Hi All,
I'm trying to make a capacitive touch switch that I can integrate into large metal objects and turn them into sensors. Think of there being a 400lb engine on a pedistol (assume the entire object body is conductive), I want to have it where when a person touches the engine a video is triggered elsewhere in the room.
Now, I've done my research on capacitive touch sensing, and I've also played around with the QProx capacitive touch sensors -- and from what I can tell all of these sensors hedge on the fact that the human body touching the sensor will have more surface area / capacitance than the object itself. In other words these things are really geared towards cellphone screens.
So my open question to the forum is, if anyone knows of a circuit that can be designed to do capacitive touch sensing on large metal objects? Or if there is a way to calibrate existing sensors to function on large objects. I tried adjusting the resistor and capacitance on my QProx sensor with no real luck as I couldn't get enough info on HOW I should be adjusting my resistor / cap. I just tried various values which quickly got rediculous.
To give you an idea of the size of the objects I'm talking about:
- 700lb 40"x36"x48"
- 4,000lb 18"x75"x75"
- 1,500lb 22"x22"x22"
- 300lb 76"x20"x24"
So while all of these are more or less cubic/rectangular and compact in form, none are small.
Thanks!
Thanks for the info.I will
Thanks for the info.
I will experiment with that circuit, but one thing right off the bat that strikes me as concerning is that the antenna is a thin wire. The big question I have is what will happen with the antenna is a 4,000lb block of steel!
Back in the old days…
Back in the old days, (70’s), we used the NE555 timer chip for about everything, including a touch switch:
http://www.google.com/search?q=touch+switch+using+555+timer&tbm=isch&tbo=u&source=univ&sa=X&ei=yBHmUcbIIqGpigLwhYGYCA&sqi=2&ved=0CC8QsAQ&biw=1016&bih=575
Here are just a few examples, I will see if I still have the schematic of the one I ended up using.
It looks like what you really want is a touch switch and not a capacitive switch, two different things
Another thought…
There is another way to go about this: Once again, in the old days, there was security technology called an “E Field Fence”. I would think you could use a similar circuit for your application. You would build up a low power high frequency RF generator configured as a “grid dip meter”, such that when it is connected to your mass, (antenna), you could adjust the L/C portion of the circuit for resonance. Assuming your mass is insulated from ground in some manner, a person touching the mass would detune the circuit, drawing additional current which you should be able to sense to trip your circuit. The circuit shouldn’t be too difficult and in fact, the simpler the better since you actually want circuit loading when it is out of resonance so you can detect the additional current draw. Your sense circuit could be as simple as the diode portion of an opto-isolator in the emitter circuit of your RF output and take your output off the transistor junction.
E Field Fence
Well I think it’s safe to assume that people won’t be walking around barefoot, so there should be some form of isolation.
This actually sounds the most promising (or at least interesting) to me, but I’m going to admit that this is out of my current range of electronics knowledge. I’m pretty good at throwing together digital components and controlling through microcontrollers but my background in electrical engineering is minimal.
So let me see if I can recap correctly. I need to create a simple low power high RF generator. I would tune this generator based on the mass/surface area of my antenna. If additional mass/surface area contacts the antenna, my circuit would increase the current load in order to compensate. I then create some form of current meter to test if the current draw changes. I could use part of an opto-isolator emitter circuit, if I tune it correctly. I believe this is what they use in solid state relays. Anyway I could use that or I could use some form of current monitor, and if I tune the whole thing right I get a trigger.
What I am confused about is how my HF low power RF generator would be able to adjust itself given the added mass/surface area. I’m not familiar with creating such circuits. Does the circuit aim to keep a certain frequency, and with added mass it needs more power to do that? How does it know the frequency of the signal it’s generating?
thanks!!
