Build Guide: Underwater Magnetically Activated Light
Overview
The Underwater Magnetically-Activated Light is a test unit with the same magnetometer and LEDs as the Pipeline Sensor Lights used in RoboSub 2026 and RobotX 2026.
This is presented as a standalone unit that teams can build to practice and train their systems. This unit is battery powered, whereas the competition unit is powered from shore.
This unit does not have to be duplicated exactly. The components that should be the same are the magnetometer and the LED ring.
Key specs:
Magnetometer
Adjustable distance from outside of the top cover: 0.2" (5mm) to 0.5" (13mm)
Centrally located, inside the LED Ring
LED Ring
16 WS2812B LEDs
Full brightness
RED or GREEN
Triggering Conditions
TDB mT
Magnet held at magnetometer for 0.5 seconds
Source Code
GitHub repository: Underwater-Magnetically-Activated-Light
Parts List
Enclosure, ABS Plastic Electrical Box 4.9 x 4.9 x 3 inch(125 * 125 * 75mm) See note regarding seal and testing
1
Magnetometer, Adafruit TLV493D Triple-Axis Magnetometer, Part number 4366
1
18650 3.7V Lithium Battery
2
Self source
Resistor, 10KΩ ¼ Watt
1
Self source
Resistor, 470Ω ¼ Watt
1
Self source
Component Mounting Plate, 2.5 mm (0.09") thick X 108 mm X 115 mm Acrylic sheet
1
LED Supply Wiring Harness (male connector) BTF-LIGHTING JST SM 3 Pin Connectors for WS2812B
1
20 Pair Pack Amazon.com Contains male and female connectors
LED Driver Wiring Harness (female connector) BTF-LIGHTING JST SM 3 Pin Connectors for WS2812B
1
Use above link
Thermistor Wiring Harness
1
Uses 10K resistor (self source) and female Dupont connector terminals See photo in this guide
ESP32 Power/Ground Jumpers, Dupont Female-Female jumper wires 2 (PWR/GND) x 20 cm long. Can substitute with other wires
1
Reference 240 pcs set Amazon.com Check DigiKey, Mouser, Adafruit, etc.
Magnetometer Jumpers, Dupont Female-Female jumper wires 4 (PWR/GND/SDA/SCL) x 10 cm long. Can substitute with other wires
1
Use above link
M3 x 8mm Long Pan Head Machine Screw
4
Comes with DFRobot power bank
M3 x 12 mm Long Flat Head Machine Screw, steel or stainless steel
4
Reference Amazon.com May be available at local hardware stores in smaller quantities
M2 x 5 mm Long Nylon Pan Head Machine Screw
2
Reference Amazon.com May be available at local hardware stores in smaller quantities
M2 Nylon Hex Nut
2
Comes with kit linked above May be available at local hardware stores in smaller quantities
M3 Nylon Insert Locking Hex Nut, steel or stainless steel
4
Assorted kit Amazon.com May be available at local hardware stores in smaller quantities
Male pin headers, 1-position. Standard 2.54mm pitch type compatible with Dupont headers
7
Reference Amazon.com Wires may be soldered directly LED ring instead of using header pins on ring and Dupont terminals on wires
Parts Notes:
The following parts may be substituted:
The enclosure listed is IP67 and was tested at 10 psig for 4 hours.
The microcontroller was chosen for size and convenience connecting the wires.
The thermistor is optional. The thermistor is used to measure the internal air temperature and the LEDs are powered off at a set threshold in code.
The capacitor can be substituted for other physical sizes and ratings; 470uF to 1000uF is recommended.
The 3D printed parts may be redesigned or eliminated completely. The LED ring and magnetometer can be taped to the enclosure cover.
The power bank was leftover from another project and is well suited, but may be easily substituted.
All wiring harnesses may be substituted. The LED power and driver wiring harnesses are standard LED connector pigtails
3D Printed Part Files
Fabricated Parts
Enclosure - Additional Information
Various enclosures were tested. Only the enclosures with silicone seals will seal against water pressure. Enclosures with black foam seals leak at low pressures. The enclosure referenced in the parts list was tested at 10 psi (equivalent of 23 feet in freshwater) for 2 hours. Longer period tests were performed at lower pressures.
ESP32
System Schematic
Note: SDA and SCL on the ESP32 were not used for the magnetometer due to a library conflict. The magnetometer's SDA and SCL pins are connected to GPIO6 and GPIO7 respectively and configured in the source code.
Construction
Solder pin headers as shown
Solder two sets of male pin headers to the 5V rail of the power bank. The photo shows three sets. Add more sets as needed for additional power connections.
Remove the power bank's base plate if it was pre-installed.
Mount power bank on outside of the short flange of the base plate with screws supplied with the power bank.
Mount the magnetometer board to the sensor/LED mounting plate using two M2 x 5 nylon pan head machine screws and hex nuts.
Set the LED ring on the sensor/LED mounting plate. Use hot glue to anchor the LED to the mounting plate. Hot glue can be applied on the bottom side in the holes around the pin headers.
Mount the electronics assembly to the base plate using four M3 x 12mm flat head machine screws and nylon insert lock nuts. Use hot glue to anchor any Dupont connectors that feel loose.
Testing
A 10mm x 3mm thick neodymium magnet will trigger a 1.5mT threshold at 23mm from the face. An 18mm x 3mm thick neodymium magnet will trigger a 1.5mT threshold at 39mm from the face. Magnet orientation (face vs edges) of course matters. You will need to experiment and choose the magnet.
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