These are the exact circuits I use for FSK, PTT and CW keying. The PTT & FSK circuits work great with MMTTY. These circuits are connected to the same DB9 connector which allows all three circuits to be controlled by the same serial port (DB9 pin-out shown here).
The transistor can be just about any general purpose NPN transistor. I use an ECG 123 (same as NTE 123). The 2N3904 transistor works fine too. The resistor value can be just about anything from 500 to 2k ohm or possibly lower or higher. I use 1k ohm resistors.
These simple transistor interfaces will work for most radios that are capable of transmitting FSK. If you find your FSK transmit signal is inverted, you can normally change the polarity of with a menu option in your transceiver. Some radios, such as the Ten-Tec Omni Series and Kenwood TS-930, do not have the capability of changing FSK polarity. These radios require either a PNP transistor or straight through keying circuit. These circuits are also shown in the next section below.
Opto-isolators can also be used instead of transistors. Bob, W3YY, has produced a very nice high-quality circuit board using opto-isolators. His boards are available for purchase in kit or assembled/tested form at a reasonable cost. Check his interface page at http://w3yy.com/fsk.htm.
Here is a circuit that can be used as a simple FSK interface for the Kenwood TS-930.
Ten-Tec radios Omni 5, Omni 6 and Omni 6+ can key FSK simply by coming off pin 3 of the DB9 serial connector through a 1k ohm resistor (see circuit below). Please note that the Ten-Tec Orion and Orion II both need a transistor or opto isolator circuit and a straight resistor should not be used (Thanks Marsh KA5M). If you are using a Rigblaster with the Omni 5, Omni 6 or Omni 6+, try this circuit from K3DI here.
Ron, ND5S, built his interface into a very nice looking box (shown below) with a short pigtail coming out to the DB 9 connector (which is female).
The box is Radio Shack p/n 270-1802. He used 3 chassis mounted RCA jacks.
Says Ron “The label was made on PowerPoint on plain paper affixed with two way carpet tape and covered with clear contact paper.”
Ron’s PowerPoint file is available for download. The template is pictured below. Download here.
Sometimes you can modify your existing interface for FSK. Frank, K7SFN, has an excellent write-up on modifying a Signalink USB for FSK transmission. Check out this great page!
Erik, K5WW, shows how to build an FSK interface inside a RigBlaster Nomic – details here!