Which capacitor to use to stabilise capacitive sensing circuit

Hi all,

I am building an interactive sculpture which uses capacitive sensing to take readings and control various outputs like sound and lights.

I have followed this topic on arduino to create my circuit: http://playground.arduino.cc/Main/CapacitiveSensor?from=Main.CapSense

I have found that it is difficult for me to get repeatable results and results that are stable enough for me to use for this purpose. The sensor pad needs to be very large and I think this is why I am encountering issues.

I have noticed that there are still a few things which I have not tried yet to improve the stability of the sensor. So before I decide to go down a different route I would like to try them but I am unclear of the following things which are quoted from the link above:

'Research has shown that a small capacitor (100 pF) or so from sensor pin to ground improves stability and repeatability.'

and

'Adding small capacitor (20 - 400 pF) in parallel with the body capacitance, is highly desirable too, as it stabilizes the sensed readings.'

I was just wondering if anyone may be able to help me out with what type of capacitor I should use to do this? There are so many different types ceramic, electrolyte etc...

Please would someone be able to email me a link to the type of capacitors I should use to stabilise the circuit?

I would be very grateful.

Many thanks,

A

There are so many different types ceramic, electrolyte etc...

Not at those values there aren't. You can not get an electrolitic anywhere close. At those values just use a ceramic. However just buying one capacitor on line is likely to be expensive on the postage, just google:- bag of ceramic capacitors to see what is local to you.

Having said that:-

I have found that it is difficult for me to get repeatable results and results that are stable enough for me to use for this purpose.

That does not surprise me. That technique is more of a demonstration of an effect rather than an effective and reliable technique.

Thanks so much.

I am going to keep trying with this technique a little longer as I would really like the piece to be touch sensitive all over.

A

Hi Aphra,

I am doing a very similar type of project, but not using the CapSense setup. I am using a design (from online sources) involving capacitance and I have been troubleshooting for the last little bit. I have also read somewhere that increasing capacitance and/of resistance might help. The version I am using uses 150picofarad ceramic capacitor (zero temperature coefficient) but, again, it's not for the CapSense.

I found this article to be helpful from a group who develop the CapSense ToolKit: http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=4144762

How big is the scale of your project, ex. how big/long/wide do you want your sensors to be? You mentioned a sculpture..do you want the entire surface area of the sculpture to be a capacitive sensor surface? What kind of software are you using to elicit changes from the sculpture sensing surfaces to the A/V outcomes? One way to possibly increase stability a little is through shielding of the various components, since almost anything can contribute to the capacitive readings. Another way is to do it through software. I am using Max/MSP6 to see if I can compensate for the stability/repeatability issues. One more thing, if you are using a breadboard (and I learned this the hard way), the stability will decrease greatly from the "noise" generated by the various components, including the breadboard itself. Once I soldered everything together, it worked better but I am still having stability/repeatability/range issues.

I am also a newbie at this and I sorted through these issues with a lot of help from fellow engineer/comp sci students from my school. I hope some of this was helpful and if you have found ways to increase your stability/repeatability/range for your cap sensor, please do post a reply!

Sincerely, J.

I want to sort out the stability of my capacitive sensors and i was just wondering what it means to add "small capacitor (20 - 400 pF) in parallel with the body capacitance".. i get the "small capacitor (100 pF) or so from sensor pin to ground" but not the parallel . Please advise.