Hello,

So I'm looking into making a capacitive proximity sensor like the picture below. I want to be able to detect the distance of about 10 cm, I have seen a video and I know you can do this, so my question is when it comes to design, how should I design the sensor pad? I dont want to use IC's.

Thanks

 

Here is how it works. Laid out in block diagrams as well as math.

There are touch sensor ICs with TTL outputs that can drive transistors, no special magic to use, connect power and ground, put wire on touch input, when it is touched, supply voltage is at output, otherwise output is low.

If you read through that app note, you can also build one using discrete components, but false triggers are a problem.

 

Havent looked at the link yet but I dont want to have to touch something I want to measure distance.

 

A touch sensor won't help if the OP wants a 10cm range.

I built a theremin years ago using Panasonic analog optical sensors, it worked well and might have had the 10cm range OP wants. Google "Theremin" and you'll see many different ways to detect hands at a distance, the majority being RF.

Google Bob Moog of synthesizer fame. His last efforts were Theremin manufacturing. After seeing his synthesizer endeavors get chewed up and spit out he backed away and did the Theremin thing. He was very smart and very nice, normally two mutually exclusive traits. The good always die young

 

Well see I know you can use capacitive proximity sensors because of the electrical field *fringe field I believe. When your hand gets closer the fringe field changes there for capacitance changes.

 

Motorola made an "e field" sensor which may have done 10cm. I have an evaluation board from Motorola for it, I'll send it to you for the cost of postage. Lemme know.

 

Well see I know you can use capacitive proximity sensors because of the electrical field *fringe field I believe. When your hand gets closer the fringe field changes there for capacitance changes.

That won't work well without an IC. The concept is the same as touch sensing, a very high frequency oscillator, detect changes in oscillator frequency due to capacitance change, look at "PLL".

 

Thanks for the replies and ideas the theremin idea seems interesting but I would need 4 different sensor withing a couple of inches from each other so I believe they would interfeer with each other.

Thanks

 

Having the sensors closer to each other than the sensing distance is possible, but requires more processing (analog or digital) to find the "pickup" that has the greatest variation.

 

I don't recall the Panasonic sensor part #. They are cheap and simple to use. Toy robot builders use them for collision avoidance. I bought mine from Maximum Robotics. They had more than the 10cm range you require, probably 20.

Maybe these will give you an idea. I'm sure you could roll your own without IC's but probably not for $35. This is NOT where I bought my Panasonic parts from:

http://www.maxbotix.com/Ultrasonic_..._Sensors.htm?gclid=CKv7rcHaqbUCFUQw4AodaVMASQ

Back in the day I made ultrasonic sewer level monitors. To compensate for the dead zone close to the sensor, I mounted the sensor horizontally about 12" away from the face of a first surface mirror mounted at a 45 degree angle. This allowed the sensor to respond to water level all the way up to sensor housing itself. Pretty cool eh?

 

That is what we get paid for...figuring out how to make it work.

 

Back to theremin, the antenna for this theremin is pretty big but I'd like to be the size of a coin. How much of an effect will it have?

 

Using the formula for capacitance, plug in the surface areas and the dielectric constant for whatever medium this will be used in. Examining whatever circuit you plan to use for this figure out the change in output for the change in capacitance.

This will make it much, much easier:

http://news.yahoo.com/blogs/technol...-gold-coin-weighs-ton-costs-57-213059944.html

 

I once was working on a projected capacitance sensor, I was looking to detect water inside a tank with no openings, sense it thru the tank by change in capacitance of a sensor pressed up against the side. I was able to reliably sense water about half an inch from my sensor. You would need to experiment and refine the sensor to suit your needs.

Here are the two most informative posts:

http://forum.allaboutcircuits.com/showpost.php?p=405600&postcount=30

http://forum.allaboutcircuits.com/showpost.php?p=405868&postcount=33