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Well, I have had a look at both your circuits and the signal one is a little complex, so I will explain that at a later date when I have worked it out myself! But the plant one I can explain now.
The circuit's main component is an op-amp. This is set up in a special configuration called a comparator. The IC compares the two I/Ps (pin 2 and 3), and then O/Ps the difference between them (pin 6).
The 10k and variable 100k resistors at the bottom of the circuit control the amount of feedback and amplification the op-amp has. In this circuit I would imagine the amplification is 1 or unity, ie: it doesn't actually amplify the O/P. The 3k3 resistor on the O/P is to limit the current to the meter, so it's not damaged. The two diodes on pin 3's I/P, are there to clamp the voltage going to the I/P so the comparator is as accurate as possible (I think!) The resistors on that part of the circuit are just there to keep the voltages and currents at controlable, safe levels. You wouldn't want too high a voltage going into your wet soil! The 1k resistor is there to protect pin 2's I/P and to try and match the I/P currents as much as possible to eachother. The +9v and -9v I/Ps at the top are to power the op-amp.
Now, for the function of the circuit. As current flows from one spike to the other through the soil, it will encouter resistance. This resistance will create a potential difference between the two spikes and it is this difference the op-amp uses as the voltages I/P to it will differ slightly. Depending on the dampness of the soil, will depend on the resistance in it. If the soil is very wet, it will have a very low resistance; if it's very dry, it's resistance will be high. This resistance is transfered through the circuit to the meter, which is an ameter (measures current). This can be calibrated however you want. So it would show very wet soil for high current (low resistance between the spikes) and a show very dry soil for a very low current.
I hope that explanation was clear and adequate enough. I will try and explain the signal circuit as well, at a later date.
I generally agree with canadianpoet2012's description of the soil meter. However, I disagree on two points.
1. The circuit is not a Voltage Compatitor, it is an amplifier as canadianpoet2012 said elsewhere.
2. The diodes are not acting as a clamp. Their purpose appears to be (in conjunction with the 15k resistor) is to provide a small voltage difference across the soil resistance and the 1k Given that the input current to the + input of the Op Amp is very small, this voltage will be small and it will vary with temperature. So it is not a good circuit design.
The secret to understanding Op Amps is that the negative feedback adjusts the output voltage so as to mininise the voltage between the inputs. Op Amps have a very high gain, so you can assume that the difference in voltage between the inputs is zero.
So the output voltage will be proportional to the resistance of the soil. Thus the current through the meter will be also proportional to that resistance.
I would be inclined to remove the diodes, connect the left hand end of the 15k to 0 Volt. It may be necessary to alter one or two resistor values, you would have to do the calculations based on a knowledge of the range of soil resistance expected.
The railway circuit looks rather clumsy and parts hungry. If you post what you want it to do, I'll design a simple one for you.
Circuit Description of the Railway Level Crossing Circuit
Numbering the 555s from left to right IC1, IC2, IC3.
The capacitors connected to pin 4 of IC1 and IC2 reset these ICs when the power is first turned on.
IC1 is configured as a monostable. When the trigger device (I assume it is a reed switch) closes, the output (pin 3) goes high for about 1 second. The amber LED will glow during this period. At the end of this period, pin 3 goes low and applies a low pulse (while the capacitor discharges) to IC2 pin 2.
IC2 is configured as a set/reset bistable. A low on pin 2 sets pin 3 high, and a low on pin 4 resets it (ie. makes pin 3 low)
So IC2 is set when IC1 pin 3 goes low. When set, the outpiut of IC2 (pin 3) applies power to IC3.
IC3 is configured as an astable. It oscillates at about 8 Hz and flashes the red LEDs alternatively.
When the train passes the Cancel switch, a low is applied to IC2 pin 4 which resets it. IC3 is switched off so the flashing stops and the circuit is now back to its initial state.