The Useless Machine is a variation on Marvin Minsky's "Ultimate Machine," which is basically a machine whose ultimate goal is to turn itself off. After building it, you will be amazed how a machine consisting of two switches and a motor and does nothing but disable itself seems to have so much personality. While it does not have much purpose, it always seems to bring smiles to people's faces.
To learn more about switches, check out my Electronics Class. You could also learn more about motors in my Robotics Class.
Step 1: Lesson Materials
Step 2: How It Works
At the heart of the machine there is a DPDT toggle switch wired to reverse polarity to a motor. This means that the direction electricity is flowing through the motor changes when the switch is toggled. This is important because the direction a motor rotates is dependent on which direction electricity flows through it. So, to put it simply, when power and ground get reversed, the direction of the motor changes.
There is also a lever switch inside the case which disconnects power to the motor, but only when it is pressed and the power is reversed.
Thus, when the toggle switch is pressed, the power is no longer reversed and
the machine is once again turned on. The arm is then free to rotate up
out of the box and hit the switch. This in turn reverses the arm, which
rotates back into the box, where it hits the lever switch, and turns
itself off once more.
This project demonstrates how a lot can be achieved by cleverly routing electricity through a few simple switches.
Step 3: Modify the Servo
First things first, we need to convert a servo motor, which is a circuit board controlled motor into a basic gear motor. The reason for this is because servos are reliable, easy to work with, and have gearboxes with a lot of torque, which is necessary for pressing the switch.
All this entails is removing the circuit board attached to the motor and attaching two wires instead. It's not as scary as it sounds, and gives us a chance to practice desoldering.
Remove the four screws to find the servo's circuit board and locate the two large solder terminals connected to the motor.
Carefully use desoldering braid to remove the solder from the two terminals connecting the circuit board to the motor.
Remove the circuit board from the case.
Solder a red wire to the positive terminal of the motor. This is typically marked with a red dot. Then, solder a black wire to the other terminal. If you mess this up or they are not marked, don't sweat it. It will just mean your motor might spin backwards when power is connected. If this is the case, just rewire it with the wires reversed.
Trim away any excess wire leads from the terminal. This will make it easier to get the lid back on.
Tie the red and black wire together in a knot such that the knot itself extends past the outside of the motor enclosure. Then, place the knot on the inside of the motor enclosure. This will prevent anything from placing strain on the wire and it getting pulled free.
Close the case back up and you are done.
Step 4: Drill and Trim the Servo Horn
The gear-looking thing attached to the servo is called its horn.
On one of its arms, widen the innermost and outermost holes using a 1/8" drill bit. This is so they are large enough that we can pass a zip tie through them later.
Then, use diagonal cutters to cut away all of the remaining arms so that they will not later get in the way of the box lid opening and closing.
Step 5: Shape Your Letter
Place the servo on top of the box and get your wooden letter. I found that the "C" worked exceptionally well.
The goal is to mark it so that it forms a hook which will be small enough to rotate fully into the box, but be large enough that it will rotate far enough out of the box to press the switch.
This might take some trial and error. Fortunately, wooden letters are cheap and easy to work with.
Step 6: Cut to Size
Cut the wooden letter into a hook shape using the markings you made in the last step.
Smooth out any rough edges with sandpaper.
Step 7: Trim the Lid
Place the motor atop the lid on the far edge from the hinges. Position the motor to figure out just how much of the lid is necessary to keep in order to mount the motor such that the servo horn is just clear of the lid.
Once you have figured this out, draw a cut line across the box.
Also make a cut line on the side of the lid angled slightly towards the edge with the hinges.
Cut the lid into two sections at an angle by following the cut line.
When you are done, the part of the lid connected to the hinges should have a slight overhang.
Step 8: Glue
Using wood glue, permanently attach the part of the lid without the hinges to the box.
Step 9: Wire the Circuit
Let's wire together the circuit as outlined in the wiring diagram above.
To begin, attach the motor to the center terminals on the switch.
Then, attach the battery back to the outer terminals on the switch, keeping an eye to line up power and ground connections. If the switch were to be thrown now, power will either be connected or disconnected, and the motor should spin clockwise.
Since we want the motor to get disconnected when it spins counter clockwise and presses the lever switch, we then connect wires to its common and normally closed pins. In this way, the switch is normally closed to allow electricity to flow, but the connection is opened (or 'broken') when it is pressed.
Finally, the outer terminals of the switch are crisscrossed to allow the motor to be powered backwards when the switch is toggled. For ground we simply use a short wire. However, for power we use the wires from the lever switch such that it can be toggled on and off.
Step 10: Drill Mounting Holes
Align the servo's lever with the base of the wooden arm, and use the servo's mounting holes to make two drill guides on the arm.
Drill these marks with a 1/8" drill bit.
Step 11: Fasten the Arm
Fasten the arm to the servo's lever using a small zip tie.
Trim the excess zip tie tail when done to prevent it from catching and getting in the way.
Step 12: Epoxy the Servo
Mix together 2-part 5-minute epoxy and glue the servo to the inside of the lid such that lever arm will sit roughly centered in the box. Also, make sure that it will be able to rotate upwards over the box lip without immediately catching.
Once you are certain on the positioning, turn the box over, and wait 30 minutes for the epoxy to fully set.
Step 13: Drill a Hole
Drill a 1/4" hole centered along the edge of the box. This is for the switch. Thus, the hole should be positioned in a spot where the lever arm can rotate up and past the hole. This will ensure that the arm will always be able to hit the toggle switch and push it far enough to activate it.
Step 14: Install the Switch
Using the switch's mounting nut, install it in place.
Step 15: Drill
Position the lever switch centered along one edge of the cube such that the body of the switch is level with the top of the cube and the lever extends up above it.
Mark the switch's mounting holes with a pencil, and then drill these markings with a 1/8" drill bit.
Step 16: Zip Tie the Switch
Zip tie the switch to the 1" wooden cube in such a manner that the lever is extending up past the top of the cube.
Step 17: Glue the Battery Holder
Use 5-minute epoxy and attach the battery holder into the bottom corner of the case below the servo. This will ensure it is out of the way.
Step 18: Glue the Block
Glue the wooden block into the box such that when the arm rotates inward, it ultimately presses down firmly upon the lever switch.
Step 19: Insert Batteries
Insert batteries into the battery holder.
The arm should ultimately rotate into the box, and turn itself off.
If it does not do this, quickly remove the batteries, and then check to see if your DPDT switch was installed backwards into the box. This is a common mistake and should typically fix things when the battery is re-inserted.
If it is still not working after you try this, again remove the batteries quickly and double check all of your wiring. Something is not right.
Likely, you might have messed up the polarity on the motor wiring. However, you should check everything carefully before switching any wiring.
Step 20: Close the Lid
Once the arm has rotated into the box, and turned itself off, close the lid to the box.
Step 21: Congratulations!
You now have a machine which does nothing. Share it with your friends and family.