# Creating A Wire Cut Detection

Discussion in 'General Electronics Chat' started by ajm113, May 16, 2012.

1. ### ajm113 Thread Starter Member

Feb 19, 2011
176
5
Hello I want to create a wire cut detection circuit for my micro controller and I'm not sure the what the best way is to do this without frying my microcontroller out.

This is basically how I kinda imagine it working:

Pump 24v through a line (since the line is maybe going to be going longer then 100 ft).

Have a EOL resistor of 4.7k at the end of the line and have my trigger/switches in paralleled within the line and of course have a 100 1/4 watt resistor in series.

Then have a some sort of ADC circuit that takes the 24 volts drops it down to 5 and gives us 3 pin binary outputs of the lines state based on voltage output.
2= We are okay, 0 = Line was cut, 3 = Alarm.

Last edited: May 16, 2012

Apr 5, 2008
15,802
2,388
Hello,

Can you post a schematic of the idea?
That way we can see how to improve it.

Bertus

3. ### #12 Expert

Nov 30, 2010
16,703
7,338
It generally works better to send a constant current into the wire and see what the resulting voltage is...but I'm just guessing. Think about whether a constant current will give you usable results.

4. ### ajm113 Thread Starter Member

Feb 19, 2011
176
5
Okay this is kinda how I imagine it working.

I know the sketch is a bit crummy, but I kinda ran through it.

The ADC basically outputs a binary format of the current state the wire is in encase if your wondering.

EDIT: I have the same idea #12, it's just I want to make or use a some sort of ADC circuit or drop down voltage circuit that wont damage the PIC. Now if I get a some sort of drop down voltage circuit that takes the 24v and converts it to 5v and have it so if 24 drops down to half, I would get 2 volt output instead from the voltage converter for a ADC pin on my PIC.

I've posted my voltage divider, but idk if it would be considered, a safe alternative.

• ###### idea.png
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Last edited: May 16, 2012
5. ### #12 Expert

Nov 30, 2010
16,703
7,338
This will give you 19 milliamps that never goes above 5 volts.

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6. ### Sparky49 Well-Known Member

Jul 16, 2011
835
417
There is a tool called a Time Domain Reflectometer which does exactly what you are asking for.

I'm not very good at programming, but I'm sure you could program it to understand a reflected pulse indicates a break?

TDRs can also judge distance to the break. With a bit of maths, your chip can tell you roughly where the cut is.

I think the theoretical 'range' of my project was about 2.5km!

7. ### ajm113 Thread Starter Member

Feb 19, 2011
176
5
Thanks #12, I'll give your circuit a try, I just need one zener, but Radioshack doesn't have one that fits my watts range, I need one at least 1/2 watts for testing reasons or if I decide to use it on something else. I don't suppose anyone knows who gives free samples of Zener Diodes?