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"Open circuit" logic help

"Open circuit" logic help

"Open circuit" logic help

(OP)
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?

RE: "Open circuit" logic help

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

RE: "Open circuit" logic help

(OP)
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.

RE: "Open circuit" logic help

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.

RE: "Open circuit" logic help

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

RE: "Open circuit" logic help

...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).

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