Watchdog Water Alarm teardown

[reagle]

It's been a while since I took anything apart and documented it, but this thing finally caught my attention. Almost retro!
Full writeup at: http://kuzyatech.com/watchdog-water-alarm-teardown
CD4069 all the way

[Gyro]

That looks like a neat little design - I like the mechanics of the separable sense contacts. It looks as if it is just using a fet to DC sense contact leakage (should be ok because they're dry most of the time) and the 4069 as an oscillator/driver for the piezo element.

I'm a bit surprised they used a separate fet - they could have just used one of the inverters in the 4069 to do the same thing.

[reagle]

I think it boils down to power- this way the only drain is FET leakage which is still lower than 4069s sleep current. And a FET is dirt cheap these days. Speaking of which, I wonder what the BOM cost of this thing is. They sell for $11 retail

[Zero999]

Most piezo alarms use a single transistor to drive the piezo and possibly an inductor to boost the voltage and make it louder.

This design uses the CD4096 to make a push-pull amplifier with the input connected to feedback, taken from the piezo.


Using the 2N7000 provides a low standby current.

[reagle]

Good catch on push-pull. The out of phase invertor stages are a neat way to do that

[Gyro]

I think it boils down to power- this way the only drain is FET leakage which is still lower than 4069s sleep current. And a FET is dirt cheap these days. Speaking of which, I wonder what the BOM cost of this thing is. They sell for $11 retail

Yes, that seems a pretty fair reason. I suspect that discrete fet adds about 30% to the board BOM cost, which of course depends heavily on volume, certainly well under $0.5 for the board... Of course you're the one buying the batteries so they needn't necessarily have bothered!

Almost impossible to guess the total BOM cost, that case molding is distinct proprietary so will be extremely dependent on total volume (on that and maybe other products.

[Zero999]

Perhaps the CD4007 would've made a more compact solution?

There again, the gates in the CD4096 are connected in parallel to provide more drive.

[TommyR333]

What would I have to do to change the freq. to 1kHz.

-TR

[floobydust]

It's a nice teardown, and you can't beat that quiescent current.

I looked at designing a water-detector that could tie into your home alarm system.
For that you need a switched contact output and 12V DC power is available.

The water-sensor's electrodes are touching a concrete floor, so you get false-trips due to noise on ground (loop) between the floor and the mains utility ground.

George Risk Industries already makes a huge line of alarm and water sensors, so I gave up 

[Gyro]

What would I have to do to change the freq. to 1kHz.

-TR

Hi,

You could try increasing the value of the 1nF capacitor but I suspect that the frequency is mostly defined by the natural resonant frequency of the sounder.