Converting 3-wire sensor to 2-wire

My dad has asked me to build him an alarm that he can leave on his windowsill so that when it rains during the night, he can get up and close the windows. Moisture alarms are common, but they're designed to be submerged in water, and don't have sensors that would respond to a few raindrops sprinkling on them. This is a sensor that seems perfect for the application:

http://www.emartee.com/product/42240/High%20Sensitivity%20Water%20Sensor%20%20Arduino%20Compatible

As I was thinking about it, I realized that, first of all, this hardly requires an Arduino. It can easily be built with a DC power supply, a potentiometer to control the sensitivity of the trigger, and a speaker module. I also figured that re-inventing the wheel is probably not the best way to go about this. I wondered if I could just take an existing moisture alarm and replace its sensor.

But all of the sensors I see online are 2-wire hookup, while the sensor I want to use is three-wire. That leads me to believe that the other sensors are basically being treated like digital inputs. The voltage is weakly pulled-up (or down) and as the sensor gets wet, resistance drops and voltage on the output pin drops (or rises) to a threshold that triggers the alarm. So now I am wondering what would be involved in converting the three-wire sensor to operate as a two-wire sensor.

As I understand the description of the emartee sensor, it is just a set of PCB traces that raindrops will connect together, with some resistance. The 3-wire nature of this sensor comes from the connection of the “S” terminal with a 1 megohm resistor to the “+” terminal. That resistor is intended to act as a very weak pullup. If this is correct you should be able to ignore the + terminal and just use S and “-” as the two wires of interest.

If you just want a buzzer to indicate rainfall, you certainly don’t need an Arduino. There are many simple circuits to be found that will generate a tone (with enough power to operate a speaker), if the resistivity of the sensor drops. I’ve used a circuit very similar to the attached example and it worked very well. Furthermore, the circuit draws negligible current when the sensor is dry, so the batteries should last for a very long time.

rain_alarm.gif

Thanks much for the input.

joshuabardwell: But all of the sensors I see online are 2-wire hookup, while the sensor I want to use is three-wire.

It appears to have a buffer transistor on it as well, and a couple of resistors. You just connect the "-" and "+" to ground and the "S" (signal) to an analog input.

There is a three pin package on the board, plus what look like two 0 ohm resistors. The 3 pin package could be a transistor but that disagrees with the written description. You should be able to discriminate between the various possibilities using the diode check/ohmmeter function of a multimeter. If you can't, try writing to the sellers

There is a PDF manual for the sensor at the very bottom of the product page, but it just repeats the product page.