In this instructable you will learn how to build an electrical switch circuit for the Monoprice Delta Mini, while this method could also work for other printers.

For this build you will need:

Previously printed Bed Clips (model available in sub-steps)
Some wire
Perfboard (best with the copper stripe pattern)
3*BC547c or equivalent transistor
3*1MΩ resistor
1*2-20Ω resistor (depending on what you have on hand and some tinkering)
3*M3~10mm bolt
1*M3~3mm bolt(might need to cut it)
3*M3 nut
3*Woodscrew 3mm thick, 11mm long (will be cut shorter a bit)
2 Binder Clips (like these)
Heat shrink tubing ~2mm diameter
Springs fitting over the feet of the build surface
Some foam/sponge as spring (I used pipe insulation)

Additionally, a 3-pin JST-SH connector avoids cutting wires → not voiding your warranty (but not having these is fine as well)

Step 1: Building the Circuit Board

In for this step you need a little experience in soldering, but it should be fine to learn it in this project.

We will build the circuit board, which does the switching logic later on. In the Monoprice Delta Mini 3 tact switches are used under the bed to detect the head crushing into it. Since we want to replace those, we need a circuit that shorts two wires, when one of 3 switches opens a loop. For this we build a 3-NAND block.

For this block we will mainly need the 3 transistors (BC547c) and the 1MΩ resistors. To mount everything together, we need some prototyping/perforated board. Here a copper stripe pattern makes the soldering easier.

In this tutorial pretty specific electrical components are used, since the voltage margin-of-error in the later integration and switching is pretty small. If you can't get the necessary components try finding alternative ones. The transistors are used due to their high gain value, so find ones that have a similar high one.

To assemble this board, a picture tells more than a thousand words, thus, I provide you with a 3D model!

(https://a360.co/2GgY8Rc)

In the case, the embedding does not work out, an image is provided as well.

Assembly

As a word of caution: Have you seen the gaps in the middle rail? Put them in place before you solder something in place, since its way harder later.

To start of, place the transistors in their places and solder them in place. After adding the transistors use the 1MΩ resistors and place them side by side with the transistors, as shown in the model. As last component, the single resistor in the corner is soldered in place. Graduations, you build a 3-NAND!

The single resistor was rated to be between 2-20Ω. This broad range can be applied and was found by testing different resistors on my printer. In the end i soldered a 15Ω resistor in place. If in the end your printer seems to be working correctly, but the red bed light is spontainiously switching to red when the bed heating is turned on, then this resistor is the cause. Switch it for higher values then. Be sure to not switch it to a too high value, since then, the whole circuit will not switch anymore.

In the next steps this board is connected to the other components in the places, where the additional wires are sticking up from the circuit board. To prepare these steps solder ~30cm wire pieces in place of the black and green wire (will be cut later on).

Step 2: Building the Switches

Before we open up your printer, please print 3 of the brackets (Clip.stl) on your printer. They are the base of your later bed switches.

BIG THANKS to Dennis Brown, who made the original version of these brackets! For this tutorial a slightly modified version (shorter and holes added) is used.

To assemble the switches, first clean up your print. Afterwards, take the binder clips and remove the metal wire part, these are the ones we will be using. Cut three 5mm pieces of heat shrink tubing and slide them over the wires. Now snap the wires into the printed clips like shown in the image. For this, you maybe need to widen the holes a bit.

Now to your printer.

Remove the old bed clips by pulling out the pin on their top. Now the bed should come lose completely. Open up the base of your printer now. Make sure to disconnect the fan while doing so to gain more space.

Now the most tricky part will be made. Insert one of the long M3 bolts into each bed clips center hole. Insert the bolts end together with the clip into the holes of the old clip and tighten the nut on the other side of the metal casing.
This is a very fiddly job. Take your time.

When that is done and all three clips are in place, put the bed back in place and make sure, that the clips are pushed furthest toward the center of the bed, while making sure, that the bed does not bind up on their edge. The metal binder clip part should now rest on the metal hexagon feet of the bed.

Again from the base, stick one of the black wires of your circuit board though each of the towers. One towards each clip. The board itself stays in the base. Insert each wire through the back hole of each clip. Strip about 5mm of each wires isolation and stick it under the heat shrink tubing. Shrink the tubing to press the bare wire onto the metal clip wire. These should make contact.

I dont quite like this method. If you find a more suitable one, tell me!

To finish up your bed clip, cut a small piece of your "spring" foam and stick it under the metal clip. As last part, screw in one of the wood screws into the small hole towards the bed. For me the 11mm screws I had been a bit to long, so I cut them a bit shorter.

Your bed clips are now finished and mounted!

Clip.stl
Download View in 3D

Step 3: Connection to the Main Board

In this step, the board is installed in the printer, as shown in one of the images. To connect it to the circuit board of the printer, the blue and red wire visible in step 2 is used (not soldered yet).

Caution: At least on my printer the blue wire is the positive and the red the negative wire.

To connect our board to the printers one, search for one of the bed switches in the printers base. They are marked in one of the reference images. Choose one, unplug it on the switches end. Unplugging the cable is not as easy as I initially thought, but with a bit of force it comes loose. Fetch a multimeter to test if the red wire is holding the negative and the blue the positive charge.

If it is the other way around, make sure to always switch the red and blue wire in the later parts of this tutorial.

After checking the polarity, unplug the other end of the cable and keep in mind, where it should go back later. Now, either your extra JST-SH plug is used to build a cable similar to the one you are taken out of the printer (only with one plug) or you just cut one of the plugs of the other. Make sure to have enough cable left on one of the ends to solder it to the circuit board as the blue and red wires. Solder the red and blue wires to your board as seen in step 2. Maybe lengthen the cables a bit to gain more placement options later on. Your board is now completely finished!

Plug in your board to the printers one. As last wire not connected, you now have to connect the green wire to the base. This is done in the next step.

Step 4: Connect the Green Wire to the Bed

In this step the last unconnected wire, the green wire, will be connected to the bed. (Notice that in the images, my green wire is actually black too)

For this, first take off your bed once more. For this you can just unscrew the wood screws from the clips.

Before we attach the wire, first we need to put the springs in place. As springs, I used springs from the home improvement store, but you could also use springs from ballpoint pens.

Now, take your springs and fit them over one of the feet of the bed. The spring needs to fit on the foot, but should not be too tight or too loose. If it is too tight, bend it open with a pair of needle nose pliers. If it is too loose, at least tighten the first loop of the spring enough to stick to the foot. Now cut the resulting springs to length and fit one on each foot. If you put your bed back in place, the springs should be able to push the bed up quite a bit, but you need to be able to push it far down enough, such that you could clamp it down with the clips comfortably. This part can be quite fiddly too, but take your time and do it right. In my case, the springs only do about 2 loops around each foot.

When the springs are in place, we can now fit the wire in place. For this, stick the green wire through one of the holes the feet of your bed were resting in. Now take the last bolt (M3~3mm). Test if the bolts head fits through the feet holes.

If it does not: File the head smaller until it fits through the hole

Screw the bolt into the feet that corresponds to the hole the green wire is stuck through. Before tightening the bolt completely, strip about 2cm of isolation from the green wire and warp it around. Tighten the bolt. The wire should now be firmly attached to the beds foot. Put the bed back in place while sticking the green wire back into the base. Clamp the bed back down. You should now have a spring-loaded bed that is attached to the green wire.

This was the last part of the assembly. Now lets test if it works!

Step 5: Test Your Build!

Without screwing your base back together, take your board, hold it by side and stand your printer back up. Power up your printer. Watch the red light on the side, which tells you, that your bed "is pushed down".

The light should not be lit at this point. You might need to fiddle around with your clips to make sure they make good contact to the bed. If you can't make the light go out, make sure all connections you made in this build are really connected. If the light goes out and stays out, you are fine.

Now, push down on the bed. The light should immediately light up. Test each of your clips for this ability by pushing the bed down next to them. Don't get too exited yet, the hard test is yet to come.

In the main menu of your printer, start the preheating of the bed and/or hotend. While the printer is performing this action, the light should stay out. If it starts "blinking" or flashing once the bed heating is started, replace the resistor on your red wire with a higher value and try again. Make sure that it still lights up in cool and heated state, as described above.

If all goes well, you should now have a hot bed that lights up the red light each time you push down on it. Your build is finished and functioning! Congratulations!

As last steps: Put a big dot of hot glue on the back of your board and stick it somewhere in your base. Pull the black wires of your clips as far into the base as possible to ensure that they don't catch up on the belts. Glue them in place with a small dot of hot glue. Close your base back up.

Your printer is now finished. Start playing around with the wood screws on each clip to get the bed level.