Hey everyone,

So I am looking to create a battery powered LED sign using approximately 30-50 LED's arranged in a pattern. The ratings on the LED's are 2V at 20mA. The battery power source I would like to use is either a 9V or one of those big 6V.

My question is what configuration would be best for wiring these together (series, parallel or both) and at what positions do the resistors need to be integrated into the circuit? It would be helpful to learn how to calculate the correct values for the resistors needed for the configuration as well.

I have also been looking for a way to find out how long the battery would last for such setup but have not been able to find out.

Any help would be great,
Thanks,
M

 

You had better like math. You have some to do.
Look here then ask again for specifics.
http://forum.allaboutcircuits.com/showthread.php?t=19075

 

I do like math as a matter of fact so I understand the equations. I found this diagram on the forums and I wanted to do something similar with a switch and 10 parallel strands of 3 LED's. In order to find out the amount of current/voltage needed for that type of diagram, how do I solve such problem? And from that what is the procedure for determining how long my power source can last.

-M

 

On your drawing the V drops for LEDs is shown = 7.5V, subtract from batterys 9V = 1.5v, with known value of R1, 75 Ω, calculate I, V/R= 20 mA. Two strings = 40 mA. Assume battery has 700 mA hr. then 700/40 = 17.5 hr.

 

Note of caution: Going over the current rating will make the LEDs die sooner but going under the current rating will not dim them very much. 17ma instead of 20ma will be almost undetectable to the eye and the LEDs will last longer. If you can't get the current just right with standard resistor values, choose the next higher resistance.

 

How long do you need for this sign to be illuminated between battery replacements?

Batteries are rated for current draw over a 20-hour period. A standard alkaline 9v battery is rated for 500mAh, which is 25mA * 20 hours = 500mAh until the battery is considered completely discharged. That would run about 4 of your LEDs for 20 hours, and they would be quite dim at the end.

Here's a Rayovac datasheet:
http://www.rayovac.com/Technical OEM/~/media/Rayovac/Files/Product Guides/pg_battery.ashx
On the 4th page, there is a chart showing battery life in hours vs current for 4 sizes of their alkaline batteries; 813 = D, 814 = C, 815 = AA, 824 = AAA. Note that the D cells last the longest.

You mentioned a 6-volt battery; I'll recommend that you use a battery holder instead, as those 6-volt cells are expensive and not all that common nowadays.