Install the wiring harness and assemble the gearbox

The gearbox houses the servo, gear train, wiring harness, microcontroller, speaker, and battery — all in the space that fits inside your gauntlet. Each component installs in sequence. Wires must be routed before sections close, and the gear timing must be verified before the cover goes on.

Servo Gear

Start by fastening the bevel gear to the front of the servo. This establishes the mechanical connection before the servo, push arms, or magnets go into the gearbox.

1. Attach the bevel gear to the servo. Seat the bevel gear on the front of the servo and secure it with the center screw. A small amount of threadlocker is recommended on the screw threads so vibration does not let it loosen over time. Use only a tiny amount and keep it off the plastic gear and the servo output shaft.

   

Push Arms & Magnets

The push arms are the mechanical linkage between the gear train and the dart rows. The short arms control the inner and middle rows; the long arm controls the outer row. A 3×1 mm magnet at each tip connects the arm to the LED housing magnetically, allowing each row to detach cleanly during removal.

1. Glue the tip magnets. Apply a small drop of super glue to the round pocket on each push arm tip, then press a 3×1 mm round magnet into the pocket. Hold each one flat for 30 seconds. Let fully cure before handling.

   

2. Seat the push arms. Install the two short push arms against the wall and the middle rack. Install the long push arm in the rack furthest from the wall. All three arms must be fully retracted and flush against the back of the guide plate before continuing.

   

Servo Installation

The servo drives the gear train. Its wire must be routed through the housing channel before the servo is mounted — the channel is not accessible once the servo is seated.

1. Mount the servo. Seat the servo in its pocket and secure it with the bottom screw.

   

2. Install the timing gear. Insert a 2 mm bar through the next set of holes and mount the timing gear. Orient it so the gear teeth for the middle dart mount — furthest from the drive gear, closest to the outer wall — face upward.

   

3. Install the drive gear. Insert a 2 mm steel round bar through the first set of holes from the servo and slide the drive gear onto it.

   

Setting the Timing Gear

The timing gear engages the push arms based on the servo movement. It must be installed in the correct orientation to ensure smooth operation. If it is off by even one tooth, the mechanism can bind or fail to fully extend.

If the mechanism appears to cycle correctly but the servo stays engaged after activation, the issue may be overextension rather than tooth alignment. In that case, reduce the servo travel in config.py before re-timing the gear.

1. Power on and arm. Connect the harness from Stage 01 and arm the system with the magnet. The servo should extend all three rows fully. If it doesn’t reach full extension, pull the timing gear axle and re-seat the gear one tooth at a time until arming produces full extension.
2. Verify the full cycle. Run a complete arm-and-disarm sequence. Darts extend in order (outer → middle → inner), retract in reverse (inner → middle → outer). No binding, no straining noise, and the servo must go quiet after each move completes.
3. Close the gearbox cover. Once timing passes, seat the cover and secure with the cover screw.

   

Power Button & LiPo Battery Optional

The power button only works when a LiPo battery is installed. If the device is powered by USB-C, the switch only disconnects the LiPo battery. That means the prop can appear to be powered on from USB while the battery is disconnected and not charging.

If you charge over USB-C, make sure the power button is in the correct state so the battery stays connected to the board. The battery-monitoring logic tries to detect when USB is plugged in, but the board cannot reliably tell whether USB and the battery are both connected at the same time. For charging behavior and normal power use, see Power and Charging. The recommended battery is the Adafruit 500 mAh LiPo, which fits the standard compartment and uses the correct polarity for the board.

If you are checking this stage out of sequence, use the Stage 01 power-button prep lengths: 3 inches on the LiPo side and 6 inches on the MCU side.

1. Install the power button. Prep the switch as shown in the Stage 01 harness guide power-button step, using the 3-inch LiPo lead and 6-inch MCU lead, then press it fully into the battery cover slot and confirm it is seated evenly.

   

2. Connect the LiPo and place it in the battery bay. Plug the battery into the power lead, then slide it into the compartment with the connector end toward the wire bay.

   

3. Route the wires to the MCU compartment. Feed the power-button wires and battery lead through the small slot into the microcontroller compartment, alongside the servo wiring. Make sure nothing is pinched.

   

4. Install the battery cover. Once the switch and battery wiring are routed cleanly, seat the battery cover and secure it.

   

Speaker & Microcontroller

The speaker is a transducer that uses the enclosure and magnet to produce sound. It does not have its own enclosure or mounting hardware — it simply presses against the gearbox wall on top of the magnet mount.

The microcontroller seats next to the speaker in the remaining space at the back of the gearbox. The board buttons align with the cover buttons, and all wiring routes through the wire bay between the speaker and the battery compartment. Route the reed switch through the front hole of the gearbox so it can be attached at the front of the gauntlet.

1. Mount the speaker magnet. Glue a 20×5×3 mm magnet into the speaker mount slot on the gearbox wall.

   

2. Attach the speaker. Press the transducer speaker flat against the magnet. It should hold without any additional fasteners.

   

3. Install the buttons. Press the tactile buttons into their pockets in the microcontroller cover so the smaller actuator tips poke through the holes on the outside of the cover.

   

4. Seat the microcontroller. Place the board into the gearbox enclosure next to the speaker, aligning the board buttons with the cover buttons from the previous step. The USB-C port should face the access cutout.

   

5. Connect and close. Plug in the terminal block from Stage 01, the servo connector, and the battery JST (if installed). Route the wiring cleanly through the wire bay, then seat the completed microcontroller cover.

   

   

   

6. Install the rear retaining clip. Seat the rear retaining clip onto the back of the gearbox and press it into place. The retaining clip will be glued to the gauntlet to ensure the unit retains its position.