Bought Yl 69 Moister Sensor

Summary

The YL-69 and YL-38 is a popular "paired" item with online electronic retailers - generally referred to as YL-69. However, like any product that does not come with a "User's Guide" - myths & misinformation become the best known features. This article will address some of those issues and allow you to make the best of this purchase.

Prior to getting the YL-69 I had already done alot of research on soil moisture monitoring. Here are three (3) major points:

  • There are basically two types of sensors - resistive and capacative.
  • Resistive sensor probes should be galvanize to reduce corrosive effects - almost none are.
  • Using a probe for each plant is unrealistic and logically difficult to maintain.

It should also be noted that the YL-69 and YL-38 can be used together, separately, or combination with other components. The pair can also be used with a embedded micro computer system (such as an Arduino, Raspberry Pi, or similar). This article will cover different configurations.

The Two Pieces

The moisture sensor YL-69 is a two lead probe that is approximately 58mm long, 20mm wide, and 1mm thick, with two (2) external connection pins. The connection pins attach directly to each probe.

The YL-38 is an auxillary board with a LM393 (dual differential comparator), a potentiometer, supporting resistors, and six (6) external connection pins. Two (2) of the connection pins are intended for connection with the YL-69. The four remaining pins functions as

  1. Vcc
  2. GND
  3. D0
  4. A0

Power to the YL-38 (and the LM393) can be between 2 volts and 36 volts. The LM393 can be fed two different power sources with different voltage levels. I will not cover that here - refer to LM393 datasheet for additional details.

D0 and A0
This quote from randomnerdtutorials.com explains:

The output (on the YL-38) can be a digital signal (D0) LOW or HIGH, depending on the water content. If the soil humidity exceeds a certain predefined threshold value, the modules outputs LOW, otherwise it outputs HIGH. The threshold value for the digital signal can be adjusted using the potentiometer.

The output can be an analog signal (A0) and so you?ll get a value between 0 and 1023.

Again, the YL-69 and YL-38 can be used together or separately. More on that in the section below….

Corrosion

I want to be upfront with this. The YL-69 should NOT be permanently embedded in soil - be it one plant or many plants.

Several online article have talked about corrosion that occurs with the YL-69. This notion comes from the misunderstanding of the purpose of moisture sensor. The impression one gets from those articles is that the moisture sensor should be a permanent science experiment and that measurements should taken on a 24 hour, 7 day a week basis. This is an incorrect notion. Some articles even extend that notion by suggesting that the polarity of the probe should be reversed, if not daily, but after each measurement.

Let me state this quite clearly, having a permanent measurement device attached to a planet is silly and wasteful. The most reasonable use of a moisture sensor is to take regular measurement with a single device. To be emphatic, the probes should NOT be permanently embedded in soil - be it one plant or many. The probe should be used to take a measurement, then wiped down for the next soil measurement or the next time used. If this is done, corrosion will be avoid and the YL-69 will give years of trouble free service.

Also, I want to correct misinformation stated in one article I read. Using galvanized probes IS the proper way to deal with unwanted corrosion. For example, galvanized nails are widely used in the construction industry to reduce and minimize the effect of corrosion on metals. Google "hot dipped wire nails" (or search for "galvanized nails" on DuckDuckGo.com) to get a better idea of this.

One thing I should add. Running an electric current through the probes does tend to attract impurities to the surface of the probe. Running a much stronger current is used for a process known as electroplating. Switching the polarity of the probe tends to reduce this effect - hence the earlier suggestion to reverse the polarity. However, this action only slows the attraction of impurities on to the probe.

Using

As alluded to earlier, the YL-69 can be used as single unit or in pieces. Here are the possible configurations

  • YL-69 & YL-38 can be used with an embedded micro system (like an Arduino)
  • YL-69 & YL-38 can be the basis for a standalone moisture sensor
  • YL-69 (without the YL-38) can attach directly to an embedded micro system
  • YL-38 (without the YL-69) can be the basis for a standalone moisture sensor

Along with these possible configurations, the YL-38 can be calibrated to give a "YES" or "No" signal, and an analog signal. The YL-69, because of it's passive nature, can only return an analog signal.

Below I will give four (4) examples on how use the YL-69.

YL-69 & YL-38 with an Arduino

YL-38 part of a stand alone moisture sensor

YL-69 & YL-38 part of a stand alone moisture sensor

YL-69 with an Arduino


References

Articles

Guide for Soil Moisture Sensor YL-69 or HL-69 with Arduino - undated
https://randomnerdtutorials.com/guide-for-soil-moisture-sensor-yl-69-or-hl-69-with-the-arduino/
Basic operations guide for unit

Using the YL-39 + YL-69 Soil Humidity Sensor with Arduino - March 6, 2016
https://www.hackster.io/nekhbet/using-the-yl-39-yl-69-soil-humidity-sensor-with-arduino-968268
As Stated

The Soil Moisture Sensor - undated
http://gardenbot.org/howTo/soilMoisture/
Extensive notes on using a simple resistive sensor similar to the YL-69. It uses an arduino and other sensors.

Why Use Galvanized Nails?
https://www.doityourself.com/stry/why-use-galvanized-nails

Moisture Detection With Two Nails - undated
https://www.instructables.com/id/Moisture-Detection-With-Two-Nails/
Video is below

Videos

An Arduino Solution

Arduino using a yl-69 and yl-38 moisture sensor
Southern Bolt - Published on Jul 14, 2015
3:17

Common Misconceptions

#207 Why most Arduino Soil Moisture Sensors suck (incl. solution)
Andreas Spiess - Published on Jun 16, 2018
10:59

The Proper & Correct Way To Connect A YL-69 & YL-38 Moisture Sencor to your ARDUINO Board
Z-HUT - Published on Jan 20, 2017
8:03

Two Nail Plant Moisture Sensor for Arduino
Electronic Shenanigans - Published on Jan 18, 2015
3:42

DIY Self-contained Soil Moisture Testers

Cheap Low Corrosion soil sensor for arduino
SilentDreamcast - Published on Jun 14, 2017
Uses copper clad PCB border as a measurement sensor
6:42

How to Make Soil Moisture Sensor
Dr SimpleHack - Published on Sep 8, 2017
LED on means add water
Uses plastic bottle caps & a button cell battery (CR2032)
2 wood screws
1 LED
220k Resistor
1 npn transistor
2:53

How to make Soil Moisture Sensor/ Indicator
SNNY Hack & Sci. - Published on Jul 5, 2018
LED on means add water
1) LED
2) Transistor- BC547
3) Resistor- any value between 220K to 1M. 470K is used in the video
4) CR2032 Cell Holder
5) Battery- CR2032 or CR2025
2 short wires or nails
hot glue
2:39

How to make a Moisture Tester Project
Rapid Education - Published on Jun 21, 2017
https://www.rapidonline.com/rapid-moisture-tester-project-kit-class-pack-of-20-sets-13-0102 project
4:53

Other Tests

Hygrometer test
RPE Ball Hacks #tublife - Published on Dec 8, 2017
Overnight test to calibrate your sensor
3:51

Homemade soil moisture sensor for house plants (2)
Pui Kwong Chan - Published on May 24, 2015
Uses bamboo skewers
4:38

DIY Soil pH Test: How To Test Soil Acidity/Alkalinity without a Test Kit
Shepherd School - Published on Jul 10, 2013
Uses vineager and baking soda
3:36

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