Here is a fun project: hook my wireless doorbell to the Internet and get SMS when someone rings it. Useful when I am not at home or am in the backyard and can’t hear it, but I carry my mobile phone with me.
My wireless doorbell is some cheap model that Google doesn’t even find, but it is powered by two 1.5V AA batteries. It is also quite bulky and I could easily fit a Funky sensor in it. It has a green LED that flashes when the doorbell is ringing, so I decided to use it to pull a digital pin high when someone is ringing. I also did setup that digital pin for pin-change interrupts, so that the Funky sleeps constantly and is only wakened when necessary i.e. that pin is pulled high:
pinMode(1,INPUT); sbi(GIMSK,PCIE0); // Turn on Pin Change interrupt sbi(PCMSK0,PCINT1); // Which pins are affected by the interrupt