[krough]

I have 2 props that run off of DC motors. One prop runs constantly and the other I would like to have activated by motion detection, they both run off the same ATX computer power supply. I know if it were AC I could hack a motion detector security light to do it. Since this is DC how can I do this, any ideas?

Krough

 

[ScareFX]

You can still use a motion detector lamp. Just add a relay interface to control the the DC circuit. Something like in the pic below. But you'll need a relay that has a coil rating of 125VAC and a contact rating that can handle your 12VDC. The one in the pic is has an AC coil and has AC rated contacts (but it would probably work fine for your DC circuit but I've not tried it). Wire the terminals into your DC circuit as a switch and you're done. BTW that's a Radio Shack relay in the pic. You can find others online much cheaper.

 

[Zombie-F]

The kit 30 is a DC motion sensor that runs off of 9 - 12 VDC, so it would be perfect for this application.

Here's a link to info on it:
http://www.kitsrus.com/kits.html#k30

I believe you can buy this at electronics 1 2 3.

http://www.electronics123.com/s.nl/sc.14/category.43/it.A/id.408/.f

Or also try this one too:

http://www.electronics123.com/s.nl/sc.14/category.50/it.A/id.448/.f