Cost estimate: $20
Here is a greatly simplified device for QRP stations that delivers electronically
keyed +12VDC to a transmitter
switches the antenna from receiver to transmitter when the code key is depressed
and back again when the key is released ["full QSK operation"].
There are very few parts and all are readily available, even from
"stand-alone" Radio Shack stores. Method of construction is not critical.
I used a prototyping circuit board from a discarded project and a small relay
that mounts on the board.
Six RCA jacks are used for the inputs/outputs: only one ground lead wire
from the circuit needs to be attached to a jack; the 5-1/4" x 3" x 2" aluminum
box conducts the ground to the other jacks [if a non-conducting case is used all
jacks have to be wired to ground].
To test, apply power: if the relay clicks when
the key is depressed, and clicks again when released, the relay works. Use a voltmeter
or even a small lamp to determine if the keyed voltage also works when the key is depressed.
Pay attention to the diagram that comes with the relay:
make sure the "nc" [normally closed] terminal from the relay goes to the receiver
antenna input, the "no" [normally open] goes to the transmitter antenna output,
and the "com" goes to the station antenna. If the relay has a built-in diode, omit the
diode in the schematic and make sure the polarity of the diode is correctly connected.
Shown above left is the project circuit board before installation in the aluminum box.
The blacked-out areas obsure the components from a previous project. Above right
is a top view of the completed controller installed in my station and
has placement of the jacks labeled.
"Simple QRP Transmit-Receive Changeover"
(ARRL Handbook for Radio Amateurs, 2005, pp.19.53-19.54)
AAs3J, "Keying Circuit for the 15 Meter Transmitter""