I have searched Google top to bottom the only circuit which fullfills my requirements is a wire break or wire loop security alarm as shown in figure 1. I soon realised that if the robber shorts the wire before cutting it (as the situation exposes the loop) my system can't sense it and thealarm will not turn on as in figure 2. Can some one tell me how to make it sense a short circuit as well?
Building on @jsotola's comment. A window comparator coupled with a resistance in the sense line would allow the circuit to detect the open and shorted condition.
Understanding a window comparator
https://www.electronics-tutorials.ws/opamp/op-amp-comparator.html
For example in the image above, if you used a 1kΩ resistor from Vin to Vcc and then your loop incorporated another 1kΩ resistor (physically out in the loop somewhere), that would create a voltage of 1/2Vcc, which is between the 1/3Vcc and 2/3Vcc limits. In this state, the output would be high.
If someone opened the wire, the voltage would go above the 2/3Vcc limit and the output would go low.
If they shorted the wire, then the voltage would go below the 1/3Vcc limit and the output would go low.
The thing you’re asking about is called in the trade a ‘supervised loop’. This can be implemented using a known load at the end of the line, such as a resistor or a Zener diode as well as other anti-tamper and fault detection methods.
This page gives an example of a Fully Supervised Loop:
So the supervision detects the following:
And I'll add, since you asked:
The control panel for such a system monitors line resistance. A possible design for this would be:
My thought would be that using an ADC instead of fixed comparators allows flexibility in how the circuit is set up. Since most microcontrollers feature multiple ADC channels, there's no additional hardware needed to do it this way.
Here's a setup that treats the sensed line as a voltage divider (simulate it here):
In the normal condition, the sensed voltage is 2.5V since the EOL resistor is the same as the pull-up at the control panel. Your uC software would monitor this voltage, and if it's 5V you know the line was cut; 0V you know the line was shorted, and some value above 2.5V but less than 4.5V you know one of the alarm switches opened but the line is still good.
That said, a smart burglar could defeat this by injecting the voltage if they could gain physical access to both sides of the loop.
The fastest way? Wire up a relay that has both normally open and normally closed contacts.
