SWING KID

You'll hang a shoebox from a broom, give it a face and a pair of long legs, and it will learn to pump itself higher the way a kid on a playground swing does: kick at the right moment, every swing. Nobody writes the rule for when to kick. A small neural network works it out by reinforcement learning in a simulator you can watch, the Gym. Then you send that brain down the USB cable into the Arduino and find out whether what it learned in the dream works in reality.

No cutting, no glue, no measuring. String, duct tape and zip ties hold everything together, and every size in this guide is "about". The robot measures its own swing when it calibrates.

Everything was tested in simulation before you build it. The firmware ran on a computer against the simulated swing, and calibration was tried on 100 simulated builds with the motion sensor taped on at random angles and the servo mounted either way round. The brain you'll train beats the textbook pumping rule. What I couldn't test is your actual box and broom. That's the experiment.

Moving its legs can't push Swing Kid anywhere. All it can do is shift its own weight, and that only adds height if the timing is right: kick at one moment of each swing and it goes a little higher, kick half a swing later and it slows down. That timing problem is what the brain learns. It's the same problem a kid solves on a playground, and the same skill lets Swing Kid stop itself dead.

The trick that makes a dream-trained brain survive reality is domain randomization. Every practice run, the simulator re-rolls the quirks: how long the strings are, how heavy the box and feet are, how weak and slow the servo is, how much air drag and cable tug there is, how much the gyro drifts. A brain that swings well in all of those worlds has a good chance in yours.

1. Stand two chairs of the same height about a long stride apart. Lay the broom across the tops of both chair backs, handle in the middle.
2. Duct-tape the handle to each chair back so it can't roll or slide.
3. You need two pieces of string, each about an arm's span long (snip them with scissors, or use two long shoelaces).
4. Tie the middle of each string around the broom handle with a double knot: one a hand's width left of the middle, one a hand's width right of it. Put a strip of duct tape over each knot so it can't slide along the handle. The ends hang loose for now.

No chairs handy? Any bar about chest high works: a closet rod, a pull-up bar, a sturdy curtain rod. The box needs about an arm's length of clear space in front and behind it to swing.

1. Turn the box upside down. Stand the servo on its end in the middle of the bottom: the end farthest from its shaft (the little white gear) goes against the cardboard, so the shaft points sideways toward one end of the box, not the front or back. The shaft ends up about 2 cm clear of the box, room for the arm to swing.
2. Tape it down hard: two strips of duct tape over the servo and onto the cardboard in an X, and one more across. Keep tape off the shaft.
3. Leave the servo's wire free. It will run up the back of the box later.

The servo is Swing Kid's hips. It has to stay put when it kicks, so press the tape down firmly onto the cardboard all around it.

1. Press the Power Supply Module onto the left end of the breadboard: its − pins on the blue stripes, + pins on the red stripes, round jack facing out.
2. Top jumper on 5V. Bottom jumper on OFF.
3. Red jumper: Arduino 5V → bottom red (+) rail. Black jumper: Arduino GND → bottom blue (−) rail.
4. Black jumper: top blue rail → bottom blue rail, so both supplies share a ground.
5. Snap the clip onto the 9V battery, but don't plug it in yet.

Why two supplies: the servo pulls sudden bursts of current. The battery, through the power module, feeds only the servo (top rail). The Arduino, powered over USB, feeds everything else (bottom rail).

The servo is taped under the box, so its own wire won't reach far. Push three male-to-male jumpers into the holes of its plug (matching brown, red and orange if you have them) and run those to the breadboard.

This little board feels rotation. ELEGOO ships it with either an MPU-6050 or a QMI8658 chip; the firmware finds whichever you have.

Use the five pins on the side marked DIN (not DOUT).

The receiver goes on wires rather than into the breadboard, so that later it can peek over the rim of the box and see the remote.

1. Use the active buzzer: sticker on top, sealed black bottom. (The passive one shows a green circuit board underneath.)
2. Push it into the breadboard. Longer leg (marked +) → Arduino pin 3. Other leg → bottom blue rail.

Three things to double-check: the capacitor's stripe faces the blue rail; the power module's + and − line up with the red and blue stripes; and nothing from the bottom rails touches the top red rail, which belongs to the servo alone.

1. Open the Gym in Chrome at michaellomuscio.github.io/swing-kid/gym. It can reach the Arduino over USB from there.
2. Or run it from your Mac, which also works offline: in Finder, open the swing-kid folder and double-click Swing Kid Gym.command. Chrome shows the Gym at localhost:8643/gym/; keep the small Terminal window open while you work. (Not on a Mac? Run python3 -m http.server 8643 in the folder.)
3. No hardware yet? In the Robot tab, Try a virtual one runs every step below against a simulated Swing Kid.

USB needs Chrome or Edge; Safari and Firefox don't support it. A Gym opened from inside claude.ai can train, run exams and drive a virtual robot, but can't reach USB.

1. Plug the Arduino into your Mac (through the USB-C adapter).
2. In the Robot tab, click Connect and pick the port named like usbmodem… or usbserial….
3. Click Install firmware. About ten seconds later Swing Kid beeps twice and the status line says hello.

Plan B, the Arduino IDE: open swing-kid/firmware/SwingKid/SwingKid.ino, choose Tools › Board › Arduino Uno and your port, then Upload. No extra libraries are needed.

1. Plug the 9V battery's plug into the power module's round jack and press its button (its LED lights).
2. The gyro says found. Pick up the gyro board and twist it: the reading jumps. Press any button on the remote and watch it appear.
3. Click Beep and Light every LED.
4. Click Legs forward, Legs back and Legs straight: the servo's shaft turns one way, the other, and back to the middle.

Something silent? See Troubleshooting. Fixing it now, with everything on the table, is much easier than once it's hanging.

1. Unplug the USB cable and the battery. Turn the box right side up.
2. Lift the breadboard and Arduino in together. Stick the breadboard to the floor along the back wall (peel the paper off its back, or use tape). Tape the Arduino in front of it and the 9V battery beside the Arduino, lying flat.
3. Tape the gyro board flat on the floor of the box, chips facing up. Which way it points doesn't matter.
4. Tape the LED matrix to the outside of the front wall, LEDs facing out, its pins at the top so the wires go straight over the rim.
5. Tape the remote receiver on top of the front rim, its dark dome facing forward.
6. Run the servo's wires up the back of the box and over the rim.

1. Hold the box under the broom, face toward you, about a hand's length below the handle.
2. Take the left string's two ends. Tape one to the box's front-left top corner and the other to its back-left top corner. Do the same on the right. Each string now makes a V.
3. For each end: tape it on, fold the end back over the tape, and tape again. A folded end can't slip out.
4. Let go. If the box tilts, peel one end up, slide the string a little, and press the tape back down.

Why Vs: seen from the side, each V is a triangle, so the box can't rock on its strings; box and strings swing as one piece. And why so close to the broom? A swing pumps best when its heavy part is high and the kicking weight is low. Part 6 shows the math.

1. Plug the USB cable and the battery back in and press the power module's button. In the Gym, click Center the servo. The servo holds its exact middle.
2. Push the long single-arm horn onto the servo's shaft, its arm pointing as close to straight down as its teeth allow.
3. Lay the stick along the arm with the stick's top end at the round hub, so nothing sticks up toward the box. Strap it on with two zip ties through the arm's holes, one near the hub and one near the arm's tip. Pull them tight and snip the tails.
4. Tape two AA batteries across the bottom end of the stick: the feet.
5. The feet should clear the floor by at least a hand's width. If they don't, retie the strings higher.

The horn's tiny screw (in the servo's bag) locks it onto the shaft. Worth it if you have a small screwdriver.

1. Run the USB cable from the Arduino up along the back-right string to the broom, with two small pieces of tape.
2. From the broom, let it hang down to your laptop, set on a chair or table off to the side, out of the swing's way.
3. Leave a little slack between the box and the string so the cable never tugs.

The cable now hangs from near the pivot, where it barely affects the swing. The dream simulates some cable tug anyway, just in case.

1. Steady the box with your hand, let go gently, and click Calibrate (or press ST/REPT on the remote). It takes about 40 seconds.
2. Hanging still: it measures the gyro's resting drift. Kicking: five seconds of kicks. The direction the box rotates around tells it how the gyro sits, and which way the box flinches on the first kick tells it which way is which.
3. Swinging freely: legs still, it times its own swing and watches it die down. Pumping and braking: it tries the textbook rule both ways to see how strong its legs are. It usually ends with a little leftover swing; that's normal.
4. Then click Fit my dream. The Gym adjusts its simulator until the simulated swing does what your real one just did.

If it says the kicks didn't make it swing, check that the feet are on, the battery is plugged in and the power module is on, then calibrate again.

First, watch one learn from scratch.

1. In the Dream tab, click New brain, then Start training. Sixty-four simulated Swing Kids, each built a little differently, practice at once with a brain that starts out knowing nothing.
2. Watch the little swings. At first the legs flail and nothing much happens. Then a rhythm appears: kick, tuck, kick. That's the moment it figured out pumping. Braking comes later.
3. Watch the curve: how far, on average, each swing is from the height it was asked for. Within a few minutes it drops below the textbook rule's dashed line. A brand-new brain needs a long practice to match the starter brain, which practiced the equivalent of about five days of swinging.

Then train one for your swing.

1. Turn on Train for my swing and click Start training. This starts from the starter brain and lets it practice on dream swings fitted to yours, so it only has to learn your swing's quirks.
2. Two or three minutes is plenty. The Gym keeps the best brain it has seen, and the starter counts as the one to beat.

1. Open Exam and click Run the exam. Every exam swing starts hanging dead still. It must reach 25°, drop to 15°, climb to 35°, then stop.
2. It's graded on the 544-byte, 8-bit copy the Arduino will run, next to the textbook pumping rule. The rule struggles to start from dead still, because there's no swing yet to time its kicks against. Watch how the brain does it.
3. Pick Best so far, then Starter, and run the exam on My swing for each. If the trained one passes more swings, send it. If not, the starter brain is already on the robot.

1. In Robot, click Send best brain. The Arduino checks every piece and stores the brain in memory that survives power-off.
2. Click Brain, set the target with the slider, and step back.

Give it a shove while it's holding a height, or ask for 0 in the middle of a big swing. Unplugged from the Mac it keeps its brain and calibration: plug its USB cable into a phone power bank in the box, switch on the power module, and press POWER on the remote.

1. In Robot, click You try. Now the ← and → keys (or the slider) move the legs. You're the brain.
2. From hanging still, try to get it to 30° and hold it there. Then try to stop it dead.
3. Then click Brain and watch how it times the same kicks.

The safety brake still works: above 55° the legs take over and slow it down, whoever is in charge.

1. In Robot, with Brain mode on, click Run the test under Dream vs reality. Swing Kid swings up from still for real, then the Gym replays every leg command it sent in the fitted simulator and draws both swings.
2. The space between the two lines is the reality gap. A small gap means the dream was a good model of your box, strings and broom.

The screenshot above comes from the virtual robot, whose physics are deliberately a little different from the dream's. Yours will tell you how good the simulator really is.

1. Skip the randomization. Turn off Train for my swing, slide How different each practice swing is to 0%, click New brain, train, and send. A brain that only ever practiced in one perfect dream often falls apart in reality.
2. Change its body. Add a third battery to the feet, or hang the box lower. Calibrate, fit, and see whether the brain copes, or train a new one.
3. Watch it learn from scratch. Click New brain and keep your eyes on the little swings. You'll see the moment the rhythm appears.
4. Brain vs rule. Run the dream-vs-reality test in Brain mode and again in Textbook rule mode, and compare.