Piezo Coaxial Cable as driveway sensor

Thread Starter

bradenjoseph

Joined Mar 6, 2016
13
If that doesn't work or proves impractical, how about using a length of rubber tube, sealed at one end and with a pressure switch fitted in the other?
Yes, that's the typical solution, but the tube usually lays on top of the ground.... googling now I see that you can bury it. sheesh. all this time wasted. I am going to look into that now :)

Thanks guys
 

Alec_t

Joined Sep 17, 2013
11,637
A car weighing an average of 4000lbs (1,814kg) would generate about 17,800N.
So it might, but not all of those N get to the cable. Only the limited area of four tyres is in contact with the ground, and I presume only two wheels could be over the cable at any time. The buried depth of the cable and hardness/compressability of the driveway would be major factors in restricting the force experienced by the cable.
 

Thread Starter

bradenjoseph

Joined Mar 6, 2016
13
If that doesn't work or proves impractical, how about using a length of rubber tube, sealed at one end and with a pressure switch fitted in the other?
Blocco, that's hilarious that you found that, after I bought the pressure switch and the hose already lol. If I can't get it to work, maybe i'll go back to the piezoelectric. :)

On the pneumatic tube attempt, I have all the mechanical parts figured out but need help with the circuitry, which is just a simple gate.

With 0 PSI, the output is 0.5V, with 360PSI the voltage is 4.5V. The output voltage is linear to PSI.

I have been playing around with BJT and MOSFET switches trying to complete my circuit with these ranges (ideally I want the circuit to close anything above 0.6V). I tried the attached, but it's not supplying enough current to my load in order for the bell to ring (25mV). Can you help?
 

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Thread Starter

bradenjoseph

Joined Mar 6, 2016
13
I think you could do this with a differential pressure switch of the type used on central heating boilers to tell the controller that the exhaust fan is running.
This type of thing:
http://www.ebay.co.uk/itm/Potterton-Prima-Air-Pressure-Switch-Honeywell-C6065A1192-2-/322031889784?hash=item4afa98ad78:g:GUwAAOSwmmxW3cAU

No electronics necessary.
So when the hose is compressed it would send a voltage up to the house? There isn't much detail on that link
 

blocco a spirale

Joined Jun 18, 2008
1,546
So when the hose is compressed it would send a voltage up to the house? There isn't much detail on that link
The pressure switch has a micro-switch fitted to the back so use it to switch power to the sounder directly. Leave the "L" (low pressure) connection open, if you block it off it won't work..
 

Thread Starter

bradenjoseph

Joined Mar 6, 2016
13
The pressure switch has a micro-switch fitted to the back so use it to switch power to the sounder directly. Leave the "L" (low pressure) connection open, if you block it off it won't work..
Do those output a constant voltage regardless of the pressure difference? The device I bought is linear as I mentioned. Either way I want just 1 cable going back to the house.

Going back to my diagrams, would you be so kind to help me figure that circuit out? I don't think it's complicated I just don't think I have it wired right.

Thanks blocco
 

Thread Starter

bradenjoseph

Joined Mar 6, 2016
13
It's just a mechanical switch, either open or closed, on your diagram it would replace the mosfet.
But what I bought sends a voltage linear to the pressure applied, so what circuitry would I need at the house (bell) in order to close the bell dc circuit and make it ring while the pressure is applied. Like I want only voltages from 0.6 and above to close the bell circuit anything below to stay open
 

Alec_t

Joined Sep 17, 2013
11,637
With 0 PSI, the output is 0.5V, with 360PSI the voltage is 4.5V. The output voltage is linear to PSI.
The question is, what will the dynamic pressure increase be when a vehicle goes over the (presumably buried) tube? Let's be generous and guess 1psi. The gauge should then read 0.5+(4.5-0.5)/360 = ~0.501V. So you're looking for a change of only ~10mV. That dictates the need for signal amplification/comparison right by the gauge, to prevent the signal being swamped by noise pickup on the 300ft cable. An LM339 or similar comparator should be able to detect a 10mV change ok. The voltage reference for the comparator would need to self-adjust according to static pressure in the tube. That could be simply done by integrating the gauge output.
The gauge you have is not ideal, unfortunately.
 

blocco a spirale

Joined Jun 18, 2008
1,546
By using a linear pressure sensor you are making this unnecessarily complicated since you only need to determine whether or not there is pressure, you don't need to know how much. I would simply "engineer" a tiny leak in the system to prevent the tube from becoming pressurised due to increases in temperature. A differential sensor will be immune to atmospheric pressure changes.
 
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