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Edited for eQSO, from an article by Peter, G4KQU, with his kind permission.

An interface unit allows one to transmit and receive digital modes using a computer sound card.  Conveniently, the interfaces below were designed to operate without an external power supply, so before rushing out and spending a fortune on a commercial interface with features you will probably never need or use, consider building your own! I have included here some simple designs that I have built, tested and which work very well considering their simplicity and economy. These circuits will perform well if you intend to run an Internet gateway using the eQSO or EchoLink software. Parts for these circuits (or complete kits) can be obtained from


This circuit, based on parts typically found in any amateur’s “junk box” is an extremely cost effective solution. In this circuit, RTS (the “ready to send” line on the computer) drives an open collector for the PTT. Any general NPN transistor can be used instead of the BC108. It is very similar to the isolated circuit (below), except it does not use audio transformers or the optocoupler, but performs splendidly. (Of course, if you just happen to have a couple of audio transformers, you could add them to this circuit in the same position as in the isolated circuit).


1 x 1k ¼ watt resistors – 2 x 2.2k ¼ watt resistors

1 x 1k Potentiometer Lin – 1 x 2.2uF 50v capacitor – 4 x 0.01uF capacitors

1 x Red LED (High sensitivity type) – 2 x Diode 1N4148 – 2 x 3.5mm Stereo plugs

1 x BC108 Transistor – 1 x 9 Pin D plug ( Com port 1 or 2) & cover

Screened cable – Project Box


This circuit incorporates two 600-ohm audio transformers (T1 & T2) and an RS232 powered optocoupler IC1. Preferably use an IC socket for IC1, for possible quick replacement! The purpose of the transformers and an optocoupler is to isolate the transceiver from the computer, keeping the interference from the PC to a minimum. Ensure that the screening on the radio and the screening on the PC are not connected together.

Stereo 3.5 mm plugs connect the line in and out on the computer soundcard. Use the tip and earth only as in this application the sleeve is not used.

To control the radio PTT, an isolated signal from the computers RS232 (RTS) line is used. If you have an available DB9 connector on your computer, use RTS (Pin 7) and ground (Pin 5). If you have a DB25 connector on your computer, use RTS (Pin 4) and ground (pin 7).

VR1 is a 1K linear potentiometer used to control the amount of audio going to the MIC and is adjusted for correct audio drive to the radio, usually converting line (0.5v) to MIC (10mV) levels. The 1.2k resistor (from the Line Out) can be changed to a greater value if you are troubled by the pot always being at the bottom or top of the range or alternately by adjusting the computers audio out slider till the correct level is achieved. Operationally, audio levels are adjusted by the computer level controls or are incorporated in the software you will be using.

The LED (high sensitivity type) is used as an indicator when the interface is in the transmit mode.

It is suggested that the finished interface is put in a metal box and that the grounding is taken from the radio side of the circuit.


3 x 1k ¼ watt resistors – 1 x 1.2k ¼ watt resistors

1 x 1k Potentiometer (lin) – 1 x 2.2uF 50v capacitor – 3 x 0.01uF capacitors

2 x (T1 & T2) 600 ohm transformers type 9000 RS Number 208-822

1 x IC1 optocoupler 4N25 RS Number 597-289

1 x Red LED (High sensitivity type) – 1 x Diode 1N4148 – 2 x 3.5mm Stereo plugs

1 x 9 Pin D plug ( Com port 1 or 2) & cover

Screened cable – Project Box


The circuit below, a special version of the Isolated Interface, allows gateway operators to run 2 eQSO Internet Gateways (say one on 2m and another on 70cm) without the need for two callsigns and wasting server bandwidth

(Modification to Peter’s original radio interface circuit, by G8SDU)

———- Edited by K0WZK, part of The eQSO Team. June 2005————–


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