#207 Why most Arduino Soil Moisture Sensors suck (incl. solution)

Intro Summer is rapidly approaching here in Switzerland and with it also the problem of watering our plants as Makers we want to measure the humidity to control it Unfortunately, most humidity sensors destroy themselves after a short while we need a better solution pretty youtubers here is the guy with the Swiss accent with a new episode and fresh ideas around sensors and microcontrollers today we will test different moisture sensors and I will show you how they work and why most sensors From China destroy themselves, and of course, we will find the solution to the problem Plants need a defined humidity to prosper This is why we want to control the watering of plants using our home automation system The controller has to measure the moisture of the soil and to give commands to a pump or a solenoid if you go to our usual purchasing platforms and enter moisture sensor or water sensor and Arduino you get these proposals a two length sensor with a separate small controller PCB It has two pins on the sensor PCP and four pins to connect it to a microcontroller It provides a digital and a analog output the digital output can be adjusted by a trimmer there are conducting tracks exposed on both legs of the sensor a Small to length sensor with control logic on the same board. It only provides a analog output a Sensor similar to number two, it connects every second copper path to emulate the two legs Sometimes the legs are of solid metal and the price of the sensor is much higher, but the rest is the same The last sensor looks different It does not have any exposed copper path and more electronics at the top It is more expensive than the cheap ones and it also provides an analog output Let's start with number one. How does it work? The sensor which is pictured like a resistor is connected to VCC via a 510 kilo ohm resistor These two resistors form a voltage divider and the analog output signal is the voltage drop across the sensor Let's check if this is true First we use our own meter to check if the water has a resistance its resistance Fluctuates a lot but it is not indefinite the more water between the legs the smaller the output voltage The rest of the circuit on this Center PCB is a comparator which compares the measured value with a constant Usually we do not use this digital output because we can do that much more elegant in software The sensor is now connected to its PCB and to five volts the analog output voltage is close to 5 volts as Soon as the sensor touches the water it starts to conduct and the voltage drops considerably The more we dive the sensor into the water the lower the resistance and the lower the output voltage We all know that we do not water our plants like that Most plants grow in moist soil, but still the principle is the same The more water between the two legs the lower the voltage efficient and Straightforward as it seems we will later see that This is not true. The next two sensors do not have a digital output and therefore to not need the op-amp But they have a transistor abort if we look at the diagram we see that one leg of this Sensors is connected to VCC The 100. Ohm resistor is just a protection against a short circuit The other leg of the sensor is connected to the base of the transistor the other two pins of the transistor are connected to VCC and Via a resistor to ground and also here the analog output is the voltage across the resistor the purpose of the transistor is to amplify the base current by a factor of let's say 50 as before if The moisture sensor sends is water it reduces its resistance and a current can flow into the base a Much stronger current flows through the collector and creates a 50 times higher current the lower the resistance of the sensor the higher the base current the higher the collector current and the higher the voltage This is precisely what we see The amplification of the transistor leads to a much smaller current flowing through the sensor Which is good as we will see later on also this sensor works But if you talk to people who used these sensors They will tell you that after a while they stop to work and I show you why Because we do not have a lot of time. I will accelerate the effect which happens in the soil I connect the two legs to my power supply precisely as it is done in the sensors The only difference I do not limit the current Now I put the sensors in water Also as intended if you have a close look you see strange things happen bubbles in the glass and The right leg starts to change its color. It loses the plating After a few minutes the current stops The right leg is interrupted because all copper was taken away If you look at the sensor, it does not look healthy and if you watch the water This is where your plants would live. It also does not look healthy I am sure one of my viewers can enlighten us about the chemical reaction and how dangerous this green stuff is Fact is the sensor is dead the same happens again if you replace it It's not a quality issue. It happens because of the water and the DC current to Prevent this from happening. We could isolate the electrodes from the If we do, so the sensor does not work at all. So this is not the solution The only solution for me is to avoid these sensors Let's continue with the last sensor This one has no copper exposed to water and it's legs cannot be dissolved good But how does it work? when we try to isolate the sensor legs before The sense it did not work at all So let's check if this one works Yes, it does. Cool Also this sensor uses a chip this time not an op-amp, but an ne555 timer The diagram looks like that the ne555 works in an a stable mode and creates a square wave This square wave goes to one leg of the sensor. The other leg is connected to ground What happens if we put the sensor into the water the two isolated legs form a capacitor? Together with water its capacity changes if we look at the resistance formula of the capacitor we see that it is reduced if the Capacitance gets bigger. This is precisely the behavior. We are looking for very good By the way, you see that its resistance is also reduced with an increasing frequency This resistance is not a real resistance, but I will not bother you with complex numbers The sensor anyway works if we do not understand these calculations in the end a diode and a Capacitor are used to smoothen the square wave and we get an analog value which changes with the humidity Without contact to the water and this sensor also does not need a lot of parts you do not believe me Look at this circuitry on my breadboard and here to my home made sensor I know it is not good looking but this is not a channel about good looks and makeup I just used an old PCP and separated two areas with trailer Now we have to isolate our sensor from the water I do this with a simple plastic pack done and we need a square wave instead of an any 555 I use my wave form generator Like that. I also can change the frequency on channel, one of the oscilloscope you see the square wave and on channel two the output of the sensor and really if I Eat my sensor into the water the output value changes now, you can believe me Here you see the effect of frequency on the range of the sensor the biggest difference between null water and fully submerged results with frequencies around 600 to 900 kilohertz the frequency used by these sensors is about 570 kilohertz because in ne555 cannot do much more We still have to solve a small issue because the supplier of these capacitive sensors use a standard PCB they leave the edges without protection and what too easily can enter here you either put your sensor into a plastic bag or Use some sort of protective lacquer like this one or that one Summarized we learned the principles of moisture sensing we know which sensors can be used over a more extended period and We know how we have to enhance them to become even more stable. So the summer can start now I hope this video was useful or at least interesting for you if true Please consider supporting the channel to secure its future existence. You will find the links in the description Thank you. Bye You

Channel: Andreas Spiess

Views: 397,840

Rating: 4.9478035 out of 5

Keywords: esp32 weather station, arduino, esp32 project, esp32 tutorial, lorawan, Moisture, esp32 datasheet, smart home, do-it-yourself, eevblog, hack, ttgo, moisture sensor arduino, esp32, greatscott, diy, wemos, hobby, moisture sensor project, iot, guide, how to, water sensor, simple, esp8266 datasheet, moisture sensor diy, beginners, wifi, nodemcu, moisture sensor, electronics, projeect, esp8266

Id: udmJyncDvw0

Length: 10min 59sec (659 seconds)

Published: Sat Jun 16 2018