What I am doing is using a car stereo amplifier, and using the output voltage from that amp (just putting wires in parallel with the wires to the sub) as the control voltage for a MOSFET. The MOSFET will then let the 12v LED string light up. It seems to be working incorrectly though. Is it possible to switch a DC source with an AC signal voltage from an amp?

Here is a photo and a schematic of my circuit. I first divide the output voltage from the amp, but I use a dual potentiometer to give me theoretically a wide range of input voltages that can cover a specific range of output voltage to the MOSFET. According to my calculations, the selected values should give me let me adjust input voltages from 8-30V to output voltages from 1.6-3.2V, which after testing is the approximate gate voltage range at which my MOSFET went from fully off to fully on. In this way, whatever volume the music is at, I should still be able to make the lights flash at the same brightness.

The problem is that the MOSFET seems to be turned on using only the negative speaker output from the amp, bypassing the voltage divider. This circuit will flash the lights to the beat of the music, and the pot will vary the brightness slightly, but if unplug the positive voltage from the amp, there is no effect on the circuit. It seems to be run entirely from the negative terminal on the amp, and I don't understand how/why this is happening. Can someone help me understand what is happening?

Note: If I remove the voltage divider, and set up the circuit as in "Circuit 2" the lights will not flash at all if both positive and negative wires are attached from the amp. However, if only negative is attached, it flashes with the beat fairly well.

 

Is it possible to switch a DC source with an AC signal voltage from an amp?
... if only negative is attached, it flashes with the beat fairly well.

Yes, and so what's the problem?

The MOSFET responds to the difference of the gate voltage over ground. Circuit 2 makes perfect sense to me and shows that the "-" speaker pole is spending some time at a voltage over ground. If the "+" pole has no impact, it must not be.

 

Well the problem is that I can't control the flashing. I want to be able to adjust the brightness and keep the lights at the same brightness regardless of what the source volume to the speakers (and coming from the amp) is. If the "+" pole has no impact then my voltage divider wont work correctly, and I can't adjust the brightness. Could I put a bigger pot in between the "-" pole and the gate, and just adjust the brightness that way?

So, if the system ground is 0 volts, then any voltage applied to the MOSFET that is positive and greater than the gate threshold voltage will trigger it? For some reason I thought that there had to be a completed and separate circuit using the gate.

 

So, if the system ground is 0 volts, then any voltage applied to the MOSFET that is positive and greater than the gate threshold voltage will trigger it?

Yes, although the voltage needs to get quite a lot higher to turn the MOSFET fully on. About 10V for normal MOSFETs and 5V for logic-level.

For some reason I thought that there had to be a completed and separate circuit using the gate.

That's true for a normal BJT but not a MOSFET. You can think of the gate as a capacitor to be charged. If it's charged up, the MOSFET is on. Discharge the gate and the MOSFET turns off. There is no current flow like on the base of a transistor, just enough to charge and discharge the tiny capacitance of the gate.

You may want to read Bill's articles here about making LEDs respond to music. Color organs is a keyword, I think.

 

Im using a 2n7000 MOSFET, which has a gate threshold voltage of .8-3 V, so low voltages do work for it.

I will definitely check out the color organ material. I researched them quite a bit about a year ago, but I need to get back into it. I don't remember much!

Thank you so much for all your help.

 

Im using a 2n7000 MOSFET, which has a gate threshold voltage of .8-3 V, so low voltages do work for it.

That's a regular MOSFET and needs to be driven to ≥10V to turn fully on. It will begin to conduct at the threshold voltage, but will have a significant resistance and will, depending on the current load, dissipate a lot of heat.

 

I will have to watch out for the heat. I was testing with only a short string of LEDs which wouldn't have drawn much current, but I did let it run for a considerable period of time and didn't notice it warming up.
I ran some test voltages to the gate and the lights were just visible at 1.6V and appeared to be fully on at 3.2V. If heat doesn't turn out to be an issue, are there any other issues to running it at that low signal voltage?

 

If heat doesn't turn out to be an issue, are there any other issues to running it at that low signal voltage?

Nope, not if it's working and not overheating.

It's easy to use a MOSFET as an on/off switch, but using it in the active region is trickier. You need to squeeze the effects you want into a certain voltage range. Sounds like you're getting close.