Build and tune the gearbox

Space inside the gearbox is tight. As you install the electronics, keep the wiring tucked close to the housing and routed through the available channels so the covers can seat cleanly and the LED assemblies have the clearance they need.

Push Arms and Hand Test

The push arms transfer the gearbox movement to the three dart rows. Before installing the servo, assemble the linkage and cycle it by hand. This is the easiest point in the build to find and correct tight spots before the mechanism is powered.

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 for the hand test. Orient the gear so the teeth nearest the outer wall face upward. With all three push arms fully retracted, lower it into the dwell of the racks below and slide a 2 mm bar through its holes to hold it on its axle.

   

3. Hand-cycle the mechanism. Extend and retract the push arms through their full travel several times. Movement should be smooth from end to end. Minor catching as a rack first engages can occur with fresh prints and will often wear in after a few cycles. If resistance remains consistent or significant, stop and correct it before installing the servo.
4. Remove the timing gear. Pull the axle and set the timing gear aside. It goes back in during timing setup, after the servo and drive gear are installed and the unit has been powered on.

Servo Installation

With the push-arm mechanism moving freely, the servo can now be installed. Route the cable before seating the servo, because the wire channel becomes inaccessible once it is mounted.

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.

   

2. Route the servo wire. Thread the servo cable through the bottom of the gearbox and along the wire channel toward the microcontroller compartment.

   

3. Mount the servo. Seat the servo in its pocket and secure it with the mounting screw.

   

   

Drive Gear

With the servo mounted, install the drive gear: it transfers motion from the servo's bevel gear into the timing gear. Leave the timing gear out for now: it goes in during timing setup, after the unit has been powered on.

1. Install the drive gear. Insert a 2 mm steel round bar through the first set of holes (closest to the servo) and slide the drive gear onto it so it meshes with the servo's bevel gear.

   

Setting the Timing Gear

Proper timing keeps each row within its intended travel and prevents the servo from pushing against the end of the mechanism. Power the unit on first, then install the timing gear against the servo's settled position, and verify a complete arm-and-disarm cycle before continuing.

1. Power the prop on, timing gear still out. 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.
2. Install the timing gear. With the unit disarmed and the servo settled, seat all three push arms fully retracted, then install the timing gear the same way as the hand test: teeth nearest the outer wall (furthest from the drive gear) facing upward, lowered into the dwell of the racks, with a 2 mm bar through the next set of holes to secure it on its axle.

   

3. Verify the full cycle. Arm the system with the magnet. The push arms extend in order (outer → middle → inner). Disarm; they retract in reverse (inner → middle → outer). Three failures are likely here:
   • The inner push arm doesn't fully extend: the timing gear is seated a tooth short. Pull the axle, move the gear one tooth toward extension, re-seat it, and run the cycle again.
   • The long push arm doesn't fully retract: the gear is a tooth over. Move it one tooth the other way and retest.
   • Nothing moves at all: the mechanism is binding. Power off, pull the timing gear, and re-check the movement by hand before trying again.
   Adjust a single tooth per attempt and retest after each change.

   

4. Fine-tune the servo angles, only if needed. The v2.5 defaults ([0, 50, 100, 160]) work for most builds; skip this step if the cycle is already clean. If a row still over- or under-travels with the gear timing right, 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

Install the completed Stage 01 harness and electronics into the gearbox, keeping the wiring compact as the compartments are closed.

1. 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 plugs into this connector from outside the case; connect it now so you can arm the prop during bench testing. Its final mounting position inside the gauntlet is set in the Installation Guide.

   

2. Connect and insert the microcontroller. Connect the battery and servo leads before seating the board. With the battery still outside its bay, plug the battery JST into the battery port and connect the servo lead. Then insert the MCU at an angle: point the USB-C port toward the access cutout, slide it in so the port end hooks into the slot, and rotate the board down flat into the compartment.

   

3. Place the LiPo. Slide the connected battery into its bay with the JST connector toward the wire bay. USB-only build? Skip the LiPo placement; the rest of the wiring proceeds unchanged.

   

4. 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, routing the leads along the edge of the LED bay. Lead lengths are set in the Stage 01 power-button rewire.

   

5. Attach the speaker. Peel the white protective backing from the adhesive side of the box speaker and stick the speaker into its mount, pressing it firmly in place. Route the speaker leads so they do not prevent the MCU cover from seating.

   

6. Tuck and inspect the wiring. Lay the wires flat inside the MCU compartment so the cover can seat without pinching them. Before closing it, confirm that no lead crosses into the LED bay.

   

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

   

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.

   

LED Housing

Finish the moving assembly by installing the LED rows. Each housing should seat securely on its push arm while leaving the short link wires free to move through the full stroke. The inner and middle rows mount on French cleats: press each into place until fully seated and release it by tilting the row down, and the outer row rides the long arm's tab connection.

1. Route the LED strips. Starting from the terminal plug, press each LED strip into its housing channel following the routing path in the figure. The fit is snug; do not force it. Pay attention to the strip orientation in the diagram (outer row faces inwards, middle row outwards, inner row inwards), and make sure each LED sits centered on its fiber-optic hole. Keep the inner- and middle-row lead exits centered between their two printed wire guides.

   

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

   

3. Attach and check the housings. Seat the inner and middle rows on their cleats, then connect the outer row to the long push arm. Test each housing on its push arm: they should stay on the arms when the mechanism is activated. Without the fiber optics through the nozzle holes to keep the rows tilted up, they can still fall off at this stage; that is expected, not a fault.

   

4. Run the completed mechanism. Arm and disarm the prop several times while watching the LED housings and the short wires between them. The housings should travel freely through the full stroke, and the link wires should hang in loose loops without catching, pulling tight, or rubbing against the gearbox. The housings may sit slightly off-axis at this stage; the fiber optics and nozzle establish their final alignment during installation.