[Elecraft] (k3) Key out redux

[N4ZR]

Thanks, everyone who responded to my earlier query - I now understand 
that the circuit in the K3 is equivalent to the old bipolar transistor 
switches we used to use for serial port CW/PTT keying.

So here's my problem - I have a small 12V DPDT relay that I want to use 
to switch the power to my SDR's preamp and active antenna - in other 
words, to cut them off when my K3 is in transmit.  What I can't figure 
out (be patient with a history major, please!) is how to use the Key Out 
jack to switch the power to the relay , while using the same 12V supply 
to power both the relay and the preamp/active antenna combo.

Ideally, the 12V relay would only be on when the K3's PTT was on.  The 
thing that's driving me nuts is that I had this working, once before, 
and then tore it apart for not very good reasons. Now I can't replicate 
what I had.

Would anyone be willing to go offline and help me figure this out?

-- 

73, Pete N4ZR
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