I want to make the alarm that can detect if someone cut the wire that made of copper. It is about 2km. Because if it is about 100m, should be not much resistance can be detect from the wire right?

I just need to make the simulation only so i thought i could just replaced it with potentiometer.
The code and circuit that i gave was just my idea on how to make the alarm.

 

You are correct, there is “more resistance” in the same (gauge, etc.) wire of 2km in length verses 100m. But 100m of wire will also have a measurable (ADC detectable) resistance.

But I have to ask, is the actual length of the alarm circuit 2km, or 100m? If 100m but you wish to use 2km worth of wire just for the added resistance, what happens to the 1.9km of wire you’re not using? If left on the spool, you’ll have quite a coil (inductor). If it’s piled up in the corner like spaghetti then I’m hoping it’s insulated!

I have some other questions:

You’re worried about the wire being cut. Is the wire of sufficient diameter to require cutting with tools, or thin enough to break if somebody (or something) simply passes through and breaks it?

Is this 2km length of wire surrounding the perimeter as one wire, or are you making a cage of wire to enclose a room or yet, a web to surround a volume of space?

If the 2km of wire is not supported correctly, it may stretch and affect the resistance, you’ll be fiddling with the PicAxe code to compensate for the changes in ADC values.

Can the single 2km length of wire be broken up into sections, each section called a zone so you may determine which zone was breached?

What are the specifications of the 2km wire - gauge, solid, stranded? Is this 2km piece of wire one piece, or is it spliced?

Off topic, but inquiring minds want to know: How much does the 2km of wire weigh, and how much money did you spend on it?

It’s not my intention to sound sarcastic but I think this one of those projects that requires rethinking from the start.

joe

 

i dont know the weight but the diameter is 0.5mm.

 

Here’s some math on your .5mm wire: (all measurements approximate)

.5mm wire is about 24 gauge wire and assumes typical commercial solid copper wire (verses aluminum coated with copper, stranded, etc.)

2km of 24 gauge wire with enamel coating (magnet wire) weighs about 4kg and offers about 170 ohms of resistance. To prevent stretching (increasing resistance) it’s recommended to support the wire every 1.05m.

100m of 24 gauge wire with enamel coating (magnet wire) weighs about .22kg and offers about 8.5 ohms of resistance.

joe