"Open circuit" logic help

[Daltondix]

Howdy! I'm re-designing a circuit and am in need of a way for notification of an open circuit (via a piezo/transducer). Something to indicate the circuit is open. Since the circuit will be in an open state, the piezo must be powered elsewhere somehow. I can't use an ASIC, FPGA, etc. as it is now; it must be accomplished using discrete components to keep the cost low. Any ideas?

 

[melone]

Most multi-meters have a continuity checker (buzzes when impedance is low, < ~.5V). Will that work?

 

[Daltondix]

Thank you Melone, but I need it built into the circuit. I received a reply from 'Viking' with instructions on how to accomplish this using an NPN transistor, which looks like it should work. I'm off to the lab to verify it.

 

[maged541946]

while we almost agree upon short circuit as a result of having DMM's seting a standered for short circuit, open circuit on the otherhand is not, while for some application a 10k resistance is considered open, in others like some touch keypads 100K will be low resistance and for humidity measurements a 1 meg is low.

The best thing is use your DMM to know the exact value of the resistance. but if you know that value you want as a threshold, a simple comparator circuit will be an accurate way to do it.

 

[VEBill]

Many piezo devices are of such Hi-Z that they're almost an open circuit to start with...

 

[VEBill]

...if so, then you could swamp the piezo device with a 10M parallel resistor to give you something to measure (assuming that the open circuit that you're looking for is in the wiring).