I'm trying to have a LED strip light up when I play my guitar. I'm told I can use a Darlington transistor to switch on and off the LEDs. It would switch using the current from the output of my amp to the speakers. I can not figure out how to wire in the transistor. I've tried a couple of combinations, but the best result I got was very dim LEDs.

Here is some more info on the set up.

Peavey 6505+ guitar amp speaker output = 120W RMS/44V RMS, selectable 4, 8, or 16 ohm

TIP120 NPN transistor = V(CE): 2.0 V(BE): 2.8 I(C) Max: 8.0A Power Dissipation: 80W

Voodoo Lab power supply = 9, 12,or 18V

LED strip = 24" blue flashing LEDs

enter image description here

  • \$\begingroup\$ Rather than ask a new question that presumes a solution, it would have been better to wait for a good answer on your previous question. \$\endgroup\$
    – Phil Frost
    Jul 23, 2013 at 11:06

1 Answer 1


The problem is the voltage taken by the blue flashing LED. Depending upon the make this should be around 24V for a 24" strip.

enter image description here

The circuit shows a very simple sound operated transistor switch.

C1 ensures only AC is fed into the circuit. R1 and VR1 form a potential divider to reduce the size of the voltage to a suitable level. VR1 allows you to set a sound (output) level that will turn the transistor on.

The base emitter junctions of the transistor rectify the positive half cycle of the AC signal. D1 prevents the megative half cycle appearing across the base emitter junction and provides a current path for the capacitor.

For the transistor to be turned on the base needs 1.2 Volts. With the values given the minimum speaker voltage output needs to be around 2.4V. (VR1 = 10k) VOX switch (more gain + smoothing/delay)

enter image description here

Q1 (+ D1) rectifies the AC signal. This switches Q1 on and charges C2 to 12V providing a current to the base of Q2 to turn it on. C2 smooths any small drop outs in signal. When the signal drops it holds Q2 on for a little longer.

I've put in a 'test' circuit (in blue) so that you can make sure Q2 / LED strip is working. You don't need to add this bit if you don't want to.

Start with 12V to see if it works with this voltage.

  • \$\begingroup\$ The LED strip is 2 12" strips synced together. When powered on with just the power source(no switch, or manual switch), 9V, 12V, and 18V all work great(very bright).I didn't have access to a 24V power supply so I tried the 18V. I wired everything per your schematic and it does sound activate, but only with VR1 turned all the way clockwise. Even then it is very dim. Is there something I can replace to get more power running through it? Or do I just have to find a way to run 24V power? \$\endgroup\$
    – killwire
    Jul 24, 2013 at 0:00
  • \$\begingroup\$ With a little more trial and error, and error and swearing, I find that the TIP120 Will switch on the LEDs. Bright and responsive. With a 12V battery to base instead of the audio signal from my amp. I don't know what this means exactly. Hope it's not completely irrelevant. \$\endgroup\$
    – killwire
    Jul 24, 2013 at 3:23
  • \$\begingroup\$ @killwire that does tell me quite a bit. If you need to turn VR1 right up it means the signal your getting is quite small and needs amplifying. I hope that 12V went onto the end of the 10k resistor and not directly onto the base connection. I'll add a second circuit (VOX switch) you can try. \$\endgroup\$ Jul 24, 2013 at 8:44
  • \$\begingroup\$ Thank you so much. It works perfectly. The only thing I had trouble with is speaker to VR1. I ended up needing speaker(+) to VR1, and speaker(-) to ground. Also I couldn't quickly get my hands on a BC557 so I switched in a PN2907. R2 ended up being 330R. I didn't have 220R so I used the closest one I had. I could not be happier with the way this turned out. Thanks again for all the help. \$\endgroup\$
    – killwire
    Jul 25, 2013 at 11:45
  • \$\begingroup\$ @killwire Well done to you for persevering - the values and transistor aren't that critical (as you found out - lol). All it needs now is a base, middle and treble filter and you're well on your way to a 70's disco lighting setup. \$\endgroup\$ Jul 25, 2013 at 12:29

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