On Thu, 20 Aug 2009 21:17:04 +0000 (UTC) Peter wrote: > Does anyone have a link to a schematic of the first modems based > apparently on LM555s ? Or anything similar ? Old RTTY modems, including > valved ones, are not quite in this class although they come close. I'm guessing you mean the NE565 (there may be an LM565 version). That chip is a PLL which can be used to decode FSK signals. It worked quite well on VHF, but the PLL was easily fooled with HF signals. John -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist