Category Archives: Electronics

Router Table safety interlock

Safety consciousness in the shop is of paramount importance – a moment’s inattention can cause lifelong damage, especially to your hands. In extreme cases, people die from shop accidents. I don’t think I know anyone who pays more attention to safety than my friend and fellow woodworker Derek Roff. Hands-free switching of the router in a router table means you never have to let go of the workpiece. A footswitch solves this problem, but introduces a new hazard; you could accidentally step on the switch and spin up the router. Several years ago Derek built an interlock using high voltage AC relays that solves this problem. In order to start the router you need to step on the footswitch and press a momentary pushbutton. The router stays on as long as the footswitch is depressed. Inspired by this device, I’ve designed a next generation interlock that takes advantage of cheap microcontrollers and low-cost relays to add features to the interlock while lowering the cost.

New safety features

Like Derek’s original interlock, you must step on the footswitch and press the start button to start the router. Unlike Derek’s, the new device enforces a strict ordering on the switch presses. You must first step on the footswitch, then press the start button to start the router.

The more important addition is a delayed start mode. If you first step on the footswitch, then double-click the start button, the router will start five seconds after the double-click. This gives you time to get both hands on the workpiece before the router spins up.

Convenience features

It is quite common to have some sort of dust collection, often a shop vac, attached to your router table. The interlock provides a second receptacle for powering your shop vac. In order to reduce the likelihood of popping a breaker, the interlock provides a 3 second delay before starting the vac. The vac stays on 3 seconds after the router stops (when you release the footswitch) to catch a few more chips.

The device provides separate jumper-selectable soft-start for the router and the shop vac, mainly to reduce the starting surge load on your breakers. When the soft-start is enabled, the router and/or shop vac are spun up to speed over a 2 second span. Because of how soft-start is implemented it will not work correctly with routers that have built-in soft-start. It also might not work correctly with variable speed routers, like the Porter Cable 7518 that I have in my router table. Soft-start is important on hand-held routers, but less so on table-mounted routers, so the loss of the ability is not a big deal.

The Build

The heart of the device is a custom printed circuit board (PCB) that I designed using KiCAD, a free, open-source electronic CAD program. The boards were fabricated by JLBPCB in China. The smarts are provided by an Arduino Nano, which was selected for its low price (<$2) and ease of use. The finished product, mounted on my router table, is at the top of the post. Here are the guts:

And here it is mounted in a box:

The front panel is 1/4″ acrylic, cut on my CNC router. The lettering is cut on the back while the cover paper is still on the acrylic. The routed lettering is sprayed with white paint. After the paint dries the paper is peeled off the back and the entire back (inside) is sprayed black.

It is beyond the scope of this blog to teach you how to build this. If you already know Arduino programming (or at least how to “burn” an Arduino) and can solder a fairly simple PCB, the files below contain the KiCAD schematic and board layout, the bill of materials, and the Arduino code.

Tip of the hat to David Strafford for tirelessly answering all my questions about the electronics I don’t understand.