So, I have pieced this one together out of two easily available items that can be found very inexpensively at the Dollar Tree or ebay
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Step 1: Things to Get
touch activated light up flashing led toy.
I found this little frog at Dollar Tree, but you can also find them on ebay,
ebay: touch activated color changing led light up toy
ebay: water activated color changing toys
Magnet activated security alarm siren, from Dollar Tree
Suction cup to hold the frog to the side of the bathtub at the desired water level
Hot knife/soldering iron
some lengths of thin wire
sugru to reseal the frog after modifications are made
Step 2: Reverse Engineering
I spent some time looking at the two circuits and redrawing them so I could understand the a bit better.
I always seem to want to combine two electronic products into one, but I'm never sure how to handle the fact that each has its own separate power supply and how to connect them in such a way that the power supply of one does not defeat/destroy the logic of the other.
I found by poking around that the power supply of the shrieker was interfering too much with the behavior of the frog circuit, so I shorted that out and relied entirely on the frog circuit power supply. This wound up making the shrieker less loud, but still audible, so I went with that.
Step 3: Combined Schematic
I poked around till I found an arrangement that worked.
I shorted the reed switch as shown.
I removed the power supply and created connections to the frog circuit there.
Soldered in the leads from the screamer circuit from one pad on the high side of the led and the other lead to ground as shown.
Step 4: Reassemble the Frog and Siren
The reassembled frog needs to be water tight. Also, I need some way to run the wires from the frog circuit to the screamer circuit, so I added some eyebrow/antennas to him.
Step 5: Add a Suction Cup to the Frog
The frog needs a good suction cup to attach to the side of the tub at the desired water level.
I used more green sugru to attach the suction cup as shown and then had to wait 24 hours for that cure completely before the final test.
Step 6: Tub Test!
In the video, the sound of the faucet came out extra loud and it's hard to hear the siren, but in reality you can hear the alarm throughout the house.
Step 7: Afterthoughts
I would like the alarm to be louder, but I'm not sure how to modify the circuit so that the siren gets more power than the frog does without overloading the little parts in some way.
If you can find a flashing touch toy that sings loudly as well, you can skip most of the steps and just attach a suction cup and you are done.