Some time ago I read about a guy who rewired his postwar engine so that the couplers would open when the whistle was blown. IIRC, this was done by wiring the coil couplers to the whistle relay, such that when the relay closed, the couplers would get power from the center rail.
I’d like to do this to my 623 switcher. But since I’ve not ruled out someday putting some sort of horn into it, I thought that rather than use the whistle/horn button on the ZW, I could use a modern Sound Activation Button as my “uncouple” button. That would require a relay that responds only to the DC output of the Sound Activation Button while ignoring the DC coming from the ZW.
I know that that Sound Activation Button produces a different polarity(?) of DC that activates the bell on Railsounds locos, while the whistle is activated from the ZW, so it seems like this should be possible.
So assuming all that made sense, does anyone know where to get such a relay?
The whistle-bell distinction in modern locomotives is indeed made by the polarity of the superimposed DC voltage. A positive center rail blows the whistle; negative rings the bell. Fortunately, postwar transformers all seem to use positive; so they always blow the whistle, even of modern locomotives.
The whistle relay cannon make a distinction with regard to polarity. So you will have to abandon it entirely. To be able to distinguish between horn and coupler by polarity, you will need entirely new electronics for those functions.
There is another way, which is what I use. I have a circuit that operates the coupler the first time the locomotive starts up after having sat for a few seconds in neutral with the track voltage turned on. So, for example, I can shove a cut of cars into a siding, stop, turn up the voltage for a moment, then back out of the siding without the cars. This scheme involves considerable rewiring of the locomotive, including rectifying the motor voltage to DC.