Need idea about pressure detector

[joe_1]

Hi:

I am working on project, part of it involves in getting signal whenever someone walks thru gate. Infrared detectors are not allowed, so the best I could think of is some sort of metal to be put on floor by the gate and when someone walks on it, it responds to the pressure by sending signal to the main circuit. My question is how do I get this type of metal, or does anyone has better solution than this?

Thanks alot for any response.
Joe,

 

[Optikon]

Hi:

I am working on project, part of it involves in getting signal whenever someone walks thru gate. Infrared detectors are not allowed, so the best I could think of is some sort of metal to be put on floor by the gate and when someone walks on it, it responds to the pressure by sending signal to the main circuit. My question is how do I get this type of metal, or does anyone has better solution than this?

Thanks alot for any response.
Joe,

I don't know of any metal that would do this with even the slightest chance of properly detecting. If it does exist, I suspect it to be very exotic and also very expensive. "metals" are excellent conductors by nature and so this makes it tough for any kind of potential difference to be established anywhere in its volume.


I think what would be better is to use a regular type of metal such as steel and attach a strain gauge-resistor bridge circuit. There are many example circuits for this type of thing and all material and parts are dirt cheap.

 

[stevez]

I agree with the strain-gage solution. An alternative might be literally making your own capacitor - two pcs of sheet metal, steel, aluminum, whatever - separated by plastic insulating material that is somewhat soft, possibly rubber. The "capacitor" would change in value as someone stepped on it - make this "capacitor" part of an oscillator circuit and use the changes in frequency to indicate the passing of people.

 

[vaineo]

I saw something in one of my books similar to what stevez said. You need two sheets of mylar, two pieces of coper and a sheet of conductive foam like the stuff they package CMOS IC's in. the two sheets of Mylar go on the outside, the sheets of copper go just inside the mylar with leads attached to it and the conductive foam goes it the middle. You would need to measure the resistance change to detect when someone is on top of it.


_________________________ Mylar Sheet
--------------------------------------- Copper (First lead)
====================== Conductive foam
--------------------------------------- Copper (Second lead)
_________________________ Mylar sheet

 

[stevez]

I like the conductive foam idea that Vaineo mentioned. Just to be sure we don't confuse Joe - the sheetmetal/rubber/sheetmetal is a capacitor that will vary. The mylar/copper/cond foam/copper/mylar is a resistor that will vary. Good idea. Only problem with either is selecting materials that will not degrade significantly with time. A foam might eventually collapse but only if enough pressure is applied.