I use BS170 MOSFETs for applications like this because it can handle .5A on and 60 volts off. We have an A/C unit at work with a drip pan that drains into a large bucket. The bucket would overflow and make a huge mess before I built an alarm for it. I built it on a scrap piece of single sided copper clad, about 1" x 3". The piezo alarm was a 2 lead type from Radio Shack. Other parts are a 1M resistor, any small capacitor to RF bypass the gate, a 9V battery clip with leads, and some scrap telco cable. The battery is just taped to the ckt board underneath the piezo buzzer, I just put a little hook in the cable so that the end hangs about two inches into the bucket. By cutting the wires to 2 different lengths, before stripping them back 1/4", you avoid the accidental contact. The MOSFET is much beefier than CMOS and you can lay out the whole board with a Sharpie. You could even use a razor knife or Dremel to isolate the traces. The only critical spacing would be for the piezo buzzer. Parts cost is < $10. Joe
