Adding an LED "Preamp on" light

[line6man]

I believe different color LEDs draw different amounts of current, so it might be worthwhile to research which color draws the least. Then, you can wire a pot between the LED and the battery, slowly adjust the pot until you get the brightness you want, then carefully remove and measure the pot to get the resistance you want.

Not necessarily the output wavelengths, but the output intensity. Many LEDs simply have a diffused lens which colors the light. Clear white and blue LEDs tend to draw the most current, because those colors often come in superbright varieties. Your standard T1 or T1 3/4 package diffused-lens red, green or amber LED from RadioShack draws 10mA at a forward operating voltage which is usually around 3V. Though voltages range from as low as 2V to as high as 3.6V, depending on the LED. The superbright whites and blues (some reds) typically run higher currents, like 20mA, on the high side of the forward operating voltage range.

Personally, I'd be inclined to choose an LED with a lower output intensity, narrow viewing angle, and diffused-lens, as opposed to ending up with a flashlight on the control plate.
Get whatever color strikes your fancy. Or perhaps a bicolor LED with DIP switches inside the control cavity to select the color.

 

[rojo412]

The more I read, the less I feel inclined to put one in. I don't want to lose battery life. I had a Hohner with one and it was just wired in simply to the preamp. Didn't have it long enough to find out how it effected the life of the battery.

But this is all good info. I appreciate the help.

 

[FunkMetalBass]

I had a circuit that flashed the LED to conserve battery life in my bass. The problem with that was that it produced a clock noise that sounded like a clock, so you had to be careful with wire routing and shielding.

I think the idea to flash to show battery life is a better idea. But I think the OP wants an indicator light.

Another way to do that is have the LED powered by a separate battery, like a AA cell or something.

Since most LEDs have a forward voltage up above 2 V, Wouldn't you need a voltage doubler of some sort in order to get the LED to any appreciable brightness?

 

[bassbenj]

Preamps can draw as low as half a mA. To make even an efficient LED reasonably bright you need one or two mA so you will lose considerable battery life if you leave the LED on all the time. You would need a flasher that kept the LED on at a 10% duty cycle or less to get the LED current drain down to a reasonable level at a reasonable brightness. I made a one flash at power on battery level checker for my bass (probably like the one in the link above) and that can be designed to have minimal effect on battery life since the LED is only on for a few hundred ms when the amp cord is first plugged in. You probably want this LED to remind you to turn the bass off so you will have to flash it.

Ken

I've got LEDs in some of my basses and they look really cool! There are some green ones at radio shack that are quite bright without too much current. But mine is a special case because I don't use batteries in most of the basses, but run them off a power supply in the amp that goes through a TRS cable. The light tells me that power is on and working correctly. But they are wired so that when I switch back to battery the pilot light quits. (batteries are just for emergencies)

One trick that might be of some use is that there are some small DC-DC converter modules that Mouser sells. They are quite efficient and convert 9 volts down to values under a volt and that will buy you some power savings as opposed to wasting power in a resistor. I haven't tried one yet (I plan to use one to power fret-marker LEDs from my Phantom power). But they look promising.

 

[ClassicAmps]

...Another way to do that is have the LED powered by a separate battery, like a AA cell or something.

(in the voice of Maxwell Smart)
"Ahh yes, the 'ol 2nd battery in the grassy knoll trick"

If you need constant power indication (and sick of changing batteries) consider doing a phantom style instrument cable. Putting your power supply in a floor (stomp) box, use mic cable (instead of guitar cable), put TRS type plugs on it, wire the 2nd conductor in the mic cable to your "R" connection and to the power (+), then replace the instuments input jack with a stereo 12A type, add David's push/pull bypass and your golden. No more batteries baby!

I put a piece of red (or other notable) colored heat-shrink on the lead sticking out past the barrels of the plugs so I can visualy note (from either end) it's the "phantom" cable. Note, you don't want to plug a "phantom" cable into a standard effect unit that grounds the (R) conection to complete the power circuit, but it's safe to go there after your floor box because the output of your phantom floor-box doesn't connect that 2nd wire (R) to it's output jack.

BTW, for those who aren't allergic to Windows (OS) I have a little helpfile on my website that lets you quickly calculate resistor values for the power LED scenario (and choose your current draw) it's here:

[Invalid or Expired Link Removed]
(right-click this link, and "save target as")

 

[khutch]

There are some green ones at radio shack that are quite bright without too much current.

Green LEDs have an advantage in that the human visual response peaks at the green wavelengths so a green LED that generates relatively few Watts of light output can still generate a respectable amount of Lumens. Even so red LEDs have historically been the brightest because the semiconductor chemistry they are based on is just that much more efficient. I think this is no longer true or not as true with the latest LED chemistrys.

All the common LEDs except white actually do generate light at the wavelength corresponding to the color they emit. The colored packages modify the LED color only slightly, if at all. I think that they exist mostly so that you can identify the color of the LED in the off condition which is important in some safety applications and very helpful if you are in the habit of keeping all your LEDs jumbled together. Many LEDs come in clear packages. White LEDs are hybrids. The actual LED chip is a blue emitter that is coated with a phosphor that converts some of the blue light to other colors and gives an overall white color.

If you want to save current in a bass pilot light you should look at the light output specs/curves versus current. The LEDs with the highest output will have the lowest current at any given brightness level since you just back the current off (ie use a larger resistor) to turn the brightness down. LED current ratings are maximum ratings, you do not have to run them at their full rated currents and you had best not if you hope to get any life out of your battery.

I guess the OP wanted a pilot light to tell him his bass was on. I use the amp cord plugged into the output jack of the bass for that purpose! My battery monitor just flashes the LED briefly when I first plug the amp cord in. The LED going on tells me the cord made contact correctly, the LED going off tells me the battery is in good condition. If the battery is low the LED will go on and stay on. I've still got 10 or 20 hours of use at that point so no need to panic, just replace it as soon as it is convenient. This way the LED current drain has no significant effect on the battery until it is already used up and I can't be caught unawares by a dead battery. Personally I think it is the better way to go but it would be possible to guild the gold by designing a more complex circuit that would flash the LED briefly every few seconds to give you a pilot light. The flashing LED would mystify/hypnotize a few members of the audience, I suppose....

Ken