How to power 10 SMD LEDs with CR2032 or a Small Lipo Battery - Page 1
Forum section: electronics / beginners

author: juanfermed
author_rank: Regular Contributor
timestamp: March 17, 2012, 06:22:51 am
content: Hi everyone!I am new to this site, not to Dave eevblog videos, but this is my first topic here. I had an idea of a project I wanted to make, but I am having problems on how to do it. I want to light around 10-15 smd LEDs with the smallest batteries possible, I think CR2O32 are just the size I would like to use. I wanted to make, for example, a Happy Facewith SMD LEDs like this onehttp://www.play.com/Toys/Toys/-/3511/2778/-/24261304/SMD-LED-Happy-Face-Kit/Product.htmlbut good looking, with more LEDs and without cables coming out of it, make a little PCB and give it to somebody as a little present. That is why I want to use small batteries. The thing is that I really dont know how I shoud connect the SMD LEDs and how to design a circuit that can light up this number of LEDs from one or two (or three in a stack) CR2032 batteries .I was also thinking that I could use a small microcontroller to light up the LEDs in patterns, or maybe a simple decade counter to light them in a loop. Anyway, I really have no idea of how to manage the voltage source, that is, the CR2032 batteries to power all that up, as each SMD LEDs requieres a voltage drop of about 1.5v and need about 15mA to light on. With the knowledge I have, I know that a CR2032 could drive 2 SMD LEDs @ 15mA for like 14.6 hours continuosly (it has a charge of 220mAh, acording to thishttp://panasonic.com/industrial/includes/pdf/Panasonic_Lithium_CR2032_CR2320.pdf). But I would like to connect 10-15 SMD LEDs and power them with one or two CR2032 batteries.I have investigated and also in some of the videos Dave has made, I have heard about boost regulators that can boost the voltage of the source they are connected to, and I guess that something like that is what I have to do, but I really dont know anything about them or if they are a solution to my design. I have no idea of what to do.Maybe somebody can suggest me how to connect the LEDs to the CR2032 batteries, how to arrange them, and how to add power also a little microcontroller from the batteries, like the PIC12F629 - 6 I/O pins- or a 12F1822 -11 I/O pins- both of which can work at 3.3v ( 1.8 to 3.6 according to datasheet) or not necessarily a microcontroller, but an IC that could make the lights lit in a pattern, the MCU would just give more flexibility.I was also considering to use a Polymer Lithium Ion which I could buy from Sparkfunhttp://www.sparkfun.com/products/10718which is 400mAh, or one from Mouserhttp://www.mouser.com/ProductDetail/Olimex-Ltd/BATTERY-LIPO-1400mA/?qs=sGAEpiMZZMtE4ePzUE8d2JwjvPtc1tM%252bwhich seems to have 1400mAh. But this would add too much cost to the project, but it would fullfill the requirements of the project in space and power, and also it is rechargable. Anyway I really dont think that using a Polymer Lithium Ion is the best solution and I am considering it because I have no idea on how to use the CR2032 and a driving circuit.I have seen some boards that, my God, light up a complete 10x5 matrix of SMD LEDs from one only rechargable coin cell battery, and that is what I want to do, maybe not to handle that number of LEDs, but to know how to do it, and do it with 10-15 LEDs.Any suggestions or questions are welcome. I appreciate your help. Thanks!JuanferI bought these SMD LEDs:http://www.mouser.com/Search/ProductDetail.aspx?R=LTST-C230EKTvirtualkey57820000virtualkey859-LTST-C230EKT,no datasheet there..

author: IanB
author_rank: Super Contributor
timestamp: March 17, 2012, 06:50:49 am
content: Some things to consider are that LEDs will still appear quite bright with a lower current than 15 mA, and that you don't need to light up the LEDs continuously. If you pulse them on and off very fast you can rely on persistence of vision to make them appear permanently on. Also, you may get a higher apparent brightness using short pulses of higher current rather than a continuous lower current.If you drive the LEDs from a micro, you can multiplex them so that only one LED is switched on at a time. You switch each of the LEDs on in sequence one after the other and do it so fast that all the LEDs appear to be lit up at once. This will help to preserve battery power.Probably the simplest thing for power is just to use a current limiting resistor with a 3 V supply to the LEDs rather than using complex boost circuitry.

author: TerminalJack505
author_rank: Super Contributor
timestamp: March 17, 2012, 07:06:25 am
content: QuoteHere's the datasheet for your LEDs:http://media.digikey.com/pdf/Data%20Sheets/Lite-On%20PDFs/LTST-C230EKT.pdfIn addition to what Ian said, I would add that using more efficient LEDs will buy you more battery life as well. Your LEDs are 6mcd. More efficient LEDs will require less current for the same intensity.

author: Psi
author_rank: Super Contributor
timestamp: March 17, 2012, 07:30:26 am
content: yes, as IanB said, you don't need 15mA. Or even anywhere near it.Infact, a CR2032 can't even supply more than ~5mA, even if you short it out.However they are a good choice for led toys, they're really cheap (~24cents each from digikey in lots of 10, 19c in lots of 100)And even at 4mA discharge they last 2+ days before the leds gets too dim.One of my recent projects involved ID tags for a computer gaming event.The simple version was just a CR2032 plus two leds and two 350R resistors which ended up at around 2.5mA per led (LED @ around 1.9V and battery volts fell to 2.8V under the load). The leds pointed inwards and illuminated the silkscreen text on the PCB.(We also had an advanced version which used four 5x7 LED array displays, a microcontroller and two AAAs.)The CR2032 was plenty bright enough to illuminate the silkscreen text indoorsHere's a group pic with people wearing them. 2.5mA is enough for a small noticeable glow in normal indoor lighting.http://psi.abcom.co.nz/tagsgroup.jpgAnd with the more advanced tag version i found the LED matrix displays only required 0.6mA to become visible indoors.(that's 0.6mA DC, i used more current due to multiplexing).An important thing to keep in mind with leds is that there's quite a big difference in efficiency between different types and colors.For a battery voltage below 3V the most efficient i found was AlGaInP (on GaAs substrate) which are in the red to orange color range.Because I never looked at leds with a forward voltage about 3V i'm not sure how white/green leds compare to that.Here's the LED i usedhttp://www.kingbrightusa.com/images/catalog/SPEC/WP710A10SET.pdfAnd here's a good online calculator to get the actual light output in lumens from an led mcd value and viewing anglehttp://led.linear1.org/lumen.wizAnd here's a pic of the two tags types close upNOTE: The second CR2032 holder was never used. it was only added to help balance the weight (the holders were cheap and i had lots)