Capacative touch sensor for a VERY large object?

[GICU812]

I've been working on perimiter security on my house. PIR is just to flakey for me.

Im working on IR laser trip beams for the front yard, the back yard is fenced. I could try and bounce a beam around with mirrors, but seems like it would be very finicky to adjust.

Is there a way to build a capacitive touch sensor that would work with such a large object? That way when someone touched the gate, or grabbed the fence to jump it, it would trip.

Would it even work with the fence being grounded? If not, what if I insulated the top bar, which would be doable, it would then not be grounded, could I build a CT sensor that would
sense maybe 200' of 1" mild steel pipe?

If not, I wonder what would happen if I energized the bar with a mild DC source, like 5VDC, not enough to even feel, but could I monitor it and sense a dip when someone touched the pole?

Im not horribly paranoid, just some activity in the neighborhood got me thinking, and im in need of a project. I definitely dont want motion sensors \ PIR because I want this to be a 99% accurate system.

Unfortunately I dont know much about capacitive touch other than plug and play chips. Maybe you guys can give me some pointers, or just tell me its impossible.

[SRLM]

You could use a laser and shine it across the top of your fence. If it's in the infrared range, an intruder would never even know that it is there. You'll probably want to modulate it so that the reciever doesn't confuse it with sunlight. If you want mechanical, take a look at the pressure force sensing area of the Parallax store.

http://parallax.com/Store/Sensors/PressureFlexRPM/tabid/177/List/0/CategoryID/52/Level/a/Default.aspx?SortField=ProductName,ProductName

Traditional security couldn't hurt either, like a thorny bush just below the fence.

[GICU812]

That gives me another option I suppose, stringing a thin steel cable above the post, with a pressure switch on one side, so if someone pushes down on the cable, it pulls the switch.

I had also thought about isolating the cable from the fence, powering the cable and grounding the fence, so when the two touch, it trips the sensor. Heck, I wouldnt mind just powering the cable with 220 and letting the intruders screams alert me, but legallity gets in the way.

Either of the cable ideas however leave me having to also alarm the gates. Touch \ laser could go right through or over the gates.

I checked, I could enclose the laser, reciever, and mirrors inside the main fence posts, just drilling holes to pass the beams so you woundlt even know it was there if it was visible, unless you noticed when you breached it. But I figured it would be very tedious to allign it. That and with the cables, there is a margin for failure \ misallignment \ etc. With a capacitive touch system, assuming it was self calibrating, it should be rather resilliant, though I dont know what would happened when it rained or snowed.

[Phil Pilgrim (PhiPi)]

You will need to consider things like wind, rain, snow, hail, etc., along with deer, raccoons, and pets, any of which could cause a false alarm.

Or, you could erect a bunch of inert but showy security items; then, in a conspicuous location (near a gate, maybe), place a lighted keypad with a little LED labeled "ARMED". Near the keypad, perhaps scratched into the enclosure's paint or penciled in so it looks surreptitious, would be a five-digit number. Just program the keypad so that when that number is entered (or any key is pressed, for that matter), the alarm sounds.

-Phil

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'Still some PropSTICK Kit bare PCBs left!

[Leon]

You can get piezo cable that could be attached to the fence and will detect anyone trying to get over it. It's used in UK prisons on top of the walls. I've got a length of it, it's *very* sensitive.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

[sam_sam_sam]

Leon

Thank You for sharing this with us about Piezo Cable

Here iis a Link That Sell that type of cable·····

http://www.windworld.com/products/msipu.htm

I just bought some of that kind of cable

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··Thanks for any··that you may have and all of your time finding them

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Sam

[Leon]

That's the stuff. I couldn't remember who made it.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

[GICU812]

So how much of the piezo cable would I need? Just a small piece for each section of the fence? At $1 a inch, I would hope not much more. If I went that route, I guess I could just use these from Parallax
http://parallax.com/Store/Sensors/PressureFlexRPM/tabid/177/CategoryID/52/List/0/Level/a/ProductID/89/Default.aspx?SortField=ProductName%2cProductName

The problem I see with Piezo, as metioned above, it would react to animals, wind, loud noise, and rain.

Capacitive sensing would react to animals, and possibly rain

Laser trip would react only to animals going over the fence (cats seem to squeeze through the cracks), leaves, possibly hard rain and snow accumulation.

Pull cable would react to animals going over only

Electrical contact cable would react to animals going over, and possibly snow accumulation.


Not only that, but I would have to consider the possible failure modes. I could program for obvious failures to avoid false alarm, but I dont want to have to tinker with this every week.

Piezo would have nearly zero failure mode

Capacitive sensing, if auto calibrating, would have nearly zero failure

Laser would be finicky to adjust, and possibly drift out of adjustment

Pull cable I dont know, it might stretch, and it might expand in the hot sun

Electrical contact cable should be fine as long as it doesnt stretch or expand enough to make contact


Just from that, it looks like a pull cable would be the simplest and best solution, though installing it poses some engineering challenges. Im still not convinced.


Anyone have any other suggestions? Input on Captouch?

[skylight]

Another capacitive touch sensor ic

http://www.qprox.com/assets/Download...0(A)QT1106.pdf

If i read it right you could break up your fence into approx 10 sections (3 +7)

presuming you have to keep the interconnecting wiring short you may need several ic's?

though this would then allow seperate zone's sensing

£1.50 or at todays rate $2.88 a throw


Post Edited (skylight) : 8/8/2008 6:41:42 PM GMT

[SRLM]

Take a look at the thread "Propeller People Counter Idea" An idea was posted there about using a laser to scan the area, like a supermarket barcode reader. For your application, you could set the module up on you house (or far away from the fence) and have the beam scan at about 1/2 degree measurements. To calibrate, make sure the area is clear, scan, and store the distances in the EEPROM.

You can adjust sensitivity in several different ways, it is unobtrusive (with an infrared), and can be recalibrated very easily. The downside is that it will take a while to build.

[GICU812]

I dont see how that would work though. There is no way im going to be able to detect a laser dot on a tree or person at 50 feet.

Maybe Im missing something.

Actually, I found IR leds to be much more powerful than I thought. Using a Parallax 38.5Khz IR reciever I found that just my TV remote will work from across the yard, and I have some supposedly high powered IR leds. So im going to try that first.

[GICU812]

I might have been wrong, using a IR reciever, I can pickup signals from my TV remote clear across the yard. It would definitely be the easiest thing to use, if IR LED's have the range, I just never thought it would work that far.

I cant seem to get my stamp working with IR though, I can use the TV remote to trip the sensor, but I dont get anything when I use the stamp, Im using parts and code from the Understanding Signals book that came with my Oscope.

Its a 38.5k reciever module, and using freqout 0,1,38500 its not working. And the LED is very dim when viewed with a camera, whereas my remote LED is very bright. If I hook the LED right up to 5v + resistor, its very bright. I', using a 220ohm resistor as called for.

[Leon]

You need an MCU with the piezo cable, with software to detect false triggering.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

[skylight]

GICU812 said...


Its a 38.5k reciever module, and using freqout 0,1,38500 its not working. And the LED is very dim when viewed with a camera, whereas my remote LED is very bright. If I hook the LED right up to 5v + resistor, its very bright. I', using a 220ohm resistor as called for.

You may have to lower the frequency or to allow the led to be on for longer and therefore brighter.
would you be able to adjust the mark/space ratio so the on time was longer?

Post Edited (skylight) : 8/9/2008 10:37:46 PM GMT

[GICU812]

I tried setting 0,10000,38500, which would pulse for like 10 seconds, still nothing. Maybe I'll try it with a transistior, maybe the stamp isnt putting out enough juice. Its just wierd that it wouldnt work when its in the Parallax literature

[GICU812]

Problem solved, Im using a BS2p40, so the timing is diffrent, 3.77 ms per pulse, so I had to send a pulse closer to 10000.

I still have the LED brightness problem. My TV remote will trip the IR detector clear across the backyard, the LED driven by the stamp drops dead after 10 feet. This is supposed to be a high powered IR LED, 16mW minimum radiant power @100mA.

I actually bought another IR LED that came packaged with a phototransistor. This LED is rated at 150ma, though its radiant power output is rated at 13-15mW, I would have to assume, if it will take another 50mA, it probably puts out more power, even if its not rated for it.

At anyrate, the LED seems far dimmer when run at 38.5khz from the stamp than it does when its just powered straight from power.

I'm using a 150 ohm resistor, but if I'm not mistaken, the LED should take a 39 ohm resistor (5v source, 1.2V foward voltage, 100 ma current) Is that correct?

If I bump the resistor down, it might help put out more light.

[skylight]

Be careful @100ma draw I think you are in danger of damaging your stamp you should use a transistor to switch the led.
Apologies if you are already doing so, if so then it still could be a mark space problem, you would need to make sure the on time is longer at the expense of the off time(duty cycle) you may need to increase the on time to 75% or more to gain that distance?
Experimentation will determine the duty cycle you need.
Alternatively use a 555 timer·it's easily·capable of handling that frequency and current and will allow the mark space ratio altering with appropriate wiring and then have the stamp trigger it.

Here's a link to help you calculate the Mark Space Ratio

http://www.antonine-education.co.uk/...er_circuit.htm

It looks like the Resistor designations are different to the ones in the formula, I'd take R1 to be Ra and R2 to be Rb

As the 555 is more than capable of handling that current you may get away with a 50% duty cycle(default) which will save you having to calculate the mark space ratio and just have to work out the RC components needed for the frequency.

Post Edited (skylight) : 8/10/2008 12:07:01 PM GMT

[GICU812]

I was using a transistor now, but thanks for the advice anyway.

I need 38.5khz to trip the reciever, so I dont think I can really play with on time anymore than how long I send the signal, which I am doing for 1 second right now.

However I am limiting the current to about 30ma, so I suppose that might be an issue, since I can safely triple the current. Unfortunately that means I need a 1watt resistor, or 4 1/4's and I cant even find one resistor with <100 ohms in my collection, guess I'll have to go shopping before I can see.

Any advice on how to extend the range on these IR LEDs?

Thinking more about Peizo though, I guess someone going over the fence would create a lot of vibration, as opposed to the wind or something that would produce very little. Maybe I'll order some of those Peizo sensors from parallax and play with them.

This is a very tough decesion because I have so many options, none of them the obvious answer.

Post Edited (GICU812) : 8/10/2008 6:32:39 PM GMT

[SRLM]

For the laser, the detection would be a change in distance of in the line of sight. However, since the IR LEDs work this would be much better and easier than a laser setup, not to mention cheaper.

Anyway, to extend the range of the IR LEDs the easiest would be to focus the beam. I'd look at LED flashlights at home depot or something. The only part that you need is the lens. Use this to focus the beam toward the reciever. You could even use multiple IR LEDs in one spot. I haven't tested this, but from using visible LEDs it seems the best step.

Also, there might be a way to daisy chain the IR LEDs so that you only need one BS2

BS2===IRRECIEVER---------------IRLED===IRRECIEVER----------------IRLED


My guess is that if one gets tripped, it causes the other LED to not emit, which then trips the next, and so on. This would eliminate the need for a bunch of BS2s or tons of wiring. Let me think on it for a while...

[kelvin james]

Accelerometers are widely used in car alarm systems, and yes very annoying when not set up properly. I don't know what span of fence you want to monitor, but you could check the vibration / and or the tilt with it. Basic physics will show someone climbing a fence will have a pull factor to the xy axis, thus creating a tilt by the offset weight. Accerlerometers are pretty sensitive, so you would have to experiment with the output to determine what is actually an alarm worth checking. Unless you have bears climbing your fence, most false alarms could be avoided. As far as an alarm goes, normally just some type of visual alert is enough to deter most, if the situation persists, then an audible alarm would be required. I use x10 cameras in globes placed visibly around the outside of the house, anyone scouting the place will wander off, not willing to chance being caught on video. Some type of deterent is the most effective measure. Anyway, just an idea with the accelerometer and not actually tested, i guess there are many ways to do what you want.