Electrician/circuitry buffs of Edugeek, can I borrow your brains a second please?

[Garacesh]

TL;DR I know I have all the right components but I'm terrible with forward voltage and amperage and all that stuff 'cause I didn't pay attention in school (and it was also over a decade ago). Pls help.

 

Storytime.

 

I'm a nerd (shock, horror!) who bought a lightsaber a while back. Not one of those cheap ones from Asda/Smyths/Toys R Us etc. A proper metal one with a beefy battery and sound. However! I'm never satisfied. The soundboard with it 1) couldn't have the sounds changed, and B) blared out an advertisement for the company every time you switch it on, which just annoyed me.

So! I did what any self-respecting geek would do. Looked for ways around it. Turns out there's a huge market for custom lightsaber boards and stuff. Sweet!

 

I have:

• A Nano Biscotte V4 soundboard with the Power Extender
  
• A 3.7v, 2500mah 18650 Li-Ion battery with 3c constant current (whatever that means....)
  
• A Tri-Cree XP-E2 LED containing:
  
  • 1x White: Forward Voltage @1000mA = 3.15v 124lm
    
  • 1x Red: Forward Voltage @1000mA = 2.5v 84lm
    
  • 1x Blue: Forward Voltage @1000mA = 3.4v 638mW
    

  [*]A momentary switch, Purple LED Light 2A/36VDC with a 20ma variable resistor

  [*]A DC power socket

  [*]A 3-7.4v rumble motor

  [*]A 28mm 2w 8ohm speaker

  [*]Many, many different flavours of resistor, except they're all 5-band resistors labelled 22K, 2K2, 1M, 470K, 470, etcetera etcetera.

 

My main issue is working out what resistors I need to put on the LED's.

 

I know for a fact the red LED needs a resistor else it'll go pop. But I'm a bit iffy about forward voltage etc. Does (i.e. in the case of the White LED) Forward Voltage @1000mA = 3.15v mean that at a current of 1 Amp, the LED will draw 3.15v, and how do I ensure that the battery is supplying the 1Amp that I assume you're supposed to use?

 

I'm perfectly happy to bribe anybody who takes the time to help me out with beer money

Edited October 6, 2017 by Garacesh

[SLS_]

Can't help with most of this, but a quick Google says:

 

In describing batteries, discharge current is often expressed as a C-rate in order to normalize against battery capacity, which is often very different between batteries.

 

A C-rate is a measure of the rate at which a battery is discharged relative to its maximum capacity.

A 1C rate means that the discharge current will discharge the entire battery in 1 hour. For a battery with a capacity of 100 Amp-hrs, this equates to a discharge current of 100 Amps.

 

A 5C rate for this battery would be 500 Amps, and a C/2 rate would be 50 Amps.

 

So... For your 2500mah battery, I think 3C means you can draw up to 7500mah (which would exhaust the battery in 20mins if drawn constantly).

[Chris_Cook]

Page 11 of the manual for the nano board goes into some detail.

 

This website can do the calculations for the resistor. LED calculator for single LEDs

 

Increasing the number of ohms for the resistor will reduce the brightness of the led. Don't decrease below what is recommended, or you risk burning up the led. Also, you may need higher wattage resistors than normal.

 

Essentially, the maths is

 

R = (Vsupply – Vled) / LedCurrent

 

V is in volts, current is amps (1000 ma = 1 Amp),

 

So for the white one, 3.7 - 3.15 / 1 = 0.55 ohms, or 1 ohm rounded up to a common value. 2 x 1 ohm resistors in parallel would be slightly too low (0.50), but probably ok.

 

for wattage of the resistor:

 

P = (Vsupply – Vled) * LedCurrent

 

Again: (3.7 - 3.15) * 1 = 0.55, or 1 watt rounded up. Maybe 2 watt to be safe.

 

So 2 x 1 ohm, 1 watt resistors in parallel would work for white.

 

So if you put 3.15 volts across the led, it draws 1000ma. if you put less voltage across the led - I.E. a higher value resistor, it would draw less current. The resistor is there to put the correct voltage across the led, and so limit the current.

Edited October 5, 2017 by Chris_Cook

[Garacesh]

Okay, that helps me a little bit more, thanks guys!

 

20 minutes seems a little low, though. I wasn't expecting to get hours and hours out of it, but when you throw the speaker in the mix, I'll have hardly any power on that..

 

May need to invest in a beefier battery. I know they come in 3400mah flavours at least.. Problem is the most common suppliers of 18650's are vape shops and those apparently don't come with the short-circuit/overvolt protections that these do.. Hmmm.

[JordanT91]

Before vaping came along, you only needed 18650 batteries for LED torches. I'd buy one from a place that sells good torches over a vaping shop. https://www.taclight.co.uk/search?type=product&q=18650

[SLS_]

Speaker by itself would only draw 500mah, nowhere near the 7500mah required to drain the battery in 20mins.

[difinity]

Whilst you can drop that much voltage with a resistor(s) it's hugely inefficient and not really the way to do it and powering straight from the cell is a bad idea due to the changing voltage and current.

You need some sort of LED driver. Much more efficient and small resistors such as 0805 can be used.

[Garacesh]

Whilst you can drop that much voltage with a resistor(s) it's hugely inefficient and not really the way to do it and powering straight from the cell is a bad idea due to the changing voltage and current.

You need some sort of LED driver. Much more efficient and small resistors such as 0805 can be used.

 

That's what the NBv4/Pex board is for. The LED's aren't directly linked to the battery.