varnishing however needed several attempts for an even covering.They have not yet covered practical electrical circuits in class, so that aspect was always going to be a stumbling block. The complexity has been scaled down somewhat, and we've settled on UniFoxOs suggestion of say 8 in series to see how that goes.
Thanks to all contributors, and suffice to say that appropriate words of wisdom about not leaving things to the last minute have been given.

Have you got an old piece of electronic gubbins that you could nick one from?

220Ω = red red brown or perhaps better would be a couple of 470Ω in parallel to increase the current handling, Yellow Violet Brown.

Good call GG.

I have a broken freeview tuner on the table beside me which could well become a donor. The power board has decent size components that can be un soldered while the rest of the internals are all surface mount. There's no exact match as per the colour code so will pick the nearest available value.

Thanks to all again.

Buy an LED product in Radio Shack. Dismantle it and reassemble it into the project.

Simples.

FL

Difficult to do on a Sunday when the project has to be finished for the next day, SGC

What's the point of all the LEDs coming on in strings?

Can't you put a small motor in there, with a sweeping contact down-stream of the current limiting resistor? Then, wire it to make some psychedelic patterns that hypnotically compel the examiner to give full marks.

:ok: Good on ya ant for taking the time to do this with your son, deadline or not.

Ant,

Did you manage to work it out?
If not, I found this site (http://led.linear1.org/led.wiz) which may help.

Don