Detecting cut wire (tampering) for low power project

Question

I'm trying to develop a cut wire detector that will be used in order to detect if a specific wire is broken or not (a.k.a fake bomb).

I'm using an ATMEGA328P running on batteries (3V) at the internal 8 Mhz clock. The external circuit used to detect the tamper loop, has this configuration:

In order to detect the state changes of the tamper loop, I'm using the ISR INTERRUPTS of the atmega MCU, but with this configuration the resistor draws around 3/4 mA, also, when putting in a higher resistor (R1 = 100kΩ) the power consumption is too high (30/40 μA).

What do I need? I need to find a solution that allows me to monitor if the wire is cut (the wire lenght is around one meter) without using any additional power from the circuit, and keep the circuit running on battery power without the need to worry too much about the power consumption of the tamper monitoring circuit.

The target power consumption for the tamper circuit has to be lower than 100 nA.

Any suggestions?

Answer 1

You can connect the tamper wire between two microcontroller I/O pins then only wake the microcontroller infrequently to sample the tamper connection between the two.

This answer describes the principle of doing this for a switch with the same low power goals. The actual final average current achieved will depend on your tamper connection sample rate and any other microcontroller activity.

If you tamper connection is prone to ESD, such as by human contact, you may also need to add TVS diodes across the tamper connection ends and low value series resistors to the I/O pins. But that depends on the actual implementation, so I'll leave that to you as you don't detail it in your question.