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Need help designing an LED circuit for a fancy hoola hoop?

My girlfriend has asked me to help her make a blinged up hoola hoop for a class she is taking.  I have some clear tubing to make the hoop, a small holder for 2AA batteries that fit inside the tube and have purchased 20 LED lights to go inside.  My question is, what is the best way to wire these LEDs to make use of the 3v from the batteries.  The stats on the LEDs are as follows: (Max forward current 20mA continious, 5VDC Max reverse Voltage. 4VDC Forward voltage drop)

Thanks for any help or ideas!

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The 4 VDC forward voltage drop will make it impossible to power the LEDs directly from the two AA batteries, unless you had something like a boost power supply. That would be unnecssarily complicated for a hula hoop.

You will need some kind of battery (or pack of batteries) with a voltage at least somewhat greater than the forward voltage drop. I know I have seen small batteries that are somewhat shorter and fatter than a AA that have a voltage of 12 volts, used in some cameras.

The wiring will depend on the voltage of the battery or batter pack you end up with, as the sum of the forward voltage drops of the LEDs in a series string must be less than the battery voltage. The battery voltage will therefore determine how many LEDs can be in a series string and how many of those strings must be in parallel to properly use the 20 LEDs you have.
chase32 (author)  LargeMouthBass5 years ago
Thanks. I guess I should have researched a bit before buying the parts. I already purchased a little AA charger that takes 4 batteries. I wonder if I put in two more AA batteries on the opposite side of the hoop it would work out. Would give me 6v and also help counter balance the weight so it might spin a little better.

Would 6V do the trick?
iceng5 years ago
Here is a pointer to DIY hoola hoop using lithium battery.

Have you thought of stringing two or three colors of EL rope wire lights into your hoops ?

A
frollard5 years ago
Chase: Per your reply to LargeMouthBass:

Yes, in fact, it's BEST to have 3 batteries in series to make 4.5 volts (to easily handle your 4 volt drop), but 4 battieries in series would give a good balanced number, unless you spaced the 3 batteries in 3 single holdes 120 degrees apart around the hoop).

Each led drops 4 volts, and you'll have either 4.5 or 6 volt supply. (3 or 4 batteries)

That means you can only have 20 parallel strands of 1 led with a resistor on each.

...easy enough!

Calculating the resistor, I'd suggest using the led wizard (google it)
20 leds
4.5 v or 6v supply
20ma
4v led.
chase32 (author)  frollard5 years ago
Awesome, thank you so much!