Basic LED Circuit

[nobertm]

Hi all - I need a ton of help - Im good at soldering but not so much with circuitry and know nothing about PLC,PIC,IC or any of that... but my circuit is fairly simple...

I need to arrange lights on a plywood board -

14 Blue 8mm LED
6 White 8MM LED
No sequencing, flashing or anything required... perhaps a switch...

needs to be battery powered and as long life as possible on as few batteries as possible

eventually Id love to add features like a scoreboard and such - but one thing at a time! lol

Thanks!

 

[Hero999]

Easy, just connect each LED in series with an appropriate resistor and connect them all to some batteries via a switch.

Use the following formula to calculate the required series resistor value:
[latex] R = \frac {V-Vf}{If}[/latex]

For example, the blue and which LEDs will have a typical forward voltage drop of 3.5V, a forward current of 10mA to both prolong the life of the batteries and the LEDs and four NiMH AA batteries in series, a total voltage of 4.8V.

V = 4.8V
Vf = 3.5V
If = 10mA = 0.01A

[latex] R = \frac{V-Vf}{If} = \frac{4.8-3.5}{0.01} = \frac{1.3}{0.01} =130 \Omega [/latex]

As the LEDs will be fine with a bit more than 10mA, you can easily use 120Ω, the nearest E12 standard value.

 

[nobertm]

Thanks for the reply, but im horrible at math and such... hense the reason Im here... any chance at even a crude diagram on that? the 14 White LEDs will outline the 2' x 4' board with the 6 Blue ones in the middle... If possible Id absolutely love to do 20 White on the outside and the 6 Blue in the middle, but I worry about power...

 

[nobertm]

I did this on a bread board and it worked... it just sucked a lot of juice... i had 27v on it at one point (series) then went paralled and cut it way down... unfortunately my design got wrecked and I dont remember where I left off....

 

[Hero999]

Where did you get 27V from?

20 LEDs at 10mA each should use a total of 200mA which should last for over 12 hours connected to four 2500 NiMH rechargeable batteries in series.

Here's a circuit, repeat the LED and resistor bit 20 times.

Some LED tutorials
Light Emitting Diodes (LEDs)
LED Circuitry tutorial: how to hook up LEDs.

An LED calculator.

 

[nobertm]

poor calculations i guess... lol - i really want to do this - but im really bad at the schematics of it... I have a breadboard and multisim - I cant seem to work this circuit out on either....

 

[philr]

Hi Nobertm

Have not been in learning environment for 2 decades (old git) so struggled to get my head around ohms law, it gets much easier if you can spend a little time on this, i found this link to a great web site that helped me get my head round series and paralel led connections and has a handy calculator program to help work out resistors etc, have a look it should appear to be much easier.

Light Emitting Diodes (LEDs)

Hope it helps


Phil

 

[Hero999]

I've already posted that tutorial.

nobertm,
I think you need to look up series and parallel circuits.

Here's a circuit showing two LEDs wired up to a battery, for 20, you just need to add 18 more resistors and LEDs wired up as shown below.
**broken link removed**

 

[nobertm]

Hero,

Thanks for the diagram - thats exactly what I needed - only issus is Im horrible at the math on these things... any help on the voltage/number of batteries and the size resistors to use? I was hoping to use a 9 volt batt due to it general size... also using white/blue... can they all be in the same parallel circuit with the same resistor size on each LED?

 

[Hero999]

You could use a 9V battery but I wouldn't recommend it because it won't last as long as four AA cells or even AAA cells. It's not just about voltage but the Ah rating of the battery (Google it if you don't know what Ah is) and a typical 9V battery has a lower Ah rating than an AAA cell.

The resistor value for 9V will be different than 6V, I posted a link to a calculator a couple of posts ago, I suggest you use it.

 

[nobertm]

Thank you for all this wonderful help - I still am struggling with the calculator - but Ill keep plugging away... 4 AA's it is then too!

Thanks again and any further help is greatly appreciated!

 

[nobertm]

Oh and a guy I know told me to ask about using a LM317T Voltage Regulator to prolong batt life... any wisdom in that or is he full of crap?

 

[Hero999]

I shouldn't make any difference to the battery life.

The LM317 just keeps the voltage constant so the LEDs don't dim as the battery discharges.

Unfortunately, the input voltage needs to be a couple of volts higher than the output in order for it to regulate properly so it isn't an option for four AA cells and the reduced minimum operating voltage could reduce the battery life of a 9V battery.

 

[audioguru]

If you make the LEDs dim because the current is low then the battery will last longer.

 

[nobertm]

Im thinking white and blue LEDs - Parallel - 4.8 V with a 150 Ohm Resister (all I have avalable)

 

[nobertm]

i dont need nearly that much resistance with 20+ LEDs do I? the calc seems to say less than 10 Ohms....

 

[audioguru]

Since every LED has a different forward voltage even if they have the same part number then in parallel, one might be very bright and burn out while the other one is dim or not lighted.

Flashlight manufacturers order LEDs with exactly the same forward voltage or they test them and sort them all themselves. Can you do it?

The battery will be 4.8V with only brand new alkaline cells for only a few seconds or minutes. If the LEDs have as forward voltage of 3.4V then with a 150 ohm current-limiting resistor the current will be (4.8v - 3.4V)/150 ohms= only 9.3mA.

 

[nobertm]

i dont know how to check forward Voltage so I guess id probably have to rely on them being close...? suppose I can use a single resistor per LED and use the 150... is 9.3 going to be that differnt than 10?

Showing my greenness here - I just dont know what to do, getting frustrated and just want to build what was supposed to be a simple circuit for a silly game.... ???? dont know where to go from here....

 

[bychon]

Checking the forward voltage is accomplished by connecting up one resistor, one LED, and one battery. When you get the resistor to supply the current you want, measure the voltage across the LED. That's the forward voltage under the condition of current that you are trying to end up with.

 

[bychon]

ps, 9.3 instead of 10 isn't much difference in this case.