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 servo clip 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 and LED Housing

The front modules are designed to enable retrofit into existing gauntlets by disconnecting and separating from the base unit — feed the fibers through the gauntlet from below, then reattach, with no need to remove the top enclosure. The push arms are the mechanical linkage that makes that disconnect possible: the two short arms now retain the inner and middle rows with printed hooks, those inner and middle housings now add wire guides on both sides of the lead exit, and the long arm drives the outer row through its own hook-and-tab connection.

1. Route the LED strips. Starting with the inner row, press each LED strip into the housing channel. The fit is snug — do not force it. The blue diode on each LED must face forward (toward the nozzle). Work from inner to outer row, and keep the inner- and middle-row lead exits centered between their two printed wire guides.

   

2. Secure the LED covers. Apply glue to each dart-mount cover, then press it down over its row's LEDs to bond the cover to the housing.

   

Gear Train

Seat the push arms, then install the timing and drive gears and test the gear train by hand before adding the servo. This catches binding from debris, layer lines, or fit issues while the system is still easy to fix.

1. 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.

   

2. Install the timing gear. Orient the gear so the teeth nearest the outer wall (furthest from the drive-gear position) face upward. Lower it into the dwell of the racks below, then insert a 2 mm bar through the next set of holes to secure it on its axle.

   

3. Install the drive gear. Insert a 2 mm steel round bar through the first set of holes (closest to the servo position) and slide the drive gear onto it. The drive gear meshes with the timing gear.

   

4. Test the gear train by hand. Push and pull the push arms through their full travel. The timing gear should rotate smoothly, the drive gear should spin freely, and the arms should move without significant binding. Some minor catching when a push arm first engages is normal — these parts are 3D printed, and small surface imperfections often need to wear in. Cycle the gear train through its full range several times; light catches usually clear after repeated operation.

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 mounting screw.

   

   

2. Install the servo clip. Snap the servo clip down over the gear train. There is no screw — the clip retains itself by friction.

   

3. 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.

   

Setting the Timing Gear

The timing gear must be aligned with the servo's "off" position so the push arms rest fully retracted when the prop is disarmed. The cleanest way to achieve this is to lift the timing gear out of mesh, power the prop on so the servo settles to its default off position, then drop the timing gear back in at the correct alignment.

1. Lift the timing gear out of mesh. Pull the timing gear's axle so the gear can rise off the racks. The drive gear stays in place. The timing gear can rest loose in the housing for now.
2. Power the prop on. Connect the harness from Stage 01 and turn the prop on. The servo rotates to its default off position. Wait for it to settle and go quiet before moving on.
3. Reposition the timing gear. With the servo at its off position, lower the timing gear back into the racks at the correct alignment — outer-wall teeth facing upward, all three push arms fully retracted. Reinsert the axle.
4. Verify the full cycle. Arm the system with the magnet. Darts extend in order (outer → middle → inner). Disarm — they retract in reverse (inner → middle → outer). No binding, no straining noise, and the servo must go quiet after each move completes. If a row doesn't reach full extension or doesn't retract fully, pull the axle and try one tooth offset.
5. Calibrate the servo angles. Arm the prop and fire each row in turn. Watch how far the servo brings each push arm back during retract. If a row over- or under-travels — or the servo strains at the end of a move — fine-tune the matching value in _SERVO_ANGLES in config.py and retest. The four values correspond to the off, outer, middle, and inner servo positions.

Installing the wiring harness

The power button toggles the board on and off through the microcontroller's EN line. The LiPo plugs directly into the board's battery JST and stays connected to the charging circuit, so USB-C charges the battery whether the prop is on or off.

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, refer to the Stage 01 power-button rewire for the 3 3/4″ board-side lead length (the switch-side is trimmed for an exact fit, typically 1 1/2″).

1. Mount the speaker magnet. Glue a 20×5×3 mm magnet into the speaker mount slot on the gearbox wall. Doing this first lets the glue cure while you work on the wiring harness.

   

2. Install the power button and connect it to the MCU. Press the switch (prepped in the Stage 01 harness) fully into the slot on the side wall of the battery compartment. Plug its JST onto the long lead from the microcontroller's EN/GND header.

   

3. Place the LiPo and route the power wires. Slide the battery into its bay with the JST connector toward the wire bay. Route the power-button leads along the edge of the LED bay to keep them clear of the gear train, then plug the battery JST into the microcontroller's battery port. USB-only build? Skip the LiPo placement — the rest of the wiring proceeds unchanged.

   

4. Route the servo wire and plug it in. Run the servo cable along the side channels of the gearbox toward the MCU compartment, then plug the servo connector into the microcontroller.

   

5. Install the reed switch JST into the front of the case. From inside the gearbox, slide the harness-side JST connector into the JST mount at the front of the case so it seats firmly. The reed switch then connects from outside — mount the reed switch itself as far forward as possible inside the gauntlet (this positions it closest to the hand-plate magnet), then plug its mating JST into the connector at the front of the gearbox.

   

   

6. Insert the microcontroller, then attach the speaker. Lower the MCU into its compartment with the USB-C port facing the access cutout. Press the transducer speaker flat against the magnet on the gearbox wall — it should hold without any additional fasteners.

   

   

7. Tuck the wires. Position all wires in the MCU compartment so they sit flat and won't be pinched — this is what enables the cover to seat clean and flat. Keep the LED bay clear of any wiring or stray leads.

   

8. Install the buttons in the MCU cover. 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.

   

9. Seat the MCU cover. Insert the front edge of the microcontroller cover under the front lip of the gearbox first. Then carefully lower the cover so the button guides fit over the on-board buttons without pinching any wires, and snap the sides into place.

   

10. Attach the LED housing to the push arms and test. Seat the inner and middle rows fully into the short push-arm hooks, then connect the outer row to the long push arm. Verify all three rows attach and detach cleanly. Arm and disarm the system — the housing should travel smoothly through the full stroke with no binding.

    

11. Install the battery cover. Seat the battery cover over the battery bay and secure it.

    

12. Test arming and disarming — check the LED-row link wires. Cycle the prop through arm and disarm and watch the short wires that link the LED rows. They should hang down in a small loose loop, and as the housings travel forward and back, those wires should not catch or hang on anything. The LED housings themselves may shift or sit slightly off-axis at this stage — the fiber optics and nozzle in Stage 03 keep them straight. What matters here is that the wires between the arrays move freely through the cycle.