Capacitive Sensor Problems

Why don't you just use the small piece of foil, and attach that to a larger plate that is non-conductive?

Or maybe make the entire plate out of something that is less conductive than aluminum.

It's also possible that I'm not getting it.

What would be an example of a less conductive material for the plate?

Sorry i don't know too much about electronics so I'm really confused with this device...

Dan

Is this a DIY device, or something you purchased? What is it?

Maybe a piece of aluminum screen instead of solid foil? Galvanized screen would be less conductive.

It is a device that was made for me so I'm not really sure what goes into it or how it works. I just know it has a capacitive sensor attached to a foil sheet...

Sorry for the lack of info...

You probably should contact the person who made this for you.

I don't think there's much we can do for you.

...or re-tune the input circuit connected to the foil so that it responds only to the change in capacitance on the larger piece of foil?

How could I retune it?

The circuit diagram cannot be seen from here.

Not very much to go on.

Try piezoelectric film.

How Capacitive Proximity Sensor works"Capacitive proximity sensors can be used to detect metallic and also non metallic targets like paper, wood, plastic, glass, wood, powder, liquid..etc without physical contact. The capacitive proximity sensor works on the capacitor principle. The main components of the capacitive proximity sensor are plate, oscillator, threshold detector and the output circuit.
The plate inside the sensor acts as one plate of the capacitor and the target acts as another plate and the air acts as the dielectric between the plates.
As the object comes close to the plate of the capacitor the capacitance increases and as the object moves away the capacitance decreases. The detector circuit checks the amplitude output from the oscillator and based on that the output switches.
The capacitive sensor can detect any targets whose dielectric constant is more than air."

What is your application meant for? What exactly you want to achieve. Get the circuit from your friend & please post it. I am sure, one of our members will help you.

Unfortunately, because of hurricane sandy here on the east I cannot get the circuit diagram...

The circuit seems to work well when I have a small piece of foil on the sensor however when I placea large piece I get the sensor's indicator light to always be on, hence the effect always being triggered.

The circuit does have a 10 second delay between effect cues so someone doesn't trigger the sensor twice by accident.

It's still too sensitive for the larger piece of foil. What does the individual that designed the circuit have to say on the topic?

I cant reach him at the moment because we have been affected by a bad storm here on the east coast and phone/cable lines are out.

I'm having trouble understanding why you need a conductive sheet on a capacitive sensor, at all.

Is there some particular reason the sensor in place must remain in place? From the little information provided, it sounds as if that selection should be reconsidered.

It sounds like the 5" x 7" foil is just a target for the foot.

Tape your small piece of foil in the middle of a 5" x 7" piece of plastic.

The plastic is the target and it will be hard to miss the bullseye with a foot.

Good luck,

Mike W.

This is the sensor I'm using:

http://gordonproducts.com/PCA25.htm

The mat i am trying to make out of foil will go under a real door mat so guests can step on the mat and trigger the effect.

Are you using a long connecting wire between the sensor & the foil, resulting in high capacitive coupling input for the sensor. Perhaps input capacitance is more than the required value of the sensor.

Is a pressure sensitive floor mat, with an area larger than the mentioned 5" X 7", contraindicated for this application?

That is to say, is the existing design for a wheel unacceptable?

Looking at the device on that site, there is a 15-turn sensitivity adjustment pot on the end the wires come out of. You could try adjusting that. Keep track of how much you turn it and in which direction, so you can return it to the original place.

However, the site also says " ... PCA25 has an 8-32 threaded insert for the addition of an expanded sensing element - sensor size is limited to three (3) inches in diameter." Your 5x7 sensor is about 5 times that big. I suspect you have too much capacitance connected. You could try connecting a smaller piece of foil with a wire to get it close to the center of your mat. Or possibly a wire loop.

I think a pressure-sensitive floor mat might have been a better choice. Good luck!

A possible fix is to buy a 1 meg Ohm pot and a 10 k fixed resistor.

Connect them together both between the sensor and battery(?) negative or circuit ground.

Adjust till the sensitivity is gone.

Start at 1 Meg Ohm and work slowly down. The 10 Resistor is just to stop you causing a short across the sensor....

A photograph might be helpful.

The foil is screwed directly into the device

<sigh>

Okay.

^Unsubscribe

You have a capacitive proximity sensor that probably uses a tuned circuit to determine the presence or absence of a person. Unfortunately when you added more foil to your sensor you increased the effective capacitance of the sensor in exactly the same manner as having a human near it. The only way around this is to retune the circuit for the larger piece of foil.

Quite frankly your sensor is overkill, you could easily use a pressure sensitive mat that provides a switch closure to your effect:

http://www.amazon.com/Swann-Wireless-Doormat-Chime-SW351-WMA/dp/B003XQPN7S/ref=pd_cp_e_0

I'd say you need to introduce a high ohm resistor into the circuit. Somewhere between the foil and the actual circuit.

Sounds like your device is too sensitive for the application. Things could be worse. I would replace your foil with a piece of 1/4 inch mesh screen (chicken wire). If that doesn't solve the problem, you will have to reduce the sensitivity of the transducer. Try introducing resistance from the sensor output to ground.

If you have overhead fluorscent lighting or other high noise generating lighting the foil is most likely getting charged by induction/noise from the lighting.

Try turning the lighting off and observe if the larger foil sensor works as needed.

If indeed the lighting is the root-cause, installing an inductive filter lighting cover on the fixtures should alleviate the issue.

These are available from several vendors on their internet websites.

If lighting is not the source, look for high electrical current consuming devices/equipment and/or high static generating devices such as a computer CRT display/console within the near proximity of the sensor and relocate them as far away from the sensor as possible.

It would help if you could send a photograph of the mechanical arrangement( showing the sensor, aluminium foil and the mat ). A sketch showing the lay out will also do.This will help us understand how the sensor is expected to sense the stepping on the mat.

Altering the oscilator inside the sensor seems not feasible, for the internal components are most likely SMDs and you'll need special equipment, parts and skills.

Adding a capacitor in series, between the large plate that you need to use and your sensor, will effectively decrease the total capacitance to a working range, but you will have to try different values.

Series capacitance is smaller than the smallest capacitor used. (inverse of the sum of both capacitances' inverse).

So this effect you are talking about....I guess it's a booby-trap for your meth lab in Kansas? Or is it more along the lines of some sort of bomb plot?