Discussion in 'The Projects Forum' started by sukichan, Jul 17, 2015.

1. sukichan Thread Starter New Member

Jul 17, 2015
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I have a circuit for detect water sensor.

I make a PCB and use oscilloscope measure signal.
But i don't understand how to this circuit operation, when L1 change inductance -> frequency will change.

2. alfacliff Well-Known Member

Dec 13, 2013
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water is conductive, a tuned circuit sensor immersed in water will shift frequency when it is in water.

3. Alec_t AAC Fanatic!

Sep 17, 2013
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Pure water is a pretty good insulator, but impure water may conduct.
The circuit in post #1 is an oscillator. U1d is biased at its switching threshold (about mid-rail) so behaves like an inverting amplifier. U1e and R3 provide positive feedback to sustain oscillation at a frequency determined by the tuned LC circuit.
If the LC circuit is immersed the tuned frequency is likely to be affected. If it shifts enough to prevent oscillation for some reason, then the circuit is indeed a yes/no water sensor as is. However, if oscillation persists at a shifted frequency then further circuit components will be needed to detect any change in frequency.

4. sukichan Thread Starter New Member

Jul 17, 2015
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when i make a pcb circuit for test. I try to change inductance of L1 -> frequency outut change, but i don't understand how to calculate formula for output frequency f=?

5. DickCappels Moderator

Aug 21, 2008
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Even pure water is a dielectric constant of about 80, which would cause the resonant frequency to drop when immersed.

6. sukichan Thread Starter New Member

Jul 17, 2015
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sorry for my mistake (LC circuit not put in a water), by my water sensor is a water level sensor. when the level of water change -> the pressure will change -> output frequency change. I use MCU to measure output frequency and detect level of water.

7. AnalogKid AAC Fanatic!

Aug 1, 2013
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Now that I see what you are trying to do, I don't think it will work. When going from dry to immersed, the oscillator frequency might change enough to detect the change. But once it is immersed, I don't think the frequency will change at all, or very, very little when the water pressure changes. There are others on ths forum that have a lot more experience in industrial controls, and maybe one of them knows about this technique.

ak

8. sukichan Thread Starter New Member

Jul 17, 2015
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AnalogKid, This circuit is ok. Many washing machine use this technical for detect level water (Ex. Sanyo, Panasonic...), when level water in washing machine change -> presure of air in the tube change -> value of inductance L1 change -> output frequency change.

9. Alec_t AAC Fanatic!

Sep 17, 2013
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f=1/(2*pi*sqrt(L1*C3))

10. kubeek AAC Fanatic!

Sep 20, 2005
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How does change in air pressure change the inductance?

11. Alec_t AAC Fanatic!

Sep 17, 2013
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Don't know how it's done industrially, but one way would be to have a coil with a ferromagnetic core moved by a diaphragm in response to pressure changes.

12. sukichan Thread Starter New Member

Jul 17, 2015
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When I use L1=220uH, C3=0.1uF ->F(caculator) = 33.9KHz but i use oscilloscope and measure F(measure) = 125.24KHz
When I use L1=1mH, C3=0.1uF ->F(caculator) = 5KHz but i use oscilloscope and measure F(measure) = 17.1KHz

13. sukichan Thread Starter New Member

Jul 17, 2015
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OK, when air presure change -> core of inductane will move -> value of inductance change.

14. Alec_t AAC Fanatic!

Sep 17, 2013
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Interesting.
When I run a simulation I get 33.3kHz for L1=220uH, C3=0.1uF, and 15.3kHz for L1=1mH, C3=0.1uF. Both values are consistent with the formula.
Try increasing R1 to ~150k and R3 to ~10k.

15. sukichan Thread Starter New Member

Jul 17, 2015
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Thanks,
I have test again and this formula is ok.