Plant moister detector

[pimster987]

I built a plant moister detector for one of my classes. I know what is happening in general circuit but I don't fully understand what is happening at the part I circled.

 

[MrAl]

Oscillator for blinker?

 

[SneaKSz]

The output(IC1D)will go high and low depending on the components around IC1A and B. C3 will charge/discharge accordingly to the output of IC1D. R4 sets the resistor for charging and D1 will conduct for discharging so R4//R5.

 

[audioguru]

The circuit is described at www.redcircuits.com in Home Related Circuits.

 

[pimster987]

The circuit is described at www.redcircuits.com in Home Related Circuits.

Thanks the site helped me somewhat. I have a couple of questions. First, from what I can see the probes and resistors R2, R3 and capacitor c2 make some sort of bandpass? When the resistance is high enough the filter will let the signal go through and when the resistance is low the signal will be blocked. Also since there are two oscillators in the circuit one would have to work faster than the other. So the first oscillator outputs a square wave and this helps save power by not having the LED light up all the time. The second oscillator with the diode would have to oscillate faster than the first one. Not sure what the purpose of this one is but I was thinking that this also helps save power just like the first one. Can anyone correct me if im wrong.

 

[Boncuk]

Hi pimster987,

forget about the bandpass filter.

If the soil is dry its resistance is high allowing for C2 to be charged. Vice versa the 2KHz rectangle wave will be diverted to ground at a low resistance of the soil. If C2 can't be charged IC1B pin9 will remain low resulting in a steady high output at pin10. IC1C is wired as inverter which controls the gated oscillator IC1D.

R2 and R3 make a voltage divider allowing to adjust for the desired trip point of the circuit.

As soon as the gated oscillator is activated by a high signal it starts oscillating at a low frequency determined by the values of R4, R5 and C3.

So Q1 switches on the LED in bursts of the 2KHz square wave and a flashing rate being produced by IC1D. Since the human eye is too slow to recognize high frequency switching the LED appears to be lit steadily at a flashing rate determined by IC1D.

With a few changes the circuit might be converted for automatic watering switching a solenoid water valve.

I'll post a circuit simulation later if my PC decides to work properly.

Boncuk

 

[pimster987]

Hi pimster987,


As soon as the gated oscillator is activated by a high signal it starts oscillating at a low frequency determined by the values of R4, R5 and C3.

I'll post a circuit simulation later if my PC decides to work properly.

Boncuk

Thanks for the explanation but im still a little confused. So IC1D turns off when the low part of the square wave is passing and turns back on on the high part. I analyzed the circuit with an oscilloscope but I couldn't get a simulation to work on PSpice. I couldn't find a schmitt trigger NAND gate and I looked everywhere on the net for help but couldn't find any.

 

[Boncuk]

So IC1D turns off when the low part of the square wave is passing and turns back on on the high part. I analyzed the circuit with an oscilloscope but I couldn't get a simulation to work on PSpice. I couldn't find a schmitt trigger NAND gate and I looked everywhere on the net for help but couldn't find any.

Yes, exactly that way. The gated low frequency oscillator is gated on and off 2,000 times per second. You can use any NAND gate to simulate the circuit.

Boncuk

 

[audioguru]

You can use any NAND gate to simulate the circuit.

No.
A Cmos Schmitt-trigger oscillator will [not[/b] work with an ordinary NAND gate. Ordinary NAND gate oscillators have a different circuit and use at least two gates or inverters.