To sense current and turn the vacuum on I used a variation of the circuit that I developed for miter saw dust collection (find a discussion of that circuit here).
In this case, though, I need two current sense circuits: one to switch on the shopvac and the second to activate the solenoid on the vacuum valve when required. Here is a schematic:
Note the way the current transformers are wired. The line feeding the table saw only goes through the current sense transformer that activates the shopvac. The line that feeds the router table goes through both current sense transformers to activate the shopvac and the solenoid on the vacuum valve.
When connecting the vacuum lines to the valve, I must make sure to connect the table saw to the default (solenoid unactuated) port and connect the router table to the port active when the solenoid is actuated.
Before I could wire the current transformers, I had to know the minimum amount of current each tool drew. I used a clamp-on ammeter to measure the table saw at 8.8A no load and the router at 3A no load.
The current transformers have 1000:1 ratio – for every amp through one turn on the primary, you get 1 milliamp on the secondary. The secondary feeds the Solid State Relay (SSR) and it needs 10 milliamps to activate. The way to increase current in the secondary is to wind more turns in the primary. To get enough current to guarantee to turn on the SSR’s I need to use 2 turns on the table saw circuit and 4 turns on the router table circuit.
I rescued an electrical box from the scrap bin at work. Design changes had rendered this box obsolete and they were just going to throw it away. It has a few holes I will have to plug but it is the perfect size for this. And it has a clear plastic top so I can see the indicators on the SSR’s.
The power input will come in through the middle wire clamp on the bottom. Wire clamps on either side of that will go to 2 outlet boxes: one to provide power to the table and router table, the second to provide switched power to the shopvac and unswitched power for other things. The black cable clamp on the side will go to the solenoid on the vacuum valve.
-- "Find out what you cannot do and then go do it!"