hie guys ,am working on a cable anti theft system and intend to use voltage sensor and current sensor to detect voltage drop when my cable is cut and is in open circuit. i understand voltage drop is directly proportional to distance.any suggestions will be much appreciated
Hi,
What kind of cable? Length? Applied voltage and current?
Look at using a Time Domain Reflectometer. [LINKS](http://arduino TDR)
pontiuspilate:
hie guys ,am working on a cable anti theft system and intend to use voltage sensor and current sensor to detect voltage drop when my cable is cut and is in open circuit. i understand voltage drop is directly proportional to distance.any suggestions will be much appreciated
You can't measure the voltage drop after someone has cut and stolen your cable, because there is no longer any path for your test current to flow. (Unless the thief happened to leave the cut ends shorted together).
Making a Time Domain Reflectometer as suggested by Terry King would be a good solution, however you will need some additional hardware designing/building as you need to be able to measure time differences in nanoseconds.
1ns = 10-9s
Arduinos can only measure down to microseconds.
1µs = 10-6s
Note that there is a difference of 1000 times in magnitude.
thanks for the insight terry king and johnlincoln ... the cables are standard twisted pair cables used in telecoms . i want to measure up to a lenth of 20m for this is only a prototype for my project.
standard twisted pair cables used in telecoms
How many pairs? If this is ethernet cable you may be able to dedicate a pair to this, shorted at one end.
terryking228:
How many pairs? If this is ethernet cable you may be able to dedicate a pair to this, shorted at one end.for my demonstration i am using 3 lines using the same concept , and yes i may use ethernet cable for this.
i may use ethernet cable for this.
typical ethernet cable has 4 pairs but only two are used for usual ethernet connection.
terryking228:
How many pairs? If this is ethernet cable you may be able to dedicate a pair to this, shorted at one end.
Eh? How would that work? They wouldn't be shorted after the cable had been stolen.
They wouldn't be shorted after the cable had been stolen.
Exactly. So the "OK LED" would go out. Or the LED inside the optical isolator would alert the Arduino.
Most wired alarm systems have a more sophisticated system called "Supervised" sensors. The current is actually measured, and so a short OR an open is seen as an alarm.
if i get you well guys an " ok LED" means the current is flowing and i guess my current sensor can detect and read this current , and if there hapens to be acut and my cable is in open circuit , the current sensor will sense nothing thus triggerinf an alarm and switching off the "OK LED".
using this method how then can i locate the point of cable cutting in this case
I can not imagine how you could pinpoint the location by measuring anything. Interested in his this will develop, looking along.
I can not imagine how you could pinpoint the location by measuring anything.
The only way to find out how far down the cable the break/short is located is to send a signal down the cables and wait for it to be reflected back. This needs high speed circuits: the propagation delay in the cables is approximately 2 Nanoseconds Per Foot. So 4 Nanoseconds Per Foot round trip.
If the fault was 100 feet away the delay of the reflection would be about 400 nanoseconds. SO you need added fast electronics hardware in addition to Arduino.
People DO know about this. See THESE LINKS
As above posters have indicated, TDRs are now the way this is done.
You can buy small hand held ones like this TDR900 - Hand-held Time Domain Reflectometer/Cable Length Meter
which will do exactly what you need.
The Arduino is way too slow in so far as clock rate to be able to do this.
Hand held TDRs are expensive though, usually around $300 upwards, depending on what accuracy range you need.
mauried:
As above posters have indicated, TDRs are now the way this is done.
You can buy small hand held ones like this TDR900 - Hand-held Time Domain Reflectometer/Cable Length Meter
which will do exactly what you need.
The Arduino is way too slow in so far as clock rate to be able to do this.
Hand held TDRs are expensive though, usually around $300 upwards, depending on what accuracy range you need.
Hie there, getting this kind of device like you mentioned is expensive , and also , it would have done all the work that am supposed to do , so the issue here is to make a laboratory solution to mimic the circuits of the megger, that can be used with the arduino, thats the challenge at hand.but now comes the issue of timing mentioned several times by my fellows in this thread, any idea of a workaround on this
pontiuspilate:
Hie there, getting this kind of device like you mentioned is expensive , and also , it would have done all the work that am supposed to do , so the issue here is to make a laboratory solution to mimic the circuits of the megger, that can be used with the arduino, thats the challenge at hand.but now comes the issue of timing mentioned several times by my fellows in this thread, any idea of a workaround on this
Not expensive at all! A fast pulse generator matched to the cable impedance and an oscilloscope and you can see the initial pulse and the reflected pulse and see to times on the display.
Paul
Well looks like there have been some projects like this already looking at the link provided by Terry
Hardly surprising.
It is a "standard" problem, solved long ago and now incorporated into common instruments.
The point is that it is not one amenable to a microprocessor approach as such as it involves essentially RF/ VHF techniques and hardware. As is so often the case, a microprocessor is a part of the equipment, to monitor the hardware that actually performs the measurements.
Paul__B:
Hardly surprising.It is a "standard" problem, solved long ago and now incorporated into common instruments.
The point is that it is not one amenable to a microprocessor approach as such as it involves essentially RF/ VHF techniques and hardware. As is so often the case, a microprocessor is a part of the equipment, to monitor the hardware that actually performs the measurements.
came across this concept of measuring length of a cut(open circuit) twisted pair cable, am not sure how i can measure the frequency then using Arduino analogue pins and covert the frequency to represent length of cut cable. Please assist
terryking228:
Hi,What kind of cable? Length? Applied voltage and current?
Look at using a Time Domain Reflectometer. [LINKS](http://arduino TDR)
thanks for the Links TerryKing..... i came across a 555timer based reflectometer circuit. however am not sure how i can use the arduino analogue pins to note and display the frequency for a particular twisted cable length as well as using the frequency characteristics to calculate the length