Here is a contribution from Chris Stelloh that seem to be really useful.
Chris says:
This is an updated version of the Demedash Incandenza micro-controller
free clickless relay true bypass switch. It takes up a bit more room
than a simple 3pdt, but because of the friction-free operation of the
SPST button, it will last WAY longer than that same traditional,
mechanical 3pdt switch. I realize that most folks will not want to build
an extra board where just a footswitch will do... but for those of us
that DO, here's my drawing of a super-slick, silent, clickless relay
that doesn't require any coding or microcontrollers. There are a few
earlier versions of this circuit floating around the web, but this one
has a very important update from Steve Demedash himself. The power
filtering and 100 ohm resistor between the 555 and the relay is super
important. I found that the power spike when engaging the flip flop was
enough to mess with other circuitry that was close to the switch. For
example: I had a dual circuit build with independent on/off switches and
triggering one would sometimes trigger the other one unintentionally.
Anyway - here's a boring utility layout that has ended up being the
thing I build the most, since I put these in just about any circuit that
will permit it. This layout also allows you to omit the polarity
protection diode for the main board and the status LED as well since
those are both contained here on this switch daughterboard. The 5v DPDT
relays you can use for this are not limited to just these three -
however, these three are the smallest and most inexpensive I've found so
far. Also - I don't like to use standing axial components or 3-row
resistors but in the case of this board, I wanted to keep it as small as
possible. Be sure and notice the anode of D3 connected to ground!
