You can build a machine that draws wonderful geometric curve patterns using Crickit, Circuit Playground Express, TT motors, and LEGO. This type of harmonograph drives the pen armature from two motorized hubs running at different speeds, generating and mapping complex harmonic motion!

Materials

In addition to the parts listed above, you'll also need the following:

• LEGO bricks and Technic pieces, as listed in the Bill of Materials on the Build the Drawing Machine page
• Glue, such as hot glue or super glue (CA)
• Fine point roller ball pens
• Paper
• Optional 3D printed Crickit to LEGO plate

A harmonograph is a type of drawing machine that creates geometric illustrations. There are a few kinds of harmonographs, we'll be making a type called a pintograph, that employs a pair of motorized rotating hubs to drive an armature with a pen attached to it.

The relative motion of the hubs, which move at slightly different speeds, typically, generates harmonic relationships that drive the curve continually off of the original path, yet bounded by four extreme points. This is in contrast to a roulette drawing machine, such as a Hoot Nanny or Spirograph, which is bounded by a circle and generates petal-like lobes.

Here is an excellent online simulator you can use to explore pintographic curves.

A terrific resource for learning more about pintographs is this page by Wayne Schmidt.

The key features we need to create our drawing machine are a pair of variable speed motor drives and an armature to move the pen across the paper. The two drives rotate at different speeds, pushing the armature (and the connected pen) along a repeating, yet slightly changing path.

There are nearly limitless ways to do this with LEGO blocks and Technics parts. You can adapt this to work with parts you have on hand, or follow these instructions to build one that uses very few parts, most of them easily obtainable in a moderate sized Technics set and a few pick-a-bricks.

This gear reduction section is the key to slowing down the fast motion of the TT motor and increasing the torque. Without this, the machine would draw way to fast, get out of control, and probably tear itself to pieces! You can use actual gears to accomplish this, but pulley wheels and belts are easier to connect, more forgiving of slop, and dampen the motion a bit.

Watch this video for a CG timelapse build of the parts.

This video provides step-by-step build info.

If you'd like to take a close look at the model, download the files below and open them in Lego Digital Designer or any LDraw compliant program!

This is the Bill of Materials (BOM) for the parts used:

Most of these parts can be found in any larger Technic-based kit. If you have any trouble finding certain parts, you have a few choices. First, search for them on the LEGO Pick-a-Brick site or at a LEGO store.

If you have access to a 3D printer, you can 3d print them -- just search for the part on a 3D model hosting site such as Thingiverse. Here's a very nice looking collection of the key components.

You can also purchase them on an aftermarket brick site, such as Bricklink.

Base Build

Following the video build instructions above, build the base of the machine. The two stacks hold the axles for the drive and the gear reduction pulleys, which will be connected to the TT motor later.

Armature

Again, following the build instruction video, make the scissor armature as shown. You can then set it onto the two drives with the short axles. You can use pegs, but this makes it harder to lift the arm on and off, which you'll do every time you want to cap your pen!

TT Motor Mount and Shaft Adapter

In order to introduce our Crickit controlled TT motors into the world of LEGO, we'll need to do two things:

1. Mount the TT motor chassis to a 2x2 flat tile
2. Adapt the motor shaft to LEGO cross axle

Now, this motor can attach to the baseplate of the drawing machine -- or any other LEGO for that matter!

Motor Connections

Next, you'll connect the two motors to the Crickit's Motor 1 and Motor 2 ports.

Pen Holder

The pen needs to be connected securely to the armature. You can place it in the holder and then wrap rubber bands or Technic belts around it to hold it tightly in place.

Now, plug in the 5V 2A power plug to the Crickit and get ready to program the Circuit Playground Express in MakeCode!

Crickit Install for MakeCode

You can code your Crickit and Circuit Playground Express using MakeCode! Head on over to the Crickit with MakeCode guide to get set up!

Code Functions

The primary goal is to run the two motors at different speeds, with the ability to adjust one motor's speed and direction while the other remains constant.

Here are some of the ways we can achieve that goal:

• Set initial speed of both motors to 70%
• Graph the motor 1 speed with the CPX NeoPixel ring
• Indicate the motor 1 direction with the Crickit's on-board NeoPixel
• Decrease or increase motor 1 speed by clicking either the A or B button
• Reverse direction of both motors by clicking both buttons at the same time
• Pause both motors by flipping the on-board CPX switch to the left
• Resume both motors by flipping the switch to the right

You can replicate the blocks as shown, and customize it if you like. 

When you're ready, plug in your Circuit Playground Express and upload the code to the board. Next, we'll make some drawings!

Place a stack of paper underneath the pen, remove the cap, and turn on the Crickit! You can adjust the height of the pen relative to the armature by pushing it up or down inside of the holder.

Once the Crickit Harmonic Drawing Machine starts up, press the B button on the Circuit Playground Express two or three times to increase the speed of Motor 1. Then, watch your drawing develop!

You can stop at any time by moving the Circuit Playground Express slide switch to the left. Some beautiful images may emerge quickly. Or, let it run until you have a very dense image!

Adjust the Armature

Try another drawing, but this time change the location of the attachment point to the hub from one of the armature legs. This will skew the curves to one side and elongate them.

Offset Pen

Another interesting variable is the placement of the pen in relation to the end pivot. Here you can see the shorter, simpler armature with the pen quite a distance from the pivot. It creates a very different curve.

What sort of variations can you create by adjusting the pivots and motor speed?