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Ultra low power tamper indicator

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hyperinflation

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I am trying to design a circuit to detect when a chassis has been physically compromised. The requirement is that the quiescent current can be no more than 900 nA when operating on battery power.

The basic idea is there is a battery that is connected to the circuit through an array of redundant microswitches. If the case is opened, then the current from the battery will be cut. Since the power to the machine may be turned off when the case is compromised, the circuit has to latch the event, so that it can be reported when the power comes back on. An indicator will light when an event has been detected.

I started trying to use a low power latching comparator, but the ones that latch seem to draw about 2 uA, which is much too high for this application. I found that the AUP series logic from TI uses only about 500 nA for a gate, and as a bonus is pretty cheap. I believe this circuit should latch. After the circuit is reset, both inputs are high at the AND gate. If an event occurs, power should be cut to the gate. When power is restored, a small capacitor should keep transients from causing the gate to flip on, and thus it should stay permanently in the off condition until Vcc is restored and it is reset.

Can anyone see a reason why this won't work, or can anyone think of a better way to do this?
 

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  • Tamper.pdf
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Wouldn't just sticking a single pyroelectric sensor inside the case be easier?

EDIT: Oh, you need power off operation too. Hmmmm. MY question is...would be the power be cut before, after, or right as the case is being broken into? Because if it is is before then I don't see how any circuit would work.
 
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