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Resistor for LEDs

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Bromosapien

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Hey guys,, i am new to the forum & fairly new to the whole electronics realm//

Heres my story.. I am a musician & play bass guitar.. I have two instruments that use an active tone preamp . One uses one 9v batt and the other uses 2 9v. I have seen other guitars with a LED light to indicate battery life.. so id like to install them on mine. plus i think they look cool

Soo.. I went to radio shack and bought 2 2.1v, 20ms green LEDs. unfortunately nobody there knew anything about LEDs or resistors.. :/ of course

my question is what kind of resistor would i need??
i used an OHM law page and came up with a 390ohm 1/4watt resistor for 1 9v battery, and a 819.99 1 watt resistor for 18v. is this right???
My other question is are the LED lights going to effect the output or drain the battery faster???

I am good with alot of electronic installs, as long as all the right parts are there. I know to wire LED & resistor in series at least :)

Thank you so much for any feedback..
 
Use a standard value 820 ohm resistor for 819.99 it's close enough :) 1 watt will run cool. Multiply the calculated wattage by 2 or 2.5 to derate the resistor to run cool.
1/4 watt is a little to close for the 390 ohm job. Make it 1/2 watt.

Any thing that draws current from the battery will lessen its life.

If these LED's are going across the power source they shouldn't affect the sound.
 
You can hook the lights to you batteries but it won't tell you anything about the state of the batteries. Run your LEDs at 10 ma and your batteries will last a bit longer and you'll still get your light. 18 - 2.1 = 15.9 V / .01a = 1.59K(1.6K) and 15.9 V X .01 A =.159 watts, 1/4 watt will be fine. 9 - 2.1 = 6.9 /.01 =690 (680) ohms & 6.9 v x .01a = .069 w 1/8 or 1/10 watt will do. Just my opinion.
To get it to tell you what your battery state is, you would need a comparator or opamp and some circuitry to cause the to turn off or on or change.
Kinarfi
 
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great thanks alot for the info man! i figured that they would drain a little power but thats the beauty of LEDs they dont use very much at all ( nothing that will be noticable )


And as far as going across the Pwr source,,, that just means bridging across the positive and negative wires yes??
 
yes, try using even more resistance and see if it keeps you happy, maybe 5 ma instead of 10, FYI the leads on an LED are usually different lengths, the longer one is usually +.
 
The LEDs simply glow and waste battery life. A little 9V battery has a very low amount of power and an LED will probably drain it more than the circuit does.

The LEDs will not indicate battery life and will still glow when the battery voltage is so low that the circuit does not work any more.
 
although i can understand your comment, id have to disagree with ya on that one.... Countless guitar builders intall LEDs to indicate battery life with no problem at all.. and also in the neck as fret markers.. Lots of people install these themselves, it has been done before. I just needed info on resistors.

If the cable is plugged in the light will come on,, if it is dim or doesnt come on time to change batteries. its not going to give you an exact readout but it will let you know if you have any power.
 
A little 9V PP3 battery runs down to only 6V or less fairly quickly if the current is fairly high.
LEDs discharge the battery faster.
The extremely simple LEDs circuit will not tell you if the battery is brand new or if it will fail in 10 minutes.
 
I just installed the LEDs in both instruments with no problems.... i went from the positive feed from the battery and the ground on the output jack.

the guitar with 18v had some old batteries in it so when i installed the light i noticed it was dim,, so i changed batteries and it was bright like the other one.. It works perfectly

All and all the system works exactly how i wanted it to.. :) thanks soo much you guys for the help, i dont think i would have been able to do it otherwise!!!
 
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