Electronics/Electricity help please

[weebl]

I'm trying to fix my 3 year old daughter's LED battery light which she's attached to:

It has a fairly simple circuit, and using a tester, I've isolated that the LED is faulty.

The light uses 4 AA batteries connected in series. The resistor appears to be 68 ohms (colour bands, when the gold quality band is on the right appear to be blue, grey, black going left to right - colour of the first band was hard to distinguish, but seems to be a dark blue).

I've found the following LED at a local The Source store:

http://www.thesource.ca/estore/product.a...&tab=3#more

Will this work as a drop in replacement? Or will I need a different resistor to use this LED. Thanks!

 

[Dave Sherman]

I'm surprised that it would only use a 68 ohm resistor. It's possible the one in there is an array of LEDs in series, or it's a superbright LED that can take significant current. Those 20 mA LEDs will not last long if you put one in there:

(6 volts - 2 volt LED voltage drop)/68ohms = 58.8 milliamps.

If it really is just one LED, then you'll either need one that can handle more current, or change the resistor to something else.

 

[TMoto]

I have nothing of value to contribute to saving her light, but you should save this thread so that when she's 16 and you've become the evil uncaring parent in her eyes, you can provide proof that you do indeed love her.

GL with the light.

 

[TooManyWheels]

This gets my vote for BEST BITOG PHOTO...

 

[JimPghPA]

Probably not. That LED looks to be too low of a light output, and also probably has a forward voltage drop of only 2.3 volts. 4 AA would put out 6 volts, and the resistor would have 6-2.3 = 3.7 volts across it. 3.7 volts devided by 68 ohms = 54.4 milliamps, the led would burn out.

If it were me, I would purchase similar item, and remove the LED from it and put it in the one you want to fix.

 

[Scienceman]

Originally Posted By: TMoto
I have nothing of value to contribute to saving her light, but you should save this thread so that when she's 16 and you've become the evil uncaring parent in her eyes, you can provide proof that you do indeed love her.

Absolutly! That is a great idea!

Originally Posted By: TooManyWheels
This gets my vote for BEST BITOG PHOTO...

Duh! This is better than the ones of the AmTran Genesis and Ward!

 

[weebl]

I think I will print this off! I can already see the battles we'll have when she's 16...

"But Daddy, if you don't let me keep my cell phone, then you don't love me."

"Well, if I don't love you, then why on earth would I have fixed this for you when you we're 3?"

Great idea about replacing the internals with another light source altogether. I can probably even rig something from the dollar store to work.

 

[Jim 5]

You need the forward voltage of the LED, and using that you can determine the resistor you need to drive the LED from 6v at 20MA.

I bought some blue LEDs at the source and it didn't have the forward voltage listed on the package and I lucked out and found it online.

If JimPGHPA's estimate of the forward voltage is correct, then you should be able to pick up a 190ohm resistor that should put you around 20MA. A pair of 100ohm resistors soldered in series will also work. You should be able to pick these up for pennies if you can find them.

But with all the driving around, you're probably better off just buying a new light, as I did with the "Princess" LED light my 3.5yo daughter goes to bed with each night.

 

[JHZR2]

Cant he put the resistor leads to a multimeter and figure out exactly what it is?

Or take a high-quality picture of the resistor

 

[Jim 5]

If he's replacing the LED, the old resistor becomes largely irrelevant unless the replacment LED happens to have the same forward voltage as the failed one, which is likely unknown.

 

[helix_rider]

Originally Posted By: weebl
Will this work as a drop in replacement? Or will I need a different resistor to use this LED. Thanks!


You could use 3 of those in parallel, that would draw 60 mA - roughly what you're getting with the resistor you have. Otherwise, they do make higher-current resistors - but they're a little more expensive and are harder to find. For the price, 3 of the ones you listed would be the easiest route to go, and might even give it a little more light than you had - but that all depends upon how it's built.

Most regular (cheap) LED's are rated for 2 to 2.3 volts and around 20-mA of current consumption - unless otherwise noted. So, you shoud be just fine with these.

 

[PandaBear]

You probably don't need those high current LED, they will be too bright and consume too much power.

If you absolutely have to do the rework without buying new resistor, you can try to run the battery in parallel, so instead of 6V you have 3V, 3V - 2.3V = 0.7V, 0.7V / 68Ohm = 10.25mA, half the brightness. If you have 2 resistor already, try to run them in parallel so you can get 0.7V / 34Ohm = 20.5mA

A trick that many cheap electronic uses to give bright LED light is to over current it to trade life with brightness. Usually the rule of thumb is if you stay below certain current (I think it is 20mA off of my head) it will last almost forever.

You have to really look at the spec sheet of the LED to see what to run it with.

 

[JimPghPA]

Two of those LEDs in series would drop 4.6 volts.

4 AA = 6 volts.

6-4.6 leaves 1.4 volts droped across the resistor.

1.4 volts devided by 68 ohms = 20.58 milliamps

This would run the leds at a slight amount over rated when the batteries are new, but you would probably get away with it.

So if you do want to try the LED you posted a link to, get two of them and put them in series and also use the 68 ohm resistor in series.


Or you could go to a larger value resistor.

 

[JimPghPA]

Heres a freebee you coud probably still try.

HarborFreight stores have been giving away a two pack of LED flashlight. If they are still giving them away all you have to do is walk in the door and clip the coupon from the store flyer and ask for the free LED flashlight.

The are powered by 3 AAA batteries. All you have to do is break one open and remove the circuit board holding the 9 small LEDs. If the entire little board fits use it complete with all 9 LEDs.
You could put the 68 ohm in series with the board conections and run it from the 4 AA.

If the bord does not fit you coud salvage some of the LEDs from it. You might try several of them in parallel, or you could even go for two strings of two in series, and put those two strings in parallel.

It is free, if they are still giving them away.

 

[Pop_Rivit]

There's always the option of buying her a new one for less than $8.

Hello Kitty Night Light

 

[weebl]

Originally Posted By: Pop_Rivit
There's always the option of buying her a new one for less than $8.

Hello Kitty Night Light


Very true.

Some other options I toyed with after seeing I may need to chase down parts that aren't worth the effort:

* Dollar store LED puck light, and swap out the internals
* LED tea light candle, but rig the switch to use the Hello Kitty's light switch instead (and the AA battery holder becomes unused)
* Rig it up to use an incandescent flash light bulb (rechargeable AAs power the light anyway)

Leaning towards the first two, but may end up doing the last. I honestly don't know where to get another one of those Hello Kitty lights locally - we picked it up on our travels last summer at The Forks in Winnipeg.