I also wanted to use my existing shop-vac, knowing from experience that the 5.0(peak) HP unit was more than adequate for the machines I have, as i have used it on each, manually changing the hose.
The last requirement was it had to turn on automatically, I was getting tired of "aw crap", forgot to start shop-vac first!
With all the above, the photos pretty much explain where I went from there. 1-1/2" PVC water pipe with 1-1/2" electrical conduit wall brackets made it easy to setup. I opted for ball valves instead of blast gates, mainly because it was going to take more work to match fittings for the blast gate. The standard 2-1/2" vacuum hose did not quite match up to the outside diamater of the PVC, but as always, the "handyman's secret weapon" duct tape, provided the solution. I wrapped duct tape on the PVC pipe end to the size where I could use a hair dryer to expand the vacuum hose and slide the hose over the duct tape. When the hose cooled, it made a perfect fit.
I decided early on to use the Dust Deputy cyclone and capture most of the dust and chips before they made it to the shop-vac so that I would not have to clean so often. The other driving factor was that this allowed me to put the shop-vac above the ceiling. More floor space and less noise. The unit mounted easily on the drum lid with six 1/4" metal screws(not included). I have to say, at $40 I was buying a bit of a pig-in-a-poke, but it truly works as advertized. I have half filled the 30 gallon drum with sawdust and have yet to empty the shop-vac or clean the filter.
I decided to anchor the PVC pipe going through the ceiling by using a floor flange for electrical conduit and drilling a clearance hole for the PVC, then gluing the PVC in place. I also used a pipe clamp above the ceiling on the PVC.
I looked at a few commercial current sensing products to automatically turn on the shop-vac when I started one of the machines, but the cost was about $50 per machine for the remote nodes, plus the controller itself. I, luckily, have all 5 of the machines I use the shop-vac on, on one circuit, which simplified the automation requirements as I just had to monitor one point.
I settled on a design that did not limit the current in case I did try to run multiple machines, using an Aprilaire 51 current sensing relay that is normally used for humidifier circuits triggered from the furnace fan motor current. It is only rated for 50 watts so I used it to drive a relay. When I first prototyped the circuit, the relay was energizing immediately. After a replacement current sensor gave the same result, I started experimenting with loading, and determined that although the specs said 50 watt max, it should have also said 10 watt minimum load. The particular relay I used only draws 10ma and measures 5k ohms resistive load, so I added a 2k ohm 20 watt wirewound resistor in parallel with the relay coil to increase the load and keep the relay from a false start.
The system has been running about a month, and works to my expectations, with more than enough vacuum and velocity. One gotcha in my system is my paper barrel. I closed all of the valves with the shop-vac on and it imploded. Nice ad for Shop-vac I suppose.
I'm not good at writing conclusions, or writing at all for that matter, so I end here I guess. I'd be happy to answer any questions.
Update - 10/30/2012
I was still having trouble after 3 months with occasional false triggers from my garage door opener. I modified the circuit by replacing the original relay with a single contact relay and replaced the 2000 ohm resistor with a 40 watt light bulb. It has ran well since then, with no false triggers, but I have procrastinated updating this site.
I also added a wall switch directly across the Aprilaire current sensor to aid in shop cleanup. The switch energizes the relay to start the vacuum directly, so I can connect a 20ft hose to clean the floor.
Update - 10/24/2014
I added a 25 foot hose to clean the shop floor. Looking around the one that worked the best was a swimming pool skimmer. I added another valve and elbow, and this hose fit the inside of the elbow with duct tape where the 2 1/2" went on the outside of the pipe.
Live right I guess. The hose reaches everywhere in the shop, even the cobwebs in the corners, and standard attachments all fit.
Update - 12/4/2014
Replaced the small dedicated vacuum system on my mitre saw with an extension ran across the ceiling. The $20 vac has never quite handled the air flow generated by the saw. I left the valve on the other side of the room, true it's not quite as handy, but can I reach it easy this way and this lets me put the flexible vacuum hose high on the wall for the swivel path of the saw. I also ran a dedicated outlet on the wall for the saw from the sensed circuit across the room for the other tools on the vacuum system. Again, since I only use it for one tool at a time, there is plenty of capacity of both the electrical and vacuum system.