Understanding and fixing an LED toy

Improvolone

I recently bought some LED balls to play with. They are cheap, and cheaply built. After playing with them for only a day or two, one of the three LEDs inside burned out. So I pulled the light system out, and this is what I'm looking at.

There is one resistor; red white black silver/29ohm.
It uses three 1.55v watch batteries.
I=160mA (I think)

The reason I don't watch to trash this is because the control board has some great functions built into it that I want to take advantage of. It will dim one LED at a time in order to create a slowly shifting color, it can freeze at any point on a color, and it can flash all three LEDs repeatedly for a neat seizure effect. Right now, a green LED lights up, a red LED lights up, and the third one is I assume blue, given the blues and violets the other balls shift through. I'm sure that with some careful soldering I could just remove the busted bulb and insert a new one, but I'm not sure how to go about buying a matching LED.
I'm tempted to scrap all three and just wire in my own red, green, and blue LEDs actually. Figuring hey, might as well experiment and make this thing as bright as I can. I'd rather not futz with the resistor for simplicities sake, but I might have to to get the most out of the four and a half volts I'm working with. The power source on the other hand is all I've really got.

First things first, I'm gonna need some really fine soldering tools for this job. I suppose I'll just check out ACE or a Home Depot for that since I don't know of any electronic hobby shops around me.
Next I'll need to buy some LEDs. I've been advised to Mouser, Superbrightleds, and a few others. I just simply have no idea what LEDs will work given these limitations. Will any LED dim?
Do all of my LEDs need to have the same forward current?

If you've got any advice for me dealing with this little project of mine, it would be absurdly helpful. I can try to get more pictures of the circuitry if need be.

Unknown

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Improvolone

So if my math is right, I have 160mA of current. I'm looking at three LEDs right now all with a forward current of 20mA. I have a feeling this will burn out the LEDs given the huge difference. Since the current doesn't change in the circuit, I need to find LEDs with a forward current of around 160mA?
That sounds... wrong (if not impossible).
What do my LEDs need to have in common in order to not be burned out?

Lodbrok

First, are you sure that the LED is burned out? This is unlikely I think, since there is no way three watch cells should be able to provide enough current to actually destroy the LED. Have you tried just changing the cells? The fact that the blue one is the one that has stopped working also indicates the cells, since blue LEDs needs a much higher voltage than the red/green ones. It is possible that there is enough juice left to light up the reg and green, but not the blue.

Second, normal high-intensity LEDs are rated for 20 mA. How are they wired in your circuit? If they can be individually modulated by the circuit, I guess that they are not in series. Are you sure that the resistor is part of the LED-circuit? When running LEDs from button cells, the internal resistance of the cells is often enough, especially since the voltage will sag from the load the LEDs put on the cells. Have you measured what the voltage is under load?

Anyway, as long as you keep the button cells as the power source, my gut feeling is that any blue LED should work without destroying anything. As for matching the output of the LEDs, there is no easy way of doing that. If the circuit uses PWM to control the brightness, as it most likely do, it probably uses different values for all the LEDs to achieve an even brightness,

Dunadan019

Lodbrok wrote: »

First, are you sure that the LED is burned out? This is unlikely I think, since there is no way three watch cells should be able to provide enough current to actually destroy the LED. Have you tried just changing the cells? The fact that the blue one is the one that has stopped working also indicates the cells, since blue LEDs needs a much higher voltage than the red/green ones. It is possible that there is enough juice left to light up the reg and green, but not the blue.

Second, normal high-intensity LEDs are rated for 20 mA. How are they wired in your circuit? If they can be individually modulated by the circuit, I guess that they are not in series. Are you sure that the resistor is part of the LED-circuit? When running LEDs from button cells, the internal resistance of the cells is often enough, especially since the voltage will sag from the load the LEDs put on the cells. Have you measured what the voltage is under load?

Anyway, as long as you keep the button cells as the power source, my gut feeling is that any blue LED should work without destroying anything. As for matching the output of the LEDs, there is no easy way of doing that. If the circuit uses PWM to control the brightness, as it most likely do, it probably uses different values for all the LEDs to achieve an even brightness,

thats my take on it too, check the batteries first. its easy and quick.

also, if this is such a cheap toy why not just buy another one? it sounds like you would have to go buy a soldering iron to replace any LEDs which probably would cost more than the toy.

Improvolone

Cheap being about $18 a pair. It's cheap compared to the alternatives. I know I'll have to buy equipment to do what I want and that it still might not work, but I figure this learning experience and the off chance that it will work is totally worth it. And hey, I'm sure a soldering iron and multimeter would come in handy sometime again in my life.
I changed out the batteries for three new ones and it's the same. Since these are poi balls that I'm also using as juggling equipment, my guess is that some of the impact caused the bulb or a connection to break.
I can't imagine the resistor is apart of anything but the LED circuit, but I suppose I'm not sure how to tell 100%. I have also not measured the voltage under load, I don't have a multimeter.

I wish I could get this circuit board off it's mount, doesn't look like an easy way to do that without cutting some soldered points.

DigDug2000

It seems really unlikely to me that you could actually "bust" the LED. More likely that a connection slipped loose somewhere.