Sound Card DAC - Ideas for safety?

My problem: Hotplugging the soundcard pushes all channels to max output
The goal: To run a safe show

I have tried so far:
a) Relay linked to +12v and the usb 5v line ---> sucks power from USB making card useless
b) Solenoid to +5v usb ---> too delayed
c) Using usb 5v to power the correction amp chips --> they react funny and won't give me over +1v
d) Relay to the power supply 'swich' - too delayed

I am using http://laserpointerforums.com/attachments/f47/29050d1282955974-sound-card-dac-tutorial-dac-laser-driver.jpg driver. I have seen talk of an inverting amp of some sort? Would this be easy to add on with my current board?

Any ideas? I feel I may be missing something obvious!

Thanks!

You can very easily use an unused half opamp to make an inverter like this:

http://www.radio-electronics.com/info/circuits/opamp_inverting/op-amp_basic_inv.gif

This will invert the signal, but probably not solve the problem: Now, if the output of the soundcard goes low, the laser will fire at full power. This is okay if you have a show running with blanking inverted, but if the lasershow software crashed, it would set it to full on instead of to off.

I think we should be able to come up with some safety mechanism that works better, but when does it go to maximum:

- when you connect the usb cable
- when you disconnect the usb cable
- or both?

My signal is NOT selected to be inverted in LFI so when I unplug the card, everything pops to max.

As far as I am aware the two options each have their drawback:
Non-inverted: Safe as long as the usb card stays connected
Inverted: No sofware crash protection, but safe against mechanical disconnection

Tuning is simple, so whichever of the above would be easier to add protection to.

Thanks for the response ben

What you would need to deal with either issue is a circuit that:

- delays powering up the lasers after connecting the usb cable
- immediately powers down the lasers when its disconnected

Doing that also ensures the laser powers down if the soundcard output goes to zero in case the projection program or soundcard driver crashes.

The first part isn't that hard to build - it could be done with a relay, or solid state switch, as long as there is some delay between sensing the 5v on usb, and enabling power. This is fairly simple circuitry to build.

The second part might be more difficult to realize: I think there is a small buffer capacitor on the soundcard board in the +5 power supply line, that could retain supply voltage briefly after yanking the cable out. You would need to reduce that capacitor to a point where shutdown is achieved before MPE is exceeded, but the soundcard keeps working as it should.

Alternatively, you could put a skottky diode between the usb supply connection and the soundcard circuitry, hoping that it will not drop so much voltage the soundcard stops working properly.

If your USB card is powered by the computer itself, you could use the power lines from the USB connector (the +5V/GND) and use that as an interlock powering a transistor. Use the transistor to switch the relay, not the direct signal.