Audio Mosfet

Use audio signal to switch a laser?

I would like to use an audio signal to switch on or off a laser (or led); basically make it flash with the beat. I would like to use the sub-woofer signal but I'm open to suggestion. The problem is the laser can not receive more than 3 volts or it will damage it. Right now I will continue using batteries to power the laser but I need a way to switch it that will not add any voltage/current.

Thoughts so far:
arduino (I don't think this will work as there will be too much of a delay)
relay (I would like to keep it small)
MOSFET (I'm not sure what I need)

Any help is appreciated!

Considering an LED first, the arrangement could be similar to that for operating an LED from a battery. If the sub-woofer has 100W with 8 ohm speaker, the voltage V = sqare_root(100W x 8ohms) = 28.2V (RMS). The peak is something like 1.5 times that, considering we ae not too sure about the waveforms, so maybe 43V.. Now the resistor for 20mA current with 43V. We can ignore the voltage across the LED for this situation. R = V/I = 43V / 0.02A = 2150 ohms. The nearest standard value is 2200 ohms. The maximum power in the resistor is V^2/R = 43V x 43V / 2200 = 0.8W. A 1W resistor will suit, considering the signal is varying. One point, the LED cannot have more than about 5V in the reverse direction, so there should be two LEDs in parallel with each other, one reversed, with a common series resistor. Use the same method to recalculate for other sub woofer amplifier powers and speaker impedance.

When it comes to a laser diode the story is different. The signal needs to modulate the laser. This is more complicated, and I suspect laser pointers are not very suitable for that. Generally laser diodes are prone to going off the rails from over-current or over voltage, even for a microsecond. Even the inductance of the connecting wires can cause a spike that destroys them. There are special modulator circuits that control voltage and current, possibly regulating them and controlling the optical output, and accounting for temperature. Probably laser pointers are not going to be reliable in this situation, though laser modules similar to laser pointers will suit. You will need to develop an on/off voltage signal to drive the modulator. Perhaps you can use an op-amp as a voltage comparator to get on off pulses from audio signals, or go a step further with a PWM signal generated from the audio..Look up PWM and comparators using a 555 timer too. The link below has all you ever wanted to know about laser diodes. Maybe more even.

Can"t find what you are looking for? Check out these products from Amazon.