Solving the Christmas Tree Problem: A Wireless, Rechargeable eInk Design

Background

Last year, I developed a basic device to monitor the water level in a Christmas tree stand using transistors as switches, which controlled LED indicators. My purpose was to solve the “Christmas Tree Problem”, which refers to the safety hazard that dry Christmas trees pose. Dry trees can ignite and become raging fires in less than a minute, a risk amplified by surrounding electric lights, fireplaces, radiators, candles and more. The device I created was designed to reduce the fire risk by alerting homeowners when their tree reservoir was low on water. While this device had promise, there were several limitations, including: wired connections, rapid drain of a single-use battery, imprecise measurement detail (only 3 LEDs),  and cumbersome casing.

Purpose

The purpose of this project is to make meaningful improvements to last year’s design and create an innovative device that will help prevent Christmas tree fires.

Procedure

Research low-cost, miniaturized microcontrollers that can transmit/receive Bluetooth. Seeed Studio XIAO nRF52840 fit these criteria at $15 CAD.

Become familiar with pin map for Seeed chip, E-Ink display, and eTape

Plan Seeed chip circuits with eTape and E-Ink on temporary breadboard

Develop eTape code in Arduino IDE to process the eTape input signal, calculate water level height, and transmit data packet using Bluetooth

Develop E-Ink display code in Arduino IDE; this receives Bluetooth data from eTape sensor circuit and outputs this data onto eInk display

Spend countless hours debugging steps 4 and 5.

Once the temporary circuits work, commence the “BSJ” (Big Soldering Job), where all components are soldered to a PermaProto board

LiPo battery soldered to the BAT+ and BAT- terminals on Seeed chip. On/off switch added to preserve battery when not in use.

Design custom 3D prints on Tinkercad. Two designs are necessary for this project: the sensor unit case and an ornamental enclosure for the display unit. Decided to use a box-shaped “present” design for optimal space efficiency. Design sensor unit case to contain loose wires (Figures 1. and 2.).

Import design into a slicer software (Anycubic) and then export .STL file into 3-D printer. Repeat this dozens of times until satisfaction.

Final assembly of sensor and display units: 3M VHB tape used to bond PermaProto board to cases; brass heat inserts to affix E-Ink display; rare-earth magnets glued into enclosures

Materials used

Lithium Polymer (LiPo) battery

Seeed Studio XIAO nRF52840

Temporary breadboard

PermaProto board

Magnets

E-Ink display

eTape

Glue

Jumper wires

3-D printing plastic (PLA)

3-D printer

Soldering iron

On/off switch

Wires

Tinkercad and Anycubic slicer software

Brass heat-set inserts

3M VHD tape

How it works

My new system consists of two wireless units that communicate over Bluetooth Low Energy. The first is the sensor unit, which would sit inside your Christmas tree reservoir. Last year, I discovered I could use three transistors to detect the presence of water at three different levels. This year, the sensor unit uses an eTape liquid level sensor, which provides continuous data. The eTape is a very interesting electrical component because it functions as a variable resistor. What this means is that its resistance changes depending on how much water it is submerged in. When you put the eTape in your tree base, the hydrostatic pressure pushes its two conductive layers together. This shorts out the resistor below. The higher the water level, the more compression on the eTape, resulting in more of the resistor being shorted out and creating a lower resistance.

My device then uses the eTape voltage divider circuit to convert resistance to voltage, which can be read by the Seeed chip. The circuit divides the voltage coming out of the 3.3V battery between a fixed resistor and the variable resistor. As the water level changes, the variable resistance changes, and ultimately the midpoint voltage changes according to the following formula.

Midpoint voltage formula

V_midpoint = 3.3V x R_eTape / (R_fixed + R_eTape)

Seeed chip

The 12-bit analog to digital converter (ADC) is the first step inside the Seeed chip, which can convert the analog voltage into 4096 (2^12) possible digital ADC readings that the Seeed chip can understand.

The raw ADC number has to be translated into an actual water level. Initially, I calibrated two points (full and empty) by measuring the ADC output at these points. However, when I tested my device, the readings were inaccurate. I realized that this was because the readings were nonlinear as the water level changed, with large ADC value swings at low water levels. I solved this issue by building a calibration table at multiple points along the eTape. The code then interpolates between those points to calculate an accurate height and percentage displayed.

The Seeed chip broadcasts this data via bluetooth, which is received by a second Seeed chip inside the display unit case.

eInk display

The eInk display is the same low-power display technology that is used in e-readers like Kindles. It only consumes power when the screen is updating, and it uses essentially zero power while displaying. The eInk display replaces the LED lighting from last year's design, conserving battery and displaying data from the eTape in a more detailed way.

Battery

Both units run on a 400 milliamp-hour rechargeable lithium polymer battery. This replaces the disposable 9V battery from last year's design, which is better for both battery life and the environment.

Last year, my device was only able to last a couple hours with the 9V battery. With the upgrades implemented this year, my design lasts for several days.

The goals of this project (to eliminate wires, miniaturize the design, incorporate eTape and an eInk display and on/off switch, and replace the 9V battery with a LiPo battery) were achieved. After a thorough redesign process, this device is now an accurate, innovative tool to ensure a Christmas tree stays hydrated, reducing the risk of house fires and death.

Dry Christmas trees can turn into raging fires in less than a minute. Most trees are surrounded by electrical equipment, candles, light bulbs, or fireplaces. Hydrated Christmas trees can still catch on fire, but the fire will be much slower and less intense, providing time to react. This device may help prevent fire-related injury and death.

This year's project solved many limitations of last year's design and revealed new opportunities for improvement.

I would like to replace the Bluetooth microcontroller with a WiFi microcontroller, allowing the device to connect to a home's WiFi network and send water level data to a smartphone app.

I am working on code that implements the deep-sleep function of the Seeed chips. The two units have trouble recognizing each other when they wake, but I expect this to extend battery life to several weeks.

The 3D-printed enclosures are not waterproofed. Sealing them would make the device more practical for real-world use.

Thank you to the Annapolis Valley Regional Science Fair coordinators — Christine Popowich, Caitlin Mitchell, and Rob Davies — for providing so much support and volunteering to organize the regional fair.

I would also like to thank my family, for being there for me throughout the numerous hours of work it took to get this project to CWSF.

Thank you to my dad, for helping me pay for all the materials I needed to create this device!

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