Robot Butler

by littleBits

Published on April 23, 2014

This smart little creature roams the table on a central wheel that is connected to a dc motor. Little plastic arms activate three roller switches on the side as they bump into cups and bowls, causing the robot to turn and try elsewhere. This happens because there is an inverter in between the first dc motor and a second dc motor positioned on the edge.

How To Make It


Let’s assemble the circuit first: power + wire + roller switch + wire + roller switch + wire + roller switch + wire + dc motor + inverter + wire + dc motor.

How it works:

The circuit is pretty simple yet still interesting. Three roller switches are in series and are all set to open mode. This means that the circuit is open (the signal is transmitting) when the roller switches are NOT pressed and the first dc motor will be turning. When one of the roller switched is pressed, the first dc motor will stop turning. However, there is an inverter between the first dc motor and second dc motor. This means that the two motors will behave opposite of one another. To recap, when no roller switch activated (pressed down), the first dc motor is rolling and the second one doesn’t move. When any roller switch is pressed, the first dc motor holds its position and the second dc motor runs.


Cut out the body with all the holes for the Bit feet, wheels and screw positions. You can download our template and laser cut it from acrylic like we did or, in the case that it is difficult to access to a laser cutter, you can print our file on a regular paper or film and use it as a template for placing the Bits Modules and drawing holes.


Attach two wheels to the dc motors and two lego wheels at their position (bottom face of the lower body). The wheels we used for the dc motors have a 3mm cutout that matches the d-shaft on the motor. You can find them here: The Lego parts that we used can be found here:


The alignment of these wheels is the secret to how this simple circuit makes smart movements. The wheel on the first dc motor is located at the very center of the circle. When this wheel is running, this and two lego wheels will be aligned, pulling the robot straight forward. The wheel on the second dc motor is positioned close to the edge of the circle. This wheel along with the two lego wheels are also aligned in circle. When the outer wheel spins, the center wheel doesn’t move, making it a perfect pivot point, the turning the counter cruiser in circles.
Note: We wanted our Counter Cruiser to be somewhat small as our table is somewhat small. Our Counter Cruiser measures 6 inches in diameter. We, therefore, had to angle our outer wheel slightly (17 degrees) so that everything fit is a small space. However, if you have a larger table and are not using our template can make the second wheel perpendicular to the first in order to minimize drag when the Counter Cruiser proceeds forward. (We will leave it up to you to figure out why.)


Attach a 5th wheel for balancing. This wheel should be able to change its direction or it will generate drag both when the robot moves forward or spins. We found a small ball caster from our material stock but you can use a Lego turntable (something like this: ) and a Lego wheel instead (we unfortunately lost our precious lego turntable piece). Adjust the height of all five wheels so they are even. We used two thin square blocks for each of lego wheels and one ¼” acrylic piece for ball caster.


Place all the Bits Modules in their correct positions on the top side of the lower layer.


Cut out the sensing arms and spacers from ¼” acrylic. Here is the other secret of this project; these sensing arms are stretch out to follow the direction of the spin (if they do not, you should change the direction of the second dc motor). You will notice that the arms overlap slightly. This configuration makes it possible for the robot to keep sensing any obstacles until it turns completely away from them. The spacers we made both help the sensing arms to move smoothly as well as meet directly with the height of the roller switch. If you don’t have access to a laser cutter, we suggest to substitute cardboard or hard paper board for the arms and brass or plastic pipe for the spacers. You should be able to find these materials easily at any art supply store.


Attach the sensing arms and spacers using machine screws. We used M3x30mm screws and nuts. It is good idea to have two nuts in a row to prevent loosening of the parts.


Cut out another disc to make a surface for the food to sit on. Don’t forget to make holes for connecting this top surface to the lower disc.


Cut three brass tubes the same length. Ours are ¼” diameter and ¾” tall. Secure the upper disk in place using these tubes by passing the machine screws through and fastening them on the bottom of the lower surface with nuts.


Put dessert, gravy, sugar, milk, or whatever you like on the cruiser.Turn on the power and let it roam. Grab a piece of desert as it passes by.

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