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water leak alarm

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Take a closer look at the literature and you will see that the circuit is not open when the sensor is dry. That's why it's made that way - the resistance has to be within a range - outside the range, regardless of whether wet or dry, suggests a failure.
 
Fail-Safe?

I finally came up with a design. The circuit checks for continuity and will fault if the cable opens.

Not sure if it is considered fail-safe. Would like to have circuit approved for UL991.

Thanks
 

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Not sure if it is considered fail-safe. Would like to have circuit approved for UL991.

How does UL define fail-safe? Using MTBF?

If you're going to produce this be sure to get Statutory Invention Registration for a few bucks from the USPTO. Otherwise some sociopath CEO will steal your idea, patent it, and then sue you for infringement. :mad:
 
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Not sure how UL defines fail-safe but I'll give you some very simple examples:

In colder climates the valves that control steam or water to heating systems are arranged in a fail-safe manner - so that if the control or control power is lost the valve opens turning the heat fully on. The "safe" mode, in this case, is heat on so that freezing is prevented (causes lots of damage) - and one accepts that the building can overheat to an unpleasant but not dangerous level.

In many electrical circuits current flow is either on or off. The problem - the "off" condition might be unintentional - the result of power failure, wire breakage, component failure. An alternative is to have some current flowing all the time - a supervised circuit might be one description - with low current possibly indicating "off" - a higher current indicating "on" and currents at low levels or zero suggesting a failure.

Again, not sure what UL has to say - just sharing relevant thoughts.
 
I used to work for a company that made ignition control systems for gas burners. Fail safe means that if any component fails either open or short, the resulting condition will not do damage. For us, the safe condition was the gas valve had to be closed. In your case it would mean that in case of failure of any component not just the sensor, the alarm would sound if there was a leak or not since that is your safe condition.
 
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