K'nex Robotics: a Few Ways to Add Circuits to a K'nex Project

K'nex is versatile and easy to use to make strong mechanical structures. This means it could be a great platform for robotics projects. I have seen several of these before, but a lot of them tend to involve glued or 3D printed parts. Being unwilling to glue parts together myself, and not having access to a 3D printer, I made some alternative solutions. The main goal of this project is secure, but not permanent, connections to K'nex parts without having to create any custom components. I hope it is helpful to anyone with an interest in robotics.

Step 1: The Servo Mount

This is a trick for mounting standard size servos on a K'nex frame. It also includes an shaft connection so that the servo can directly power the model.

Attaching a shaft begins with this set of white rods and connectors.
The shaft consists of the rod, two gray connectors, and the piece from the last picture. Most rods should work, but if this is a relatively high torque application, you will want one of the stiffer ones such as a tan or black rod.
Slide the rod through the white connector so that the gray clip slides over a rod.
Attach the other gray clip to the other side of the white one and slide it down over a rod.
The servo is a standard-size hobby servo with an X-shaped horn. Depending on the project, you may want to center the servo or move it to one extreme end of its rotation range before attaching the horn. The arms on the horn will fit in the spaces between the white rods, so line up the horn accordingly. If the screw sticks out too far for the next steps to work, you can remove it. Just make sure the connections on the rod help hold it in the frame so it doesn't slide. An advantage of removing the screw is you can re-center the servo after the rod is attached and make sure it is lined up more easily.
Begin attaching the shaft by lining up one or two arms on the horn with the spaces between the white connectors.
Pull the rod into alignment with the servo shaft. This will force some white rods to slide up on the connector.
Push or pull the rods back down one at a time.
When you are done, the rod should be attached to the servo horn like this.
The frame connection starts here. A square frame of white rods will hold a standard servo.
Attach the yellow rod across the back as shown. This will keep it secure in the frame.
Slide the servo into the frame through the gap where a white rod is missing.
The body of the servo fits between the yellow rod and white rod as shown.
Fit a white rod in the gap on top to fully secure the motor.
This piece will hold the shaft.
Here is the final motor mount. The orange and yellow connectors can all be substituted for whatever you need for mounting.
Here is an in-progress machine that uses the servo mount.

Step 2: Low Profile Servo Mount

This is a smaller, rod-based frame for holding standard-size servos. Most of the time, I use it on open-loop servos because the mount is less likely to get in the way of the wheels.

The mount begins with two yellow connectors and two orange connectors on a yellow rod. The white rod will help secure the servo.
This is the frame, servo, and rod. The rod may be yellow, red, or gray, depending on how you intend to mount it to your project. The servo may have a horn attached without changing the following steps; it was just removed here to make the pictures easier to use.
The servo slips into the frame so that the mounting tab goes under the white rod.
Once the servo is in place, attach the rod over the top of the frame.
Adding an additional white rod to the other side of the frame will improve security.
Here is the mount with a 2.5 inch wheel on the motor. Note that it sticks out above and below the frame.
Here is an old obstacle avoiding bot that used a variant of this servo mount. If you use the mount for something like this, you will want to ensure that the mounting rod can not rotate. The mount is only as secure as the frame to which it is attached.

Step 3: Slotted Potentiometer Holder, Parts

This is for measuring how far a K'nex rod rotates. Since it uses a potentiometer, it does not allow 360 degree rotation. However, it does allow position control, which could be useful for some projects. The final design I ended up with is bulkier than I would like, but it is sturdy and secure. Combined with hacked motors like the ones here or here, this could be used to make a servomechanism. Here are the parts for it:

2 micro black connectors
3 micro green connectors
2 micro blue spacers
5 micro yellow rods
1 green classic-to-micro transition rod
2 classic white connectors
4 classic yellow connectors
12 classic purple connectors
11 classic blue connectors
4 classic red connectors
2 classic orange connectors
2 classic gray two-slot connectors
4 classic gray one-slot connectors
2 classic blue spacers
24 classic green rods
20 classic white rods
4 classic yellow rods
A classic rod that is yellow, red, gray, or black (depending on the length and stiffness you need)
A potentiometer about 5/8 in. (16 mm) diameter with a slotted top
A paper clip

Step 4: Slotted Potentiometer Holder: Assembly Part 1

The setup begins with an all-micro-piece coupler. Start by connecting the parts shown. Note the blue spacer between the black connectors.
Slip the paper clip over one of the black connectors and attach the yellow rod to the side opposite the first one.
Here it is with both yellow rods attached.
Add a third yellow rod next to one of the first two. The paper clip should be trapped between the new rod and the old one.
Add a fourth rod opposite the third one, again trapping the paper clip. At this point the clip may slide back an forth, but should not rotate on the axis of the connectors. This is the coupler.
The measuring end begins with the arrangement shown. The black rod may also be gray, red, or yellow; but yellow rods can only support one object (such as a gear) beyond the length of the support. This connects to whatever is being measured.
To add sturdiness, clip together 3 blue connectors with two white connectors so that there is a rectangular slot between them.
Here they are clipped together.
Slide the rectangular slot over the two orange connectors. This will significantly increase the stiffness of the assembly.
Attach the two-slot connector and white rod to the black one as shown.
The two blue spacers go on the end of the green rod.
This frame piece will go on the end.
Slip the frame over the green rod and secure with a micro green connector.
The frame is now attached. Yes, it's a bit loose; that will be fixed in a second.
A similar frame and 4 yellow rods will add the sturdiness it needs.
The frame goes over the other end of the connector as shown.

Step 5: Slotted Potentiometer Holder: Assembly Part 2

This is the cradle that holds the potentiometer.

Start with the pieces shown. Note that one is missing a white rod and one has a gray connector.
Attach them together in a ring as shown.
Attach the other gray connectors around the top as shown. This is where the potentiometer will go.
Attach 4 red connectors to the corners using white rods.
Here it is with the corners attached.
Connect 4 yellow connectors between the corners using 3 green rods each.
Here it is with all the sides connected.
These parts will be needed to keep the potentiometer tight.
They will be connected at right angles like this, but do not connect them yet! This is just to show how they will be after assembly.
The micro yellow rod fits in the triangular hole between the link with two gray connectors and the link with none. The orientation of the green connector is important.
The other micro green connector holds it in place. The orientation of the green connector may be as shown, or 180 degrees from what is shown. Either way, it should be at right angles to the other green connector.
How the wires thread through the holes to the potentiometer depends on how they connect and where they fit. In this case, it's a 4 wire bundle of female-female wires and a potentiometer with breadboard pins, so the wires fit through the slots on each side of the blue connector.
If the potentiometer is not soldered, clipped, or otherwise connected to the wires leading away from it, attach them now. Attaching them after it is in place will be too difficult.
Press the potentiometer into the cradle. If the wires just slide or clip onto the contacts, make sure they don't slide off. When you're done, the potentiometer should be neatly in the center with the micro green connectore pressed against the side or slightly over the edge of the casing.
Take the paper clip coupler and shaft from earlier and fit the coupler onto the micro-size end of the shaft, so one of the rods on the coupler goes through the hole in the green connector.
Rotate the shaft until the clip lines up with the slot on the potentiometer and set the potentiometer onto the coupler.
Now rotate the shaft until the frames line up and begin connecting the cradle to the shaft frame as shown.
This is the final connected piece.
Add any gears, connectors, or other components you need to measure.
If you use a yellow rod for the shaft, this is how much will stick out. It is enough for the gear and the tan clip under it, but it is not enough for a connector or an end cap above the gear.

Step 6: The Stepper Motor

This is a controlled motor I don't use much, so it took a while to figure out how to mount it. The shaft connects to the K'nex parts using a small binder clip. The flat-ground points on the stepper shaft let the binder clip grip surprisingly well. That said, over-torquing it could make it slip off. Here is how it all connects.

Start with the three-quarters ring of blue connectors. This will hold the body of the motor.
The motor fits snugly into the holder. Align the screw tabs with the two connectors on the sides.
These will form tab holders to keep the motor from sliding out.
Attach the gray connectors to the Y-clip in the middle of each rod. This should also work with one-slot connectors instead of two-slot connectors.
Attach these parts to the sides of the holders using the part of the rod without a Y-clip.
Clamp the binder clip onto the flat parts of the shaft. Line it up so the shaft is in the middle of the clip.
Here is the shaft connector. Any single slot clips should work on the sides, but I don't think tan clips will because of the pins.
The shaft uses a tan clip to lock rotation to the connector. The other clip is an optional spacer.
Here is the assembled shaft.
The legs of the binder clip fit over the ends of the white rod on the shaft.
Technically, this is all you need to make it work. The rest is for stability.
These three parts will add support to the shaft.
The one with the Y-clip goes on top of the holder, while the other two go in the slots above the two white rods.
A connector between them finishes off the shaft stabilizer.

Step 7: HC SR04 Sonar

So far, this has mostly been motor control designs, but there had to be at least some sensors. I can't put them all, of course, since there are too many types to be practical in a single instructable. However, the sonar can easily come up in robotics projects, so it seemed like a good idea to include it. Ways of attaching these are almost limitless, but here are a couple that I happen to like.

Method 1:

A simple frame of a blue connector, two white rods, and four gray two-slot connectors can hold the module.
The pins on the module slip through the slot on the bottom of the frame.
Turning the two connectors on the top so they fit over the module secures it. You may have to remove and reattach them.
The wires will plug in through the slot on the bottom.
Here is a shot of the module with the wires attached.

Method 2:

Method 2 begins with the frame pieces shown here.
The frame pieces attach to mirror each other.
The sonar fits into the gap between the gray connectors and blue spacers.
Pulling back the rods on top tightens the connection and pulls the module up so it faces forwards.
Electrical connections are easy, since the pins just stick out on top.
The last two pictures show some possible structural connections. The rods are pulled forward enough that a connector can fit exactly on the end, like the red connectors shown here.
The blue spacers provide a gap so that the top and bottom would be even. As a side effect, the gap allows the use of blue/purple 3D connectors to work in place of the yellow ones.

Step 8: Breadboards

Breadboards are a great way to add circuit chips, wiring connections, or certain sensors. Ways of connecting them can range from dropping them on a platform to tying them down with rubber bands to mechanical locks. Here are a few of the mechanical ways to add them.

Two connectors that stick up at least 45 degrees and are attached by a blue rod can hold a breadboard. The frame shown uses a square of blue rods and 3D connectors for this. The gray rods do nothing. They don't enhance the strength or security, but they don't reduce those or get in the way of the breadboard either.
The breadboard just presses down between the connectors. The connectors can hold the breadboard at angles, including sideways. The friction is enough for an upside-down breadboard if you want one, but I wouldn't trust it completely.
This rig uses the pins on the tan connectors to lock into the central groove on a medium breadboard.
The mechanical lock is loose, but secure. The rod on top does not get in the way of IC's, but the end connectors would get in the way of devices like a raspberry pi to breadboard connector.
This rig produces a slot that a breadboard can fit through. The yellow rods will block some holes on the breadboard, but not enough to be significant.
The board slides through the slot easily. Since it is a bit loose, you will want to either squeeze the connectors together or lock the board with strategically placed electrical components or rubber bands.
The rig does not have a defined up or down, so the orientation of the board depends on whatever is convenient.
Since the slot is the most important part of this design, alternate layouts work well.