# Using 555 timer with soil moisture sensor

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:

1. 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.

2. What exactly is the difference between the OUTPUT pin & DISCHARGE pin, since I am seeing the same waveforms from both pins?

1. Is the direct feedback mode same as the Astable mode? why is it called direct feedback?
2. 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?
3. 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

• You're using the NE555 with a 3.3V supply. That can't work in reality, the minimum operable supply voltage of the NE555 is around 4.5 V. $$\,$$ Also, the NE555 in a battery-powered long-term observation device is a terrible idea – far, far, far too high power usage. The PDF at the bottom (crop your screenshot and link to the source!) mentions the LMC555, which is a similar, but different device, and does indeed work at lower voltages; so maybe fix the top circuit! Commented May 21, 2020 at 7:57
• Then, you're asking about basic functionality of the xx555 chips – have you read the LMC555 datasheet? It comes with pin descriptions and a lot of functional descriptions that look a lot better than what you show in your whatsapp screenshot (again, when posting such pictures, crop to the relevant content and link sources). Commented May 21, 2020 at 7:59
• Actually I am using the LMC 555 timer , couldn't find it in LTspice so used NE555 for simulation. Commented May 22, 2020 at 6:29
• That makes no sense. I didn't find sugar, so I simulated my cake with salt will not make a cake simulation that has anything to do with your real cake. Commented May 22, 2020 at 7:53
• Also, I've given you 24h to crop your screenshots, -1 for lack of diligence. Commented May 22, 2020 at 7:53