How to Make Interlock Safety Switches for K40 Laser Cutter

IMPORTANT EDIT! Please don’t wire your interlocks to the mains of the machine. Instead wire to the PG pins on the PSU. Will do a full update shortly.
-Tony 7/30-19

What is one of the first pieces of advice on the internet for when your brand new, (maybe) shiny K40 laser cutter shows up? Install interlock switches!

What is an interlock switch? Basically just a switch (in our case, a microswitch), that is mounted near the lids of the machine. If the machine is running and you lift up any of the lids, the switch will disengage and cut power to the machine. The functional concept is extremely similar to an emergency stop button, except you don't, need to press anything.

So as I patiently researched which machine I would buy, I also was building up a list of upgrades that were going to happened before the laser ever turned on. Safety first! The main problem I discovered about installing interlock switches is that there isn't too many good guides that are (1) simple; (2) written for non-electrical engineers; (3) have good documentation.

So this guide is for all the people out there that want a 100% effective, quick, and safe way to wire up interlock switches for your K40 laser.

Required Parts:

• (3) End-Stop Style Microswitches — Commonly found on 3D printers, especially Prusa kits. You can in theory just use plain jane microswitches, but I decided to spend the 5-6 bucks to get these that are mounted on the boards and come wired ready to go. Also, Don from Don's Laser Cutter Things, has a schematic for an end-stop switch that has some more components on it. Honestly, not sure why, but I'm rolling with Don.
• Small, Blank PCB — I happened to have one of these Adafruit ones laying around that I used
• Jumper Wire — I like getting solid core wire for these
• Male Header Pins
• 2 Pin Screw Terminal Mount – In general, when I get to the making the PCB section, you can substitute these things for whatever you prefer or have on hand.
• 3M Mounting Tape (you can always drill holes and bolt the switches on)
• Heat Shrink Tubing (optional, but do yourself a favor and get some)

Required Tools:

• Soldering Iron (you could probably pull this off without one, but come on, you will need one soon enough if you bought yourself a K40)
• Wire Strippers, Wire Cutters, Needle Nose Pliers (when don't you need these things??)
• Screw driver

ONE IMPORTANT FINAL NOTE!

My machine came with an emergency stop switch, which I am going to be tapping into for our wiring, which also makes it very easy. If you don't have an e-stop, you'll have to proceed at your own risk in terms of slicing the positive wire that runs into the power switch. Easy enough, but I won't be showing that.

Alright, so let's get going with the electrical.

Our methods for installing interlock switches will be to tap into the emergency stop button on the machine. My crude post-it note drawings explain.

We are going to...

1. Take off the wire going into the e-stop from the PSU
2. Run that wire into one of our interlock switches
3. Run wire between each switch (parallel) so each switch, including the e-stop, will need to be closed in order for the machine to work.

There were so many other methods people proposed, using magnetic reed switches, and all sorts of stuff. This seems to be pretty foolproof.

So the first two pictures you can almost imagine that being our e-stop. Even though you press in an e-stop button to engage it, when it is not pressed down, it is actually allowing current to flow right through it.

To make sure what I wanted to do was truly the right way to go, I did a quick experiment using a breadboard and a few more microswitches I had around.

First I wired up 1 microswitch (call this our e-stop switch), and it worked. Then I added 3 more microswitches to represent each of the 3 switches we will be adding. They are wired just like my post-it note schematic in the first part. Everything worked and I was confident everything would work fine.

The final test I did, was grabbing one of my end-stops and some alligator clips and actually hooking it up to the e-stop and turning on the machine with one interlock switch to see if it worked as planned. Yep, all good.

Now let's make our little break out board.

This part does assume you have some basic electronic skills and know how to solder and make simple, hand-made PCBs.

1. The board I made was super simple. I wired up 4 sets of 3 male header pins on the PCB and soldered them down.
2. I then ran jumper wired in between each switches connection, once again, similar to my post-it note schematic, and how we wired the switches together on the breadboard.
3. I put 4 sets on there, you only need to do 3 though. I put a 4th on there as in the future I want to add a water flow sensor to my system and I can plug it right in there. I put a little homebrewed jumper those pins.
4. Solder on your 2-Pin Screw Terminal Connector and wire each end to the start and end of the interlock switch header pins.
5. If this isn't self-explanatory, please leave a comment.
6. I think just added some 3M mounting tape to the back of it. I like using this stuff because its crazy strong, and it acts as a good insulator when mounting to metal stuff like the case of this (poorly) wired laser cutter.
7. As you can see in the last pics, I just took the wire going into the e-stop, cut it in half, took the side with the wire crimp and added a length of wire to that and also added a length of wire to the other end of the cut wire.

Almost done!

And that's about it!

Plug the e-stop wires into the appropriate side of the screw terminal.

Plug in the interlocks (make sure the orientation is correct.

And this is where I will let your ingenuity and inventiveness take over. You can see where I decided to mount two of mine for the front lids. I need to add something to make them engage when the lid is closed still. If you have gottten this far, I'm sure you can figure out how to mount them. If you have a 3D printer, go over to thingiverse, there are lots of models out there as well.

Hope this was helpful and happy Chinese laser cuttin'!