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• Breadboard Development Complete

  John Opsahl • 4 days ago • 0 comments

  The win for this week was completing the breadboard development and getting towards a proof-of-concept prototype. Several new mechanical, electrical, and software features came together to make this work.
  

  Mechanical. I redesigned the enclosure to better capture the digital tire depth gauge and spring. I increased clearances between parts to ensure that the design would still print well even on a hobby level 3D printer. I also made some modifications to the plate that sits over the digital gauge PCBA to allow access to the button pads. 
  

  Electrical. I added two 2N7000 mosfets. One to control power to the digital gauge. The other to achieve a "Units" button press. 

  Software. Successfully able to read the digital signal from the digital gauge, capture current time from the RTC, and record to a SD card. I have also incorporated a sleep routine for power saving. The RTC alarm gets set, Arduino goes to sleep, and the RTC wakes up the Arduino at the time of the alarm.

  The maximum amount of time that the device can be asleep for is 4 minutes. Any longer and the digital gauge itself goes into a sleep mode from inactivity. The "Units" are toggled from "mm" to "in" and back to "mm" each time the Arduino is woken up to ensure that the digital gauge does not go to sleep. 

  Unlike other calipers, the digital tire depth gauge does not store the last state prior to going to sleep or powering down. Instead, it resets to zero anytime it turns on. The only way to combat this and not allow reset to zero for the OpenDendrometer application is to ensure that the digital gauge always remains active. 

  Next Steps. Only remaining feature to add is the battery and battery voltage monitoring circuit. Thought is that once the voltage drops too low, cut power to the digital gauge and maybe some of the other components. Save enough power to transmit the data when wireless capabilities are added.

• Breadboard Development Started

  John Opsahl • 07/25/2022 at 03:48 • 0 comments

  Big success for this week was being able to read and interpret the 24bit output signal from the digital tire tread depth gauge. 

  Then added a SD card and RTC module for data logging capabilities. Everything is looking good after I ran some initial functional tests for each module.

  I have decided to move away from the ESP32 initially because wireless data transfer isn't needed for the mechanical proof-of-concept. I have lots of experience with the Arduino Pro Mini data loggers so I can throw one of those together in a couple of hours. The most important thing at this stage of the project to get this device mounted on a tree and see if it can accurately measure tree diameter changes.

• Cost Estimate

  John Opsahl • 06/11/2022 at 15:33 • 3 comments

  Initial cost estimate for a single OpenDendrometer is $70. The 3D printed parts are the largest contributor to total cost. So if you have access to a 3D printer, you could build an OpenDendrometer for around $45.

  I have planned to develop a custom PCB to make assembly very easy, but it's likely possible to make all the connections with jumper wires. Using jumper wires would shave an additional $8-10 off the total cost. 
  

  All costs for purchased items in the table below are based off of list price on Amazon.com. I would guess that sourcing all of the purchased components from Aliexpress could reduce total cost by 20-30%. The downside is that lead time for items purchased from Aliexpress is usually somewhere between 3 and 8 weeks.

  Qty	Description	Cost Per Unit	Total Cost
  1	digital tire tread depth gauge	$8.99	$8.99
  1	ESP32 microcontroller	$6.66	$6.66
  1	lithium battery, 1800mAh	$7.18	$7.18
  1	micro SD card module	$1.00	$1.00
  1	micro SD card	$4.99	$4.99
  1	DS3231 RTC module	$3.40	$3.40
  1	logic level shifter, 4 channel	$1.60	$1.60
  2	2N7000 mosfet	$0.08	$0.16
  1	custom PCB	$10.00	$10.00
  1	custom 3D printed parts	$25.00	$25.00
  1	screw, 200mm	$1.00	$1.00
  		Total	$69.98

• 3D Printed Weatherproof Enclosure

  John Opsahl • 06/08/2022 at 04:38 • 0 comments

  I have always thought of FDM 3D printed parts as somewhat porous. As though defect holes in the wall of the part are inherit to the hobby level 3D printing process. I assumed that some kind of sealant would need to be applied to achieve any level of water ingress protection. Recent testing of the first OpenDendrometer enclosure prototype has shown that these assumptions are not true. 
  

  Now, I am only trying to develop a weatherproof 3D printed enclosure (some might call it water resistant); not a waterproof one. So the requirements are less challenging, but I still expected to encounter issues. Incidentally, I think I achieved a weatherproof 3D printed enclosure on the first iteration.
  

  Really all I did was to copy the foam cord water seal strategy used on almost all small ingress protection rated electronic enclosures. It involves a foam cord gasket between the case and the cover. With all fasteners between case and cover on the outside of the gasket seal and features in each to either capture or compress the foam cord.
  

  To test the seal, I put a piece of tissue paper inside and placed it in a bowl of water. After five minutes in the bowl of water -> voila, the tissue paper was still dry. 

  Even without applying any sealants to the 3D printed parts, it achieved a 5 minute waterproof seal. Which should be more than enough for just a weatherproof enclosure that will only see occasional precipitation. 
  

  I suspect that UV exposure may damage the plastic enough overtime to cause water ingress, but all testing is going to be fairly short at less than a month. I will worry about how to combat long term UV exposure once the design passes system level testing. 

• Preliminary Competitive Analysis

  John Opsahl • 05/15/2022 at 20:24 • 0 comments

  The following table details existing commercial, academic, and DIY dendrometer solutions: 

  Source	Model	Link
  Channel Technology	Circumference DC2 & DC3	http://channelinstruments.com/article/1349.html
  Channel Technology	Circumference Dendrometer 1 (DC1)	http://channelinstruments.com/article/1348.html
  Channel Technology	Fruit and Vegetable Dendrometer (DF)	http://channelinstruments.com/article/1355.html
  Channel Technology	Root and Aquatic Plant Dendrometer (DRO)	http://channelinstruments.com/article/1356.html
  Environmental Measurement Japan	MIJ-02 Rotary Type3	https://environment.co.jp/en/dendrometer
  Wafer Sensor, Inc.	Automated Dendrometer D6	http://www.wafer-sensor.com/html_products/D6-129.html
  ICT International	DEN1 Dendrometer	https://ictinternational.com/products/den1/den1-dendrometer-meter/
  Natkon.ch	ZN12-O-2WP Point Dendrometer	https://natkon.ch/portfolio/zn12-o-2wp/
  Environmental Measurement Japan	Absolute Linear Dendrometer	https://environment.co.jp/en/absolute-linear-dendrometer-mij-02lm-typeii-2/
  kinasmith	Dendrometers!	https://publiclab.org/notes/kinasmith/09-09-2016/dendrometers
  Phytech	Fruit Dendrometer	https://www.phytech.com/post/the-art-of-controlling-fruit-size
  OregonState	Dendrometer	https://github.com/OPEnSLab-OSU/OPEnS-Lab-Home/wiki/Dendrometer

• Initial Component Selection

  John Opsahl • 05/15/2022 at 20:07 • 0 comments

  Displacement Measurement. A digital tire tread depth gauge is the lowest cost linear displacement measurement solution that I have found so far. The most important consideration is that it has the required resolution of 10um and minimum range of 12mm. Fortunately, others have already determined how to use an Arduino to log length measurements from these digital gauges -> https://www.instructables.com/Caliper-Data-Interface/. The only real challenge being that these devices use 1.5V logic so a logic level shifter is required. 

  Microcontroller. The ESP32 is the obvious choice to meet the low-cost and wireless data transfer requirements of the OpenDendrometer.

  Real Time Clock. I have used DS3231 RTC modules in the past for data logging projects with great success. The primary reason for using the DS3231 module is that the ESP32 internal clock is not accurate enough for the month or greater operational period. The second reason is that I will likely be configuring the DS3231 to wake up the ESP32 from long periods in sleep mode in an effort to minimize ESP32 power consumption.

• Preliminary Requirements

  John Opsahl • 05/15/2022 at 19:33 • 0 comments

  The following is a list of preliminary requirements for the OpenDendrometer: 
  

  Type	Requirement
  shall	cost less than $60 in materials
  shall	accommodate tree diameters >=100mm
  shall	be able to be configured as a point dendrometer
  shall	<= 10um linear displacement measurement accuracy, point dendrometer
  shall	range of up to 12mm, point dendrometer
  should	be able to be configured as a circumferential dendrometer
  should	<= 0.025mm diameter change measurement accuracy, circumferential dendrometer
  should	be able to be configured as a fruit dendrometer
  should	<= 0.1mm diameter change measurement accuracy, fruit dendrometer
  should	be human tamper proof
  should	be wildlife tamper proof
  shall	log length measurement twice an hour
  shall	be capable of measuring both expansion and contraction
  shall	alert the user when battery charge is low
  shall	transfer data via Bluetooth
  shall	be contained in a weatherproof enclosure
  shall	operate within temperature range 0 to 55°C
  should	operate within temperature range -20 to 55°C

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