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Capacitive Proximity Sensor

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sticky01

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Im building a capacitive proximity sensor on a breadboard that will be used to determine what material was passed in proximity to the sensor. Only trouble is, i dont even know where to begin, or what electronic components to use. Any help would be greatly appreciated.
Thanks
 

Reloadron

Well-Known Member
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For what it may be worth this is a pretty good little link. When you get to the osc stage click. Then advance to the demod and all the way through the demo. Generally when I need a prox sensor or prox switch I just buy an off the shelf solution. What exactly are you looking to detect when it is in proximity of the sensor?

Ron
 

sticky01

New Member
Buying one would be easier, but it isnt an option for me. Thats the route i would go usually too. Looking to detect all manner of materials. Was hoping to connect the output to the adc on a PIC to display the material on a LCD
 

Reloadron

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If you run through the link I provided you will note that for the most part a capacitive proximity sensor just identifies the presence of a non-conductive (dialectic) object like plastic for example or conductive objects within its sensing range. They detect a change of capacitance when they "see" an object. That causes a shift in the oscillator frequency. The oscillator output is fed to a demodulator circuit and then on to a trigger stage with the trigger stage driving an output. The output is generally a 1 or a 0 as in a digital level; signal and not an analog signal. The sensor really doesn't know exactly what object is out there, only that there is an object out there.

I guess if this is a school assignment you may want to think about it this way. Create a circuit that is the front end of a prox sensor (capacitive type). Note the frequency and frequency shift for assorted objects placed in front of it. The oscillator frequency should vary. Run that frequency into a F/V (Frequency to Voltage) converter and then run that analog signal into the analog input of your PIC Chip. That is likely how I would approach it anyway. Maybe some other members have some thoughts.

Ron
 

nsaspook

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Detection with a capacitive proximity sensor is pretty simple but characterization from the results can be hard unless there is a small amount of possibilities and large changes in the sensor output to avoid false detections.

http://www.pepperl-fuchs.com/global/en/2746.htm
 

sticky01

New Member
nsaspook

I don't need to be able to distinguish between a large range of materials. If the sensor can pick up on 5 at most materials, that would be sufficient, so i'll choose materials that are not too close in the dielectric spectrum. Thanks for the link.
 

jpanhalt

Well-Known Member
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If you want something not run of the mill, you might consider a capaciflector (http://www-cdr.stanford.edu/touch/previous_projects/capaciflector/capaciflector.html ).

Basically it is a directional capacitative senosr. I have a 2005 report from Goddard /NASA on the subject. Unfortunately the report is about 4 MB, and the link appears to be dead. NASA was interested in its application for robotic inspection inside rocker engines. It might be worth searching for.

Nevertheless, here is a cross section of a sensor I made and the schematic. One down side is it might be too sensitive for your use.

John

Edit: In thinking a little more about it, Roman Black's capacitance meter might be better suited to identifying materials at close range. Here's a link: http://www.electro-tech-online.com/threads/pic-based-high-resolution-cap-meter.119900/ It is also described on his site.

Capaciflector.pngView attachment CAPACIFLECTOR2.pdf
 
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sticky01

New Member
With regards to the capaciflector, you are right. For my application it is a little too sensitive as I only need it to operate across a broad spectrum. Since I am using a PIC 16F877A to collect and convert the data, Roman Black's model looks perfect for me. It will only require changing the code for the microcontroller to recognise a different microcontroller target.

Thanks again
 
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