Geocache Riddle Box Logbook Ejector

This guide describes a fail-safe logbook ejector, designed for installation inside a Geocache-Riddle-box.

The round design allows a easy opening in a wooden board, using a standard hole saw.

Logbook container size Ø 40 x 75 mm/ Ejection hole diameter Ø 50mm

The eject mechanism is operated by an SG90 servo controlled by a microcontroller.

Operation

1. At the servo position of 0°, the logbook container is locked.
2. Moving the servo to 90° ejects the container.
3. The ejection movement from 0° to 90° and back to 0° only needs about 1 second.
4. Reinserting the logbook container by hand, automatically locks the mechanism again.

3D printer and filament

Hot glue gun

Soldering iron and solder wire

1 nail or steel pin Ø 4 x 60 mm

1 compression spring Ø 4 x 20 mm (e.g. from a ballpoint pen)

1 SG90 servo with servo horn

1 microcontroller (e.g. RP2040)

Some thin stranded wire

8 toothpicks

10 M3 x 8 screws for plastic

Sanding paper

Superglue or plastic glue

Print the STL files. I used a 0.4 mm nozzle with 0.20 mm layer height (“strength” settings).

First, assemble the “Locking-rod” and the compression spring into the “MainBody” and secure it with the steel pin (or nail). Secure the steel pin on both sides with a small dot of hot glue to prevent it from slipping out.

The locking mechanism must move without friction. If necessary, smooth the edges slightly with sandingpaper and drill out the hole in the “Locking-rod” to 4.5 mm using a drill.

The lid “Top_Lid” and the base “Mounting_plate” can be accurately positioned using toothpicks, inserted into the holes. The base should be mounted according to the final assembly so that the mounting holes are in the correct position. Then glue both plates together. If needed, use screws where the toothpicks are, after the glue is dry.

Attach the servo depending on the installation position, on either the left or right side. Secure it with 2 screws together with the “Servo_Cover”.

Adjust the servo arm correctly and shorten it if necessary.

At the 90° angle position, the locking rod should be pressed in, by approximately 95%. The servo should not be mechanically blocked during operation.

When the microcontroller starts up, the servo should be set to 0°.

To trigger the mechanism, move the servo to 90°, wait approximately 1 second, and then return it to 0°.

This should provide sufficient time for the container to release properly.

By returning the servo to 0°, the container can then be easily reset manually by pressing it back into place until it locks.

Happy Geocaching ;-)