osc. for capacitive proximity sensor

[Adam S.]

I want to make very simple capacitive type proximity sensors
for detecting the presents of water in a tank. From my
understanding of commercial units the sensors consist of an
oscillator that operates only when the target is close to
the sensor i.e. when the sensing capacitance has increased
in value due to increase in dialectic constant of the
surrounding material.

I'm looking for a design of such an oscillator circuit. I
haven't measured the values of the upper and lower range of
the sensing capacitance. What oscillator stops working
(fails to oscillate) when the capacitance of LC tuned
circuit goes below a certain value ?

Would a better approach to have a colpitts oscillator and
feed this to a comment emitter RF amplifier with a another
parallel tuned circuit on its collector. That way when the
oscillator frequency drops due to an increase in the sensing
capacitance the frequency becomes out of tune with the 2nd
tuned circuit, and so the output level falls off. ?

Adam

 

[John Popelish]

I want to make very simple capacitive type proximity sensors
for detecting the presents of water in a tank. From my
understanding of commercial units the sensors consist of an
oscillator that operates only when the target is close to
the sensor i.e. when the sensing capacitance has increased
in value due to increase in dialectic constant of the
surrounding material.

I'm looking for a design of such an oscillator circuit. I
haven't measured the values of the upper and lower range of
the sensing capacitance. What oscillator stops working
(fails to oscillate) when the capacitance of LC tuned
circuit goes below a certain value ?

Would a better approach to have a colpitts oscillator and
feed this to a comment emitter RF amplifier with a another
parallel tuned circuit on its collector. That way when the
oscillator frequency drops due to an increase in the sensing
capacitance the frequency becomes out of tune with the 2nd
tuned circuit, and so the output level falls off. ?

Adam

Remember that LC oscillator frequency changes only with respect ot the
square root of the capacitance, because both the inductance and
capacitive is involved.

I have wanted for some time to try to make one of these capacitive
switches by phase locking a CD4046 phase locked loop voltage
controlled oscillator to a fixed frequency oscillator, using the water
as part of the timing capacitors for the VCO. Instead of connecting a
cap between the two cap terminals on the VCO, you can use two grounded
caps (a pair of plates with the grounded water on the other side of a
plastic wall completing both capacitors). Then a simple comparator
watching the VCO control voltage could make the present-absent
decision.

 

[BFoelsch]

Don't know if it would apply in your case, but the simplest and most robust
liquid level sensor I have ever seen, bar none, is the Robertshaw
Level-Lance.

All it is is a plastic coated rod immersed in the tank. Another uninsulated
rod into the tank serves as a ground. The insulated rod (and the ground rod)
are used as the timing capacitance for a 555 timer. The timer runs a CMOS
divider which does nothing except flash an LED. In other words, the LED
flash rate is proportional to the capacitance, which is proportional to the
tank level. This oscillator, divider and LED are all that is in the
transmitter. The flashing LED makes the transmitter draw current pulses
proportional to the level. The receiver just uses these pulses to retrigger
a monostable, so a decision can be made as to whether the flash rate, and
hence the tank level, is above or below a predetermined amount.

There is no reason you couldn't do the same thing in just a couple of
packages.

Only trouble I ever had is on a job where the contractor put the signal
cables in the same pipe as the motor leads from a VFD.

 

[Neil]

I have used the following circuit that was suggested to me by someone in
this group. I have used it many times and love it...simple, easy, cheap.

http://www.discovercircuits.com/PDF-FILES/capgage.pdf

Kim

 

[Luhan Monat]

I want to make very simple capacitive type proximity sensors for
detecting the presents of water in a tank. From my understanding of
commercial units the sensors consist of an oscillator that operates only
when the target is close to the sensor i.e. when the sensing capacitance
has increased in value due to increase in dialectic constant of the
surrounding material.

I'm looking for a design of such an oscillator circuit. I haven't
measured the values of the upper and lower range of the sensing
capacitance. What oscillator stops working (fails to oscillate) when the
capacitance of LC tuned circuit goes below a certain value ?

Would a better approach to have a colpitts oscillator and feed this to a
comment emitter RF amplifier with a another parallel tuned circuit on
its collector. That way when the oscillator frequency drops due to an
increase in the sensing capacitance the frequency becomes out of tune
with the 2nd tuned circuit, and so the output level falls off. ?

Adam

If you only need to detect the presents of water, why not just a couple
of electodes (close togeter) and look for conduction? Only if the water
is very pure will this not conduct.

 

[Ben Bradley]

If you only need to detect the presents of water, why not just a couple
of electodes (close togeter) and look for conduction? Only if the water
is very pure will this not conduct.

Just have a circuit that depends on capacitance (in addition to
increased conductance), the dielectric constant of pure water is
surely very different from that of air. Use AC only, with no DC
offset, else there would be (excess) corrosion on one of both
electrodes.

 

[Luhan Monat]

In sci.electronics.design, Luhan Monat <[email protected]>
wrote:




Just have a circuit that depends on capacitance (in addition to
increased conductance), the dielectric constant of pure water is
surely very different from that of air. Use AC only, with no DC
offset, else there would be (excess) corrosion on one of both
electrodes.

The low dc voltage/current used to measure the resistance will not
corrode the electrodes. Its bascially an ohm-meter circuit.