The interfaces found on this page can be used for many ham radio programs such as: MMSSTV, EasyPal, FRN, Eqso and Echolink

Basic com port PTT Interface for base / mobile radio

 Isolated audio & Keyer Interface for base / mobile radio "it is best to isolate the audio"

Simple Interface for base / mobile radio

Simple Interface for Handheld Radios

Download a PDF file with more details on the above

Transformer: The type of transformer is determined by the source of the RX audio.

If you take it from the radio's microphone or data jack, you'll use a 600 to  600 Ohm (1:1)

transformer. If you take it from the radio's speaker jack (8 Ohm impedance), you'll need a 1000:8 transformer.

R1 = Resistor, e.g. 1K2, to reduce voltage on the IC pin 1
IC =  Integrated Circuit; this sketch shows an IC, such as a 4N33
or PS2603 Optocoupler, which uses a Darlington pair transistor.
(Note: to identify pin #1, look for a small embossed circle on
the top of the IC above pin 1; or looking into the notch in one
side of the IC with the pins down, pin #1 is to the right of the notch.)
D1 = Diode, e.g. IN4001, would shunt any potential reverse voltage
that might damage the sensitive diode/emitter in the IC.
(Note: the band printed on the diode marks the cathode end,
which attaches to the Serial Port/IC Pin 1 line in the sketch above.
The opposite/anode end attaches to Ground.)

The ideal components for your circuit may vary because of your radio's requirements.

For example, you might be able to use a different type of IC, such as a 4N25 or PS2601, but that may require a different value for R1.

Use the RTS ( Request to send ) pins NOT the DTR ( Data terminal ready ) pins

On a DB 9 Serial connector use RTS pins 7 ( PTT ) & 5 ( Ground )

On a DB 25 Serial connector use RTS pins 4 ( PTT ) & 7 ( Ground )

Here is some HT interfaces

• 2x 600ohm line transformers  
• 2x 1.2k or 2.2k 1/4w resistors
• 1x 2.2uF 25 or 50v electrolytic capacitor
• 1x 4N25 Opto Isolator
• 1x 1N4148 diode

And if you just can't be bothered!