If you are looking for a doorbell that you can hear from anywhere in your house have a look at this Train Whistle Doorbell by Pete Mills. He has hacked into a cheap wireless doorbell system for activation. Have a look at the video on his site for the explosive result when it is activated! Pete has a schematic on his site if you want to see how it ticks.
“Circuit wise, this doorbell came with an led on the front that blinks when the bell is activated. So, when I was designing the circuit in my head, thinking I would have to tap into the circuit board to get a logic signal; turns out I could just make a simple circuit to ‘watch’ the led; a photo transistor with a 10k pulldown works a treat.”
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cool, thanks!