The cathode keyer is a simple circuit I use to key my boatanchor rigs from my modern
keyers.  I went through various other solutions including reed relays and high voltage
transistors, but they weren't reliable.

This circuit uses inexpensive power FETs and it works well.  I got the FETs from B.G. Micro
and I think they were a dollar or so apiece.

A modern keyer is designed to pull a small positive voltage (5 to 12 volts usually) to ground.
The front end of this interface presents that requirement--the keyer just has to pull 9 volts
at very low current to ground.  The 2N2222 transistor acts as an inverter.  In key up conditions,
the transistor is ON and  there is near 0 volts on the gate of the FET, so the FET is switched
off.  When the key is closed, the transistor turns OFF and 9 volts is applied to the gate of the
FET, turning it ON. 

I'm keying an HT-40 transmitter (sometimes a DX-35) and a Heath VF-1 VFO, which is also cathode
keyed. I added a second IRF730 to key the VF-1 so I don't have to bother with switching the VFO.

The circuit is powered by a 9 volt battery.  Battery drain is 108 uA key up and 22 uA key down.
With a 120 ma-hr batter, life can be over 1000 hours with no switch.  If a power supply other
than a 9V battery is used, I don't recommend going over around 12 volts because the rating of
the MOSFET's gate is 20 volts maximum.

Parts values aren't critical, nor is the selection of the 2N2222 or IRF730.  By the way, I think
a typical cathode keyed rig (mine, anyway) will have about 70 volts or so on the keyed line.

73 and I hope this is useful to someone.

Nick Kennedy, WA5BDU
wa5bdu@arrl.net