Hi everyone,

I am happy to share my very first PCB design project with you. It is a capacitive soil moisture sensor using an ATmega328P and NRF24L01+.
The capacitance is measured as commonly done through a low pass filter using the ATmega PWM instead of a NE555.

I am not a professional in the area of microelectronics and pcb design and have thought me most of the things myself. This is also why I already have a ton of ideas for future improvements on the design with the switch to a NRF52 being on top. Still I am quite satisfied with my results so far. The pcb is tested and working (despite the fact that I soldered a wrong value capacitor and forgot the capacitor for the ftdi connector, which is already fixed). You can see example measurements on a plot below.
I have made all source files available on the corresponding GitHub repository.

I am happy to hear your thoughts and maybe some tips and remarks.

Cheers!
Ron

Hi everyone,

first of all thank you very much for this awesome project / community.

I recently got more and more interested in building sensors. My next project would be to add some capacitive soil moisture sensors to some plants around the house.
I know there are already a few sensors of this kind out there like these:
https://www.openhardware.io/view/767/Wireless-Module-for-Capacitive-Soil-Moisture-Sensor-v20#tabs-design
https://www.openhardware.io/view/672/Soil-moisture-meter#tabs-instructions

The second one however, despite it is exactly what I was looking for, does not provide any design files.
But as I wanted to learn more about electronics in general and also wanted to build my first smd pcb, I build my own capacitive soil moisture sensor based on an ATMEGA328P, NRF24L01+ and a 3V coin cell. The schematic is based on the chirp sensor and I oriented myself with regards to component selection at this project.
The good news is: It works!
But I already noticed some flaws (e.g. I forgot to add a FTDI connector besides the AVR ISP ) and I could need some help from you:

The voltage range that comes out of the sensor circuit is very limited. In air the analog pin reads about 570 while reading around 498 submerged in a glas of water.
I read this post and made my own calculations (I measured my probe to have between 20pF and 460pF). Based on these the voltage swing should be around 2.4V from minimum to maximum capacity. Now the real world is probably not exactly like the calculations but this difference seems a little bit large to me. Anything that I am missing here?
As additional information: I generate a 1 MHz square wave using the internal clock of the atmega by enabling fast pwm using the following bits:

TCCR2A = (1 << COM2B1)  // Clear OC2B on compare match
	| (1 << WGM21)  // pwm mode 7
	| (1 << WGM20);  // pwm mode 7
TCCR2B = (1 << WGM22)  // pwm mode 7
	| (1 << CS20);  // no prescaling
           
OCR2A = 7;
OCR2B = 3;

In the setup function I set pin 3 (connected to OC2B) as output and pin 14 (A0) as input (this is where the sensor output goes in).

Any help on this would be greatly appreciated.
As soon as I finish the sensor (when I am no more bothered by the remaining flaws ) I will make it accessible for everyone.

Thank you in advance for any advice or guidance.

Best regards,
Ron

Hi there,

i am reading now for some time in this forum and it really helped me out a few times, thanks for that!
But now i would like to actively participate here.

The Problem:
I have a Teufel Concept E in my livingroom. This can only be controlled with a special remote control.
Because it is not automatically activating itself when there is an incomming signal (e.g. from my tv) i always need to search for the remote control.
At the moment i can already switch a 433MHz Socket with an Arduino/Transceiver and Openhab to turn on my tv, it would be great if i could also switch the sound system with an Arduino. So i asked myself on which technology/frequency the remote control is sending and opened it up

there was a leaking battery but i don't mind because it's still working
now im not really sure if that what i want to do is going to work. I think the black chip on the right side is the transmitter? I searched google for "N51822" and found a "NRF51822 bluetooth low energy" which can send on bluetooth low energy and 2.4GHz ultra low power.
Do you think it is going to work? And what do i have to do to achive just a simple on/off remote with an arduino?

Thank you very much for every answer on this!
Ron

Hi everyone,

I have rebuild the sensor and this is the current status:

• Changed out the coincell for two AAA batteries due to higher capacity and better availability. This increased the sensor size in both height and width but this does not bother me.
• Added the possibility to attach an ATSHA204A in a SOT-23-3 package for message signing.
• Added an FTDI connection besides the AVR-ISP
• Changed from indentation to mounting holes. This makes it easier to design 3D printed cases for it.

Any nice features that I am missing here or that some of you would like to see on this? I am open for suggestions

PS: Is there any way to scale the uploaded images?

@mfalkvidd you are right, I might somehow have found an older datasheet somewhere (my one was from 2015). In the current datasheet from the microchip website the same graph is present which indicates 2.4V minimum for 8 MHz. So I probably will deactivate 2.7V BOD.

@Anticimex That was also my thinking behind that. According to the datasheet the ATmega328P works reliably down to 2.4V when using 8MHz.

@Puneit-Thukral Thanks. I am not quite sure, so correct me please if I am wrong, but I think I am already using the MiniCore bootloader with platformIO. Or do I need to configure to explicitly use the MiniCore bootloaders? Also I have set BOD to disabled as I read somewhere that no BOD can also save battery.

Hi everyone,

I am happy to share my very first PCB design project with you. It is a capacitive soil moisture sensor using an ATmega328P and NRF24L01+.
The capacitance is measured as commonly done through a low pass filter using the ATmega PWM instead of a NE555.

I am not a professional in the area of microelectronics and pcb design and have thought me most of the things myself. This is also why I already have a ton of ideas for future improvements on the design with the switch to a NRF52 being on top. Still I am quite satisfied with my results so far. The pcb is tested and working (despite the fact that I soldered a wrong value capacitor and forgot the capacitor for the ftdi connector, which is already fixed). You can see example measurements on a plot below.
I have made all source files available on the corresponding GitHub repository.

I am happy to hear your thoughts and maybe some tips and remarks.

Cheers!
Ron

@mfalkvidd you are right, I might somehow have found an older datasheet somewhere (my one was from 2015). In the current datasheet from the microchip website the same graph is present which indicates 2.4V minimum for 8 MHz. So I probably will deactivate 2.7V BOD.

@mfalkvidd said in ATMEGA328P standalone soil moisture sensor:

@Ron You can use 8MHz safely down to 2.34V according to the graph in the datasheet.

Are you sure with that? I found this information in the datasheet to the atmega328p-au, where it states that the safe operating range ends at 2.7V:

Maybe I should consider using an external oscillator instead of the internal one? Do you know something about the power consumption with an external crystal compared to the internal oscillator?

@mfalkvidd said in ATMEGA328P standalone soil moisture sensor:

For size, CR123a might be an option. They are lithium 3V, so one would be sufficient.

These ones look definitely interesting, I will have a look at them, thank you

Hi there,

I am currently thinking about a power supply solution for the sensor. The possible options with their advantages/disadvantages are:

• Single AAA Battery Cell + dc-dc boost converter
  + compact formfactor
  - not that much capacity
  - additional costs because of boost converter hardware
• Double AAA Battery
  + relatively large capacity and thus longer lifetime
  + no need of a booster which could introduce noise to the radio
  - bulky (as you can see on the photos of my last post)

What bothers me is that without a booster I can only use two AAA batteries until they reach 2.7V in total, as I need at least 2.7V if I want to use the 8 MHz internal clock of the atmega.
Does it make sense to combine two AAA batteries with a boost converter to 3.3V? This way one could use the batteries until they reach ~2V in total.
I am interested to hear your thoughts on that.

Hi everyone,

I have rebuild the sensor and this is the current status:

• Changed out the coincell for two AAA batteries due to higher capacity and better availability. This increased the sensor size in both height and width but this does not bother me.
• Added the possibility to attach an ATSHA204A in a SOT-23-3 package for message signing.
• Added an FTDI connection besides the AVR-ISP
• Changed from indentation to mounting holes. This makes it easier to design 3D printed cases for it.

Any nice features that I am missing here or that some of you would like to see on this? I am open for suggestions

PS: Is there any way to scale the uploaded images?

I have a strong feeling that I got the wrong diode for the rectification of the triangular wave that comes out after the capacitive sensor (RC circuit). I got this one:

https://lcsc.com/product-detail/Schottky-Barrier-Diodes-SBD_Guangdong-Hottech-SS14L_C190475.html

In the datasheet it says that this diode has a Typical Junction Capacitance of between 90 and 110 pF which seems pretty high compared to the capacitance of the sensor. Do I need a diode with a lower capacitance?

Can someone give me a hint if I am on the right track?
Thank you

Edit: Okey I switched the smd diode with an all purpose tht diode that I had lying around and now the voltage swing is already at 1.1V so it definitely is the diode. Any recommendations on diode selection for high frequency (1-4MHz) applications?

Hi everyone,

first of all thank you very much for this awesome project / community.

I recently got more and more interested in building sensors. My next project would be to add some capacitive soil moisture sensors to some plants around the house.
I know there are already a few sensors of this kind out there like these:
https://www.openhardware.io/view/767/Wireless-Module-for-Capacitive-Soil-Moisture-Sensor-v20#tabs-design
https://www.openhardware.io/view/672/Soil-moisture-meter#tabs-instructions

The second one however, despite it is exactly what I was looking for, does not provide any design files.
But as I wanted to learn more about electronics in general and also wanted to build my first smd pcb, I build my own capacitive soil moisture sensor based on an ATMEGA328P, NRF24L01+ and a 3V coin cell. The schematic is based on the chirp sensor and I oriented myself with regards to component selection at this project.
The good news is: It works!
But I already noticed some flaws (e.g. I forgot to add a FTDI connector besides the AVR ISP ) and I could need some help from you:

The voltage range that comes out of the sensor circuit is very limited. In air the analog pin reads about 570 while reading around 498 submerged in a glas of water.
I read this post and made my own calculations (I measured my probe to have between 20pF and 460pF). Based on these the voltage swing should be around 2.4V from minimum to maximum capacity. Now the real world is probably not exactly like the calculations but this difference seems a little bit large to me. Anything that I am missing here?
As additional information: I generate a 1 MHz square wave using the internal clock of the atmega by enabling fast pwm using the following bits:

TCCR2A = (1 << COM2B1)  // Clear OC2B on compare match
	| (1 << WGM21)  // pwm mode 7
	| (1 << WGM20);  // pwm mode 7
TCCR2B = (1 << WGM22)  // pwm mode 7
	| (1 << CS20);  // no prescaling
           
OCR2A = 7;
OCR2B = 3;

In the setup function I set pin 3 (connected to OC2B) as output and pin 14 (A0) as input (this is where the sensor output goes in).

Any help on this would be greatly appreciated.
As soon as I finish the sensor (when I am no more bothered by the remaining flaws ) I will make it accessible for everyone.

Thank you in advance for any advice or guidance.

Best regards,
Ron

I could try it with a cable of length 170mm but I am not sure if that is anything else than with the antenna.
Does the transmitter also needs more voltage when an antenna is connected? At the moment i just connected it to the 5V output of the arduino. Maybe that is too low for the transmitter with antenna. I ordered some boost converter to test with higher voltage. Maybe that helps a bit