LOL. They aren't "my" relay modules but I do recommend the $1 (or so, free shipping) eBay relay modules that even include the opto-isolator function:
But in this case, since there are some half-dozen components that must be hand-assembled anyway, I'd just go with wiring in a bare relay itself...adding whatever few components are needed to provide a suitable relay coil voltage when the +DC detector triggers the transistor. That's assuming the OP wants to pursue the traditional relay approach.
It just depends on the OP's requirements - cost, size/space, ease-of-assembly, etc.
Just my 2 cents, but what makes a whistle-detection circuit "interesting" is if you can integrate the DC detection function with the load circuit to leverage the somewhat tedious assembly task. That's why I ponder about combining this with a horn battery-replacement circuit (if applicable). For example, not quite apples-to-apples but here's a hand-built circuit which has the same DC-offset detector circuit (that big cap on the bottom is the bi-polar) and a toggling circuit to drive 2 ditch-light LEDs. Track voltage comes in on the left. So when the Horn button is pressed, the ditch lights flash/alternate. Size was an issue. The point being by knowing what is being activated by the DC detector circuit, I did not have to use a large relay or expensive SSR.
