I have a small mod idea but I don’t really know how to do it as I don’t know a lot about electrickery theory.
What I am thinking about is adding an LED in the pickquard of an on/off active bass I have which will only light up when the active circuit is turned on and the jack is connected.
This is a bass with a 9v battery.
The little bit I know about LEDs suggests it should be easy to add this to one of the existing battery wires but will require a resistor somewhere.
Can someone help with where this could sit and what value I could use.
This is hypothetical but I might do it because, er, reasons.
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So it’s a bass that has both active and passive capability? Wouldn’t the position of the switch which controls that already provide you with that info?
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Yes, it has a push pull on/off.
Not thinking about it to tell me whether it is off or on, I am thinking about it because I haven’t done it yet and it sounds cool even if useless 
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Seems like it would be totally doable. I am a reformed electrickeriologist, but haven’t looked at the guts of the active amps in a bass for a while to see how easy it would be. I suspect what you are suggesting is very easy. All you need to do is have the LED go from the + power side to the ground side in series with something around a 1k resistor for red or green LEDs. Play with resistor values to get the brightness you want and 1k might be pretty dim. You could go down to about 500 Ohms to get it brighter. If it’s a blue LED it might need as low as maybe 300 ohms. If I remember right, the jack completes the ground circuit as a switch, so you just need to be on the proper side of the jack. I could be glitching that memory though. And yes, it sounds cool. I’ve also seen where someone replaced the dots on the neck with LEDs and someone else who had figured out how to have LEDs next to each pickup that were proportional in brightness to how much emphasis each pickup had. But it will also trash your battery life. The LED will burn much more power than the audio circuits in there. If you want to get really sexy, maybe some UV leds with the DR coated Neons - those strings look amazing with UV light - shame I didn’t like how they sounded. Get ready with some nice rechargeable batteries. And don’t tell my wife about your idea. She loves putting LEDs on everything!
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It could be as simple as adding the pressure switch/contact sensor to the push/ pull knob. Just have to figure out the power source.
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Would need a schematic but I suspect you can just put it in series with whichever wire ends up going from the ring contact to the preamp. Assuming that the jack will likely be a TRS with the ring contact serving as an internal switch that actually grounds out against the sleeve when you plug in a TS cable
For the resistor, it’s there to limit the current through the diode. Its value would be based on the voltage across the diode divided by the desired current through the diode.
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Cheers,
Is there a way to find out the diode current- does it vary by type/colour etc?
I guess the voltage would be somewhere around 9V, if that is the battery I am using(?)
As noted above is this going to eat up my battery?
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The diode should be rated for current and its bias resistance.
The voltage will depend on what else is between the battery and the diode.
Yes this will drain battery faster than with just the preamp.
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if you don’t know the current, a basic value for a standard LED is around 15 to 20mA.
Let’s say you want 15mA with your 9v battery :
U = RI
R = U/I
R = 9/0.015
R = 600
So in this case you will use a 600Ω resistor. You can incrase a bit this value if you want your LED to be less bright.
Also there are ultra-low current LED’s (about 2mA) which could be a good choice if you don’t want to drain your battery too fast.
In this case :
R = 9/0.002
R = 4500
you’ll use a resistor around 4.5kΩ.
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Audere preamps do this. They have a blue light that comes on when you plug in the jack, then fades off after a few seconds so you know the battery is charged, and it doesn’t drain it. Just food for thought.
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Throwing my $.02 in here if the pull button turns on what you want to track then you should just wire it to that pot/switch. The entire guitar powers up when a cable is inserted and powers off when it is removed. If you have it on the switch then you are tracking when the switch is on rather than when the bass is plugged in.
You should be able to find LEDs with the correct resistors included on AZ. Then just make sure your polarity is right (one of the legs is longer).
Oh wow I guess I’m getting served up old articles. Sorry for digging this up! 
I’m glad you did, I missed some of the replies above.
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Here’s some ideas from chatGPT. It sounds plausible but just vet everything and test.
If you want the LED to turn on only when a push/pull pot is pulled out, you can use the switch built into the push/pull pot. Most push/pull pots have a DPDT (double-pole, double-throw) switch, which means it can control two separate circuits when toggled.
Wiring the LED to a Push/Pull Pot
Materials Needed:
Push/Pull Potentiometer (DPDT switch)
LED (any color, preferably low-power for battery efficiency)
Resistor (1kΩ to 4.7kΩ, depending on LED brightness preference)
Wiring
Soldering tools (optional but recommended)
Wiring Steps:
- Find the DPDT switch terminals on the push/pull pot. Usually, there are six lugs:
The middle two are the common terminals.
The top two are active when pulled out.
The bottom two are active when pushed in.
- Connect the LED circuit to the switch:
Solder one leg of the resistor to +9V (or +18V if using an 18V system).
Connect the other leg of the resistor to one of the top lugs on the switch.
Connect the LED’s positive leg (anode, longer leg) to the same lug as the resistor.
Connect the LED’s negative leg (cathode, shorter leg) to ground.
- Ensure the circuit completes only when pulled out:
When the switch is in the default (pushed-in) position, the circuit is open, and the LED stays off.
When you pull the pot out, the switch closes, completing the circuit, and the LED turns on.
Wiring Diagram (Simplified Text Version):
+9V (or +18V)
│
├──[1kΩ to 4.7kΩ Resistor]──┐
│ │
(Push/Pull Switch) LED (+)
│ │
└────────────────────── LED (-)
│
GND
Things to Consider:
LED Brightness & Battery Drain: If you want to conserve battery, use a higher-value resistor (e.g., 4.7kΩ to 10kΩ).
Placement: If you want the LED on the guitar body, you’ll need to drill a small hole and mount it with a bezel.
Alternate Uses: If this is for a pickup coil-split, mid boost, or kill switch mod, the LED can serve as a visual indicator for when that function is active.
Pretty sure you want the resistor in series with the LED, not like that diagram. Unless I am misunderstanding what it is trying to draw there. The text sounds incorrect too, the LED will always be on regardless of switch position.
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Yea that seemed a little weird to me too. Maybe the resiator from the hot pin of the upper pot to the resistor, to the LED, to the middle leg? But you need power to that pot so maybe that’s what it meant?