Hi. In this circuit, what is the role of the capacitor? I can see that BC548 turns on BC558 which turns on the speaker. But what does the 0.1 capacitor do?

 

bit of a stab in the dark here but may be to make the sensor less pron to false alarms in the case of say high humility and other factors ? may be some one else can correct or confirm this as i would also like to know as well.

just a head up and you may already know this but if your making a rain sensor you will also need a heating circuit.

 

What would you expect to hear from the speaker when it rains?

 

Capacitors allow ac and block dc. I am guessing that the Prob when wet with rain sends an ac signal through the capacitor to the speaker. This changes the sound of the speaker. Am I correct?

 

It is an oscillator and the capacitor provides phase-shifted feedback. The circuit, employing an inductor in place of the speaker is awkwardly described in U. S. patent 6603342 <= link

 

Thank you. I will look into LC oscillators.