DIY Devil Horns mod - how to

[xtian]

I have completed my first Devil Horns mod, in preparation for my AC/DC tribute band's gig tonight (see bonnyscott.com). I am posting results herewith, in the event this information will be valuable to someone in the future.

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[selloutrr]

RAD!

[M Fowler]

You would feel right at home with all of us Vikings in Minnesota were a horny bunch.

Love the AC/DC horn amp rig.

Mark

[Cantplay]

Horns go on the side for Viking.

John

[surfsup]

Mod suggestion...make the horns into an amp handle...

[xtian]

Horns worked well last night, though I'm going to have to do something about the battery life. Got pretty dim by end of second set…

I have an Arduino board. Should be pretty easy to go from amp's effect loop out into the Arduino, and make the horns respond to the music.

[Structo]

Oh, I didn't realize that they light up.

Cool!

Yeah if you can make them pulsate to the music that would be cool too.

If you do that be sure to post some video of it.

[DonMoose]

Horns worked well last night, though I'm going to have to do something about the battery life. Got pretty dim by end of second set…

I have an Arduino board. Should be pretty easy to go from amp's effect loop out into the Arduino, and make the horns respond to the music.

Run 'em off the heater winding?

[xtian]

I have a circuit for powering my devil horns. The prototype works great, except that the LEDs are not getting bright enough when on. I'm getting a max of 1.3 volts, where I want three or four.

See circuit. Is the NPN transistor my limiting factor? It's spec'd for 20ma out, which should be enough juice.

[Cliff Schecht]

Grr... Just lost a post answering your questions. Essentially you are missing a resistor after the DC blocking cap and before the opamp "-" pin. Something like 10k-100k should work. That pot as it sits isn't going to do much. I think the reason you aren't seeing much voltage is because the op amp has no gain and doesn't develop enough voltage to turn on the BJT fully. Those mics only output a few hundred millivolts max which needs to be gained up before it can drive the BJT switch properly.

Also look at adding a 1-10uF cap from the "+" input to ground. When you kick on the LED the battery voltage will droop for a second and so will this reference voltage. A cap here will prevent the reference from jumping around as you switch the LED on and off.

Add a 1k resistor in series with the BJT base. This helps the switching action by developing only enough current to turn the base on and off. That op amp can push out much more current than the BJT needs to switch on and will push out as much as the base asks. If you are switching 20mA then you only need 200uA to turn on the BJT (assuming a beta of 100), so any extra current pushes the BJT further into saturation and will require longer to turn off. This, like the 10uF cap above, may be a bit nitpicky for such a simple circuit but if you are only going to go half way, why bother at all..

The collector voltage should be able to hit 6.5-7V depending on the turn-on voltage of the LED (usually 2.1V for your standard 5mm guys). The low voltage should be about 0.2V (VCEsat of the BJT). Again I think the reason you aren't getting as much light as you want is because you aren't giving enough output swing at the output of the op amp. Adding in the first resistor I mentioned should give you enough gain to turn that BJT all the way on.

[xtian]

Cliff, thanks a bunch for your notes. I started with a Jameco schematic and stripped components that I thought would throttle the current, but I think I went too far. Your comments would certainly have improved the circuit.

That pot as it sits isn't going to do much.

Actually, this was the most effective mod. The 1M pot was perfect for dialing in the sensitivity.

Anyway, I abandoned my v1 circuit when I discovered that my local Radio Shack has a kit for $8. I know! Quitter!

Anyway, see next post...

[xtian]

Devil Horns Mod finished!

I used a Velleman MK103RS kit, "Sound-to-Light Unit." Runs about $8 at local Radio Shack.

Pat-on-the-back-department: I secured the horns with magnets for easy packing in guitar bag.

See video!

[xtian]

BTW, here's the schematic:

http://www.vellemanusa.com/downloads/0/ ... 3_rev1.pdf

[spiralstairs]

thats awesome, thanks for sharing