So I recently purchased 2 3v coin cell batteries from sparkfun and I need to use one for helping a certain lady with a dress.
She wants to put some UV blacklight leds in the dress so i'm helping her. I'm going to wire them all up in parallel circuits and use smd resistors so that is smaller.
I have a question though.
What is the correct resistance for this?
The forward voltage is 3.2 - 3.8 V , think it would work with 1 3v coin cell?
So I recently purchased 2 3v coin cell batteries from sparkfun and I need to use one for helping a certain lady with a dress.
She wants to put some UV blacklight leds in the dress so i'm helping her. I'm going to wire them all up in parallel circuits and use smd resistors so that is smaller.
I have a question though.
What is the correct resistance for this?
The forward voltage is 3.2 - 3.8 V , think it would work with 1 3v coin cell?
If so what sort of resistor would I need?
Won't work, two 3 volt coin batteries wired in parallel is still just 3 volts and your led(s) requires a minimum of 3.2 volts before it's forward biased and turns on. If you wire the two coin batteries in series you gain 6vdc, which is enough to forward bias an led. But you haven't stated how much the recommended forward current is for those specific LEDs, so not enough information to calculate resistor size.
Lastly but most difficult is I suspect those small coin batteries cannot support the current requirement for even one LED let alone more then one. Those batteries are designed to flow only a few milliamps at most and will be discharged very very quickly trying to power LEDs.
3V is not high enough to turn on an LED with a 3.2 Vforward.
6V would be enough. In which case, assuming the LED needs 20mA for full brightness: (6V -3.2V)/.02 = 140 ohm or higher, say 150 as a standard value. For each LED.
A fairly large coin cell battery:
Lithium Battery CR2032 ?20mm 225mAh 3V
will not last very long, maybe 10 hours with just 1 LED.
5.5 with 2.
3.5 with 3.
Is the coin powering an uC as well?
funkyguy4000:
Yea, I can get the resistors and everything i'm just wondering how i should wire this whole thing up
Were are best at answering specific questions, can't help you too much with 'this whole thing' as it include information of what you want the arduino code to be able to do with the leds, how many leds, how long you want the thing to operate given a certain battery capacity, etc.
Next world hunger solved right here on the arduino forum.
CrossRoads:
You could also go with a 3.7V flat LiPo battery.
Get a 2000mAH battery, will run quite a while.
A single cell lipo would be pretty marginal working with 3.2 to 3.8 volt forward voltage drop leds as voltage drops pretty linearly with charge state on lipos starting at 4.2 and going down to 3.0, making sizing the resistor rather difficult. I would use a two series cell lipo at minimum.
funkyguy4000:
Oh okay!
What would happen if i used a 220 ohm. I don't have any 270's on me
Wait, you can calculate that yourself now that you have been shown the example calculations. You want to learn not just wait for answers don't you? Mr. Ohms law will thank you.
I'd go worst case: High voltage in LiPo, low Vf on the LED, test them all & match the resistors:
(4.2 - 3.2)/0.02 = 50 ohm for 20mA
(4.2-3.8)/0.02 = 20 ohm for 20mA
Another option: Add a boost regulator and kick the voltage up to a known steady level until the battery is drained.