[Elecraft] [KX3] Key input equiv. circuit

[Joe White]

What is the equivalent circuit of the KX3 key input?  

The reason I ask:  If I use some electronic circuit to key the KX3, how firmly 
do I need to pull the key line down to key the transmitter reliably.  What is 
the threshold voltage and how much current will I need to shunt to ground at 
that voltage?

Thanks,
     joe
  WA4GIR