# LED Lights - Power Supply / Battery

#### THRobinson

Joined Jun 7, 2018
55
Hey guys... niece is wanting to build a shadow box with mini light sabers in it.

I got the STL files for Vader/Luke/Obi/Yoda's sabers and got acrylic rods we plan to "frost" so the light shows up. Just a small project.

I am gambling with AliExpress, fingers cross, not had much luck with them. Ordered from places with a 4 week ETA instead of an 8 week. Found 1 seller with pre-wired LEDs, and because of size, grabbed the 3mm ones (blue, red, green). Says 5-12v. I also bought a Type-C usb 5v1A charging module with safety for Lithium batteries. I also have a small switch, and a buck-converter that I can use from another order. Tonne of resistors, I think all 1/4w.

Plan is to make a box/frame for the wall with no wires that can be recharged.

I was looking at something like THIS BATTERY, 3.7v 1000mah, small and rechargeable, cheap and fast shipping. But, 3.7v not 5-12v.

Would I be better off with something bigger? or will the buck-converter set at 5-12v work or do I need this DC-DC BOOSTER? How long would 4 LED's last off that battery? Should I grab other lights? (was trying to find 3v ones pre-wired but didn't see any with 4 week ETA's cheap).

I "think" the plan is to wire the 4x3mm LEDS in parallel, run the wires to the booster, booster to the type-c charger, from the charger to the 3.7v 1000mah battery.

The switch I have on hand, I guess between lights and booster?

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#### WBahn

Joined Mar 31, 2012
27,391
I'm confused. You are saying that you want to make a box with no wires that can be recharged, but then want to use a rechargeable battery. How do you plan to recharge the battery?

How long do you want this thing to work for on a fully charged battery.

A buck converter converts from one DC voltage to a lower DC voltage. But you want to go from 3.7 V to at least 5 V. You need a boost converter.

How can we tell you how long four LEDs will last off that battery when you haven't told use either how much current each LED takes are how they are connected (series, parallel, some combination)?

Try to provide that information and we can see if we can move the needle.

#### jiggermole

Joined Jul 29, 2016
63
with it saying 5-12v they may have current limiting resistors already on the die with the led so you may have to look at that. Buck is step down, boost is step up. So I think you meant that one. Making you're own small leds with leads really isn't that bad. So it may be simpler to get like an led sampler from sparkfun and solder on you're limiting resistor right in line with the led. So then you're 3v from two AA batteries would be more than enough voltage to run the leds. Most little 3mm leds are 10-20 ma. And blue is going to be the highest forward voltage, around 3.1V so maybe 3AA batteries. Sparkfun has an LED sampler pack https://www.sparkfun.com/products/15418 . Probably quicker than 4weeks.
The aliexpress mass produced little boost converters are pretty ok in my experience, if you need one.
You need a charging circuit if you're going to work with charging that battery. unless you're going to bury the battery I wouldn't really use that type. I'd probably look at something like the rechargeable AA batteries. That would make you're life quite a bit simpler. https://www.digikey.com/en/products/detail/med/BH2A2S/16604331 would work very decent for this. Then you can just swap out the batteries as they die. If you're set on using a lipo than you'd probably want to look at something like https://www.sparkfun.com/products/15217, Its a usb-c lipo charger. It only charges the lipo.

As far as longevity. You can use watt hours for your battery capacity. Its rough. 3.7V lipo with a capacity of 500mAh is 3.7V x 0.5Ah = 1.85wh. An LED you've properly sized the current limiting resistor for will drop 3.7V and use 20mA (an assumption of forward current on most hobby LEDs) will be 3.7V x 0.02A is 0.074w. that 500mA pack will last very roughly 25hours. In this case I know you don't have to convert, but if you're converting to different voltages and stuff its good to get into the practice. A watt is a watt is a watt.

edit: I committed the sin of wrong your.

#### THRobinson

Joined Jun 7, 2018
55

#### THRobinson

Joined Jun 7, 2018
55
@Tonyr1084 Well, the box frame is almost like this frame. The white border has a 1/2" gap behind it before it hits the masonite board/backing. The middle part where the sabers go is 1/2" deep.

Plan is to grab foamcore with a black surface and replace the white border with it and lay a piece down in the middle as well. I'll add mounting blocks to the back side of the hilts to epoxy to the foamcore, and on the back of the foamcore cut v-grooves with a knife for the wiring to fit.

Then behind the border, in the gap I'll fit all the batteries and such. On the right side of the frame I was going to file out 2 holes, 1one for the USB-C port and one for the small on/off switch I salvaged from an old router years ago. Basically attach everything with hot glue to the masonite board so when the back is removed everything comes out with it.

So, with foamcore thickness and the small gap, AA batteries with a holder I don't think will fit. Coin cells definitely would, as would the Lithium battery I was linking to earlier. The small buck converters/step-ups/battery charger will have plenty of space.

Ideally though, I don't want the person to have to open the frame ever for swapping batteries. That's why I was looking at the Lith battery and USB charger option. But that all said, I don't understand a lot about electronics. Basic skills and I get ideas, but I need help with the parts because again, I don't have a bucket of parts to play with. That's why when I order stuff lately, I get extras to play with.

#### THRobinson

Joined Jun 7, 2018
55
Aliexpress still hadn't shipped my order after almost a week and said will ship within 1-7 more days, so cancelled.

Grabbed the ones in stock again at Amazon Canada, 5mm 3v-3.2v 30 degree angle, blah blah blah.

Will grab 2xAA rechargeable NiMH batteries and a holder. Plus my on/off switch. Gonna forget about chargers, lithium batteries, etc.

5mm might be too big but will try filing to fit them in. For wires to run through the hilts, my 18awg is too thick so grabbing an old Ethernet cable I was about to toss and will use the wiring from that, which is very thin.

Since using 2 x 1.5v batteries, wired in series I shouldn't need any resistors I assume or buck converters etc.

4 x LED lights though... In series correct?

Hopefully this setup is simple and bright enough? Should I go 3xAA and use the buck converter after all?

#### jiggermole

Joined Jul 29, 2016
63
in parallell. Two AA batteries at 3v total might have issue turning on an led with a forward voltage of 3.1v. I personally wouldn't trust the internal current limiting of the 2032 cells, but for testing purposes they're right. The internal resistance of the cell limits the output current so you don't blow up the led. The internal resistance of the AA batteries may be too high for the leds. However, I have opened up several cheapo work lights and thats how some of them have been. Alkaline batteries just wired directly to the leds. Its shady, but it does function and light up.
I've found those little blue and green indicator leds to be quite bright at 20ma.
I should have included this in my last post, sorry. There are a number of good calculators on line for choosing your resistor for an led. This is the one I use at digikey. https://www.digikey.com/en/resource...ors/conversion-calculator-led-series-resistor . I know the math is simple, I won't turn away a ready built calculator.
And my calculation in #3 was a rough estimate based on one single led at 20ma. Your mileage may vary. just as a guess with the nimh batteries, 1.5v apiece, 1600mAh capacity and 4 20mA leds in parallell. My guestimate is 20ish hours. the 2032 battery would last in the 6-8 hour range. I guessed at 500mAh capacity.
If you're using leds in the 20mA range than the 26 or 28 awg wire in ethernet is sufficient for current carrying capacity. The NFPA 79 goes all the way down to 30awg with a current carrying capacity of 500mA.
As far as you're power situation. NiMH is a solid choice. They're pretty robust. And

#### BobTPH

Joined Jun 5, 2013
5,748
NIMH batteries are 1.2V, so two are 2.4V , which will probably not light your 3V LEDs at all, or maybe light them very dimly.