Pulsing a model train set using HexFETs. This was an integrated systems project that describes a circuit that was built and, much to my surprise, worked.

It allows a computer to pulse a model train track and send messages through it. The same track supplies power to the model trains (HO scale I think). The circuit is 'safe' in that the power components are optically isolated from the logic components.

The really cool thing about the circuit is that it allows a short circuit on the power side, like putting a screwdriver across the tracks, and nothing blows up. It uses PALs to reduce the duty cycle of power being supplied to the track, and polls until the short is removed. HEXFETS are used to control the track voltage.

Download the postscript version here: HexFET Pulse Modulator Postscript
Download the pdf version here: HexFET Pulse Modulator PDF

The PostScript version looks best. Linux people can use ghostview or something. For Windows a very good Postscript interpreter and viewer is available at the AFPL site.