I am using an IRROMETER sensor for soil moisture measurement. This sensor requires AC excitation, so I found an interfacing module for it from online. This is the circuit recommended by them:
The circuit was easy to simulate and these were the results:
Currently I am trying to understand the exact working of the circuit, since this is similar to the astable vibrator I am trying to relate them.
From the general circuit diagram, we can conclude that R1, R2 & C form the wave shaping network.
The following are my questions:
Which of the resistances form R1 & R2 in the case of my soil moisture interfacing circuit? Basically I need those values to theoretically calculate the duty cycle & ON time and then compare them with my LTspice simulation.
What exactly is the difference between the OUTPUT pin & DISCHARGE pin, since I am seeing the same waveforms from both pins?
- Is the direct feedback mode same as the Astable mode? why is it called direct feedback?
- Why is the OUTPUT pin called totem pole? 5.Why is the duty cycle different in my simulations and the one I have observed on the CRO, when I have used the exact same values of R & C components?
- Also why is it said that the current through the circuit is of AC type when we are providing a DC supply?
The brief working of the circuit is given here by the manufacturers. Ignore the voltage regulator part and refer the circuit of the simulation. Till now I have figured out that the 390 Ω, 150 kΩ & 0.1 uF and the sensor resistance form the wave shaping network, but I am still not able to understand which ones are R1 or R2?
The same circuit I built in my lab, and observed it's waveforms on the CRO, sorry the pics are not very clear. I am also adding the data sheet of this 555 timer module. https://www.emesystems.com/smx/documents/SMX_2018.pdf