Relays

[Ken Stone]

Following the recent discussions about analog switches and relays, I decided
to build the intended "analog switch" module for my synth. I chose to use
relays, as these overcame several problems I had yet to address, such as
exposing the inputs of CMOS analog switches to static, by bringing them out
to the panel... I really think some sort of protection is needed here.

The relays worked fine, giving me two sets of normally open and normally
closed contacts per relay. The coils are driven by BC338 transistors, in
turn driven by a LM385 dual op-amp, with resistors and diodes where
appropriate. I have the op-amps wired as comparators, switching at about 1V,
a compromise between signals that work above 0v (logic/gate) and those that
go through 0v (LFO sine, etc). I avoided the old problem of supply rail
voltage fluctuating with the coils kicking in by running the relays off my
LED power rails.

There are only two problems I can see with the relays versus semiconductor
analog switches and they are the audible noise of the relays opening and
closing and the maximum speed to which you can drive them. I would like to
be able to run an analog switch at low audio frequencies for certain effects.

One of the advantages of relays, is if you felt like it, you could even
drive lights from it (low voltage of course).

Ken 

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                Ken Stone  sasami at blaze.net.au
                   ** Catgirl Paradise **
               <http://www.anime.net/~kens/>
         ** Australian Miniature Horses and ponies **
             <http://www.blaze.net.au/~sasami/>
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