Fella last week posted the links to some DIY stuff, I looked em all over. One (IIRC) ran on 12v and had an adjustable frequency. Didja guys see that? I haven't priced a commercial unit but this one looked very do-able. I read the description and schematic, but that was 'then'. It has a 555 timer chip in it, and I saw an adjustable pot, so I am thinking it is an adjustable rate.
http://www3.telus.net/chemelec/Projects/Strobe/Strobe.htm GeoB
