This quick, fun project propels string in an endless loop with a handheld device. It requires a few 3D printed parts, a battery, two small motors, and a switch.

Supplies

String! We found that nylon string worked best
Access to a 3D printer & filament
Two hobby motors
Switch or button (like this)
Two AAA Batteries
Wire & soldering iron

Step 1: 3D Print the Body

Download the 3D model from THIS LINK and print!

It is a small part, so it won't take too long.

We sized it for our FDM printer which has around a 0.2mm tolerance, so the motors fit snugly.

The wheels are a little more fickle as the diameter needs to be enough that they grip the string, but not too large where they don't let the knot pass through.

In the model, there are two versions, "RegRoller" which meant to be used as-is. Then there's "PadRoller" which was sized to fit inside a cut piece of flexible acrylic tube. In testing, both seemed to work, so "RegRoller" is just a simpler bet.

Step 2: Wiring the Battery Holder

To connect the batteries, we sprawled out some stranded wires and folded a piece of copper sheet around it to create pads for the end of the batteries to attach to. Repeat this with two wires.

Glue these pads to the insides of the battery holder and place the motors in the slots. Create a wire with pads on either end and glue it between the two battery slots

Step 3: Soldering the Wires

Solder the ‘to switch’ wire from the battery holder to one end of the switch. Then, solder the motors according to the diagram.

Step 4: Load the String and Play With Your String Shooter!

We cut about 3' of string and tied it in a loop using a square knot. You can play around with the length of the loop! Motor power and traction affects how long the loop can be before it bends and comes back.

Once your loop is tied, push it in between the wheels, and flip the switch to see your string shooter in action.

Step 5: Alternatives:

If you don't want to make a handheld one, there are numerous videos online for how to make stationary ones. We got a lot of tips from Bruce's video here!