This circuit was designed to play an audible alarm when water connects two probe contacts. I was planning on using it to sound an alarm if my water heater was leaking into the emergency drain pan so it wouldn't overflow onto the downstairs neighbors. In general it can be used as any kind of continuity alarm since the output square wave frequency is a function of the resistance between the probe contacts. Other applications include a musical instrument (resistisizer), a tester for ions in water (distilled vs. tap), an audible resistance measurement (frequency is a function of R), a lie detector (skin resistance decreases when lying), an intrusion alarm (pressure sensitive plates under a doormat), etc.
I found this design to be extremely sensitive, if two people each touch one of the probe contacts and then touch each other you will hear an alarm due to the resistance through their skin (or through multiple people). It was also able to detect ions in distilled water a few seconds after dipping a finger between the contacts.
NOTE: Your instincts may be telling you that water and electricity are a bad combination. Generally this is true, but this involves low voltage DC and you're not shorting out the entire circuit, just providing a path for the timing capacitor charge/discharge through an unknown resistance. You won't feel anything like an electric shock.
The original schematic for this design came from here. The circuit is a simple 555 oscillator connected to a small speaker. The connection between the timing resistors and capacitor is broken so the infinite resistance makes the output frequency infinitely low. Connecting the probe contacts with low resistance activates the timer and makes the output frequency reach its highest value (approx. 1kHz). Any noticeable resistance between the contacts will vary the output frequency (more resistance = lower frequency).
The board was designed with interleaved water sensor contacts at the bottom and two probe contact holes. If desired, the interleaved contacts can be deleted from the design or cut off the bottom of the board and wires can be used for the probes. I found a small enclosure with 4.5V battery stack and a small speaker that originally played Christmas sounds. This "Ho Ho Ho" box is no longer available from the Electronic Goldmine but there is a nearly identical "Gobble Gobble Gobble" box. You will need to use a LMC555 to operate from from this 4.5V battery.
The inside of this sound box is pretty simple, a 4.5V battery stack, a small speaker, and a pcb that plays the sounds:
I gave the box a brain transplant by removing the original pcb and connecting the battery and speaker to my water alarm pcb:
A few holes were drilled to pass the probe wires out and the circuit was complete
This design could be connected to the audio input of a pair of computer speakers for a much louder alarm. There is probably a way to connect it to a smoke alarm if you're looking for something much louder. I've also thought of using a real fire alarm module.