Here is my version of a moisture detector. The picture below shows it fitted to my D70 housing.

The green circuit board just below the port is the sensor and the cct itself is mounted on the inside of the housing by the left handle.

Based on a Quad Schmitt 4000 series NAND gate. There is really very little to say about it as in electronic terms it is pretty straightforward. 

At one point I did mess about with adding an audio output as well as the visual ( LED ) output but had trouble with keeping it working over the full expected terminal voltage of the single cell ( 2.0 – 3.0v ). 

R3 C1 and one of the gates are an oscillator which is gated on by the action of R4 the two sense lines and a second gate. 

A third gate and Q1 provide level shifting and drive to the LED D1. 

D1 is interesting as it is a very efficient high output LED, there are many on the market now for automotive use and data for the one that I used is at :- 

http://www.farnell.com/datasheets/46028.pdf 

Mounted in a clear housing the output of the LED is so huge that it would be difficult to not see it when triggered.

The two sense lines can be a small strip of veroboard or similar. 

Don’t forget to tie the unused inputs to the fourth gate as if left open it may oscillate and flatten the battery. 

Current consumption when the LED is not flashing is about 10uA which makes an on / off switch not required – if left unused for a period of time I just insert a piece of plastic under a contact of the cell. 

Battery wise I used the same 3v cell as my dive computer saving on spares inventory. 

Paul C