Who needs a rain detector? How hard is it to look outside to see if it's raining? But a rain detector can come handy when you're distracted; it will save you the trouble of looking out the window every second when you want to catch the start of a rain storm, or cover some water sensitive stuff on your backyard. The following schematics are simple, but they can give design ideas or serve as a part of a more complex circuit like a computer weather data collection system. It all depends on your dedication, skill and expertise level.
A handy rain detector circuit that operates on only 3 volts using common silicon transistors (2n4403 and 2n4401). When raining, the buzzer is pulsed about once per second for a very short time, giving it a "dripping water" sound.
The circuit can be built on a standard perforated circuit board and mounted in a little plastic box. The rain sensor can be built using two bare, closely placed conductors that are bridged by the rain water.
The rain detector circuit with a 555 astable multivibrator gives out a tone of about 1kHz upon detecting water.
The design uses aluminium foils secured to a wooden or plastic board via epoxy adhesive or small screws.
This simple circuit uses a sensor made of a small piece of etched PC board and a SCR to activate a buzzer, a relay, turn on a lamp, or send a signal to a security system.
National semiconductor's LM1830 fluid level detector circuit can be used as a good quality rain detector if a suitable rain sensor like an etched circuit board is connected to its input.