[Elecraft] (k3) Key out redux

[Don Wilhelm]

Pete,

Connect one side of the relay coil to your +12 volt supply, and connect 
the other side to the KEYOUT jack.  Put a diode with the cathode 
connected to the +12 volt side across the relay coil.  The negative of 
your power supply has to connect to the shell of the KEYOUT connector.
The relay will be activated when the K3 KEYOUT is active - keying, TUNE 
and PTT will all activate it.

Yes, you should be able to use the same +12 volt supply to power the 
SDR's preamp and active antenna, but that is a different consideration 
than activating the relay.

73,
Don W3FPR

On 10/14/2017 10:18 AM, N4ZR wrote:
> 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.
>