>There is also the way used in some model trains. The voltage is upped for
>a few miliseconds to change a bistable relay. It could be used by lowering
>the voltage, f.ex. by having two antiseries zeners shorted by a NC button
>for ac.
>
>Peter

Doesn't the voltage vary quite a bit as the train gets closer and
further from the feed point?

A while back (1960, Lionel) they actually did superimpose DC onto
the AC to activate electromagnetics that ignored pure AC.  As I
recall it sounded the horn.   I also recall that it appeared
to make the voltage higher.  Maybe this fooled you, too.

Barry

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