Site Home
  Site Home
  2M SSB
  CT WIN Introduction
  Emergency Power
  JPole Antennas
  Microwave & Up
  Military Scanner Freqs
  Packet Radio
  Sound Card

Testing Session
Next test session is Sept 8th

  Simple Sound Card Interface Part 1

Here is a project that I recommend for first time builders. I originally presented this project at a PARC Tech nite presentation sometime in late '05 or early '06. I keep a few copies of the schematic around to answer the inevitable question of 'do you have a ...'

It's really a simple, easy to build, sound card interface. It incorporates transformers for isolation and can be configured in several different ways to accomadate a wide range of PC interfaces.

This interface is designed to built from junkbox parts and assembled in a single evening. It will work with many different sound card programs such as MM-SSTV, MIXW, HRD, etc. It provides ground loop isolation and well as providing a serial port compatible PTT interface circuit. Let's take a look at how this thing works by starting with the schematic diagram.

Figure 1 - Schematic Diagram

Click here to download the full PDF version of the instructions: SSCI.

How it works

The central idea is that the PC sound card is used as the modulator and demodulator for the audio. To accomodate a different modulation technique or protocol only requires different software. No hardware changes are required. Many different modes are now represented with sound card versions. All the old favs, Packet (HF and VHF), AMTOR, PACTOR, RTTY, SSTV and many new ones as well, PSK, Olivia, MT63, etc.

Parts Needed

The bill of materials is listed below. Theres nothing special about the parts, order them from Mouser (cheaper) or buy from RS (expensive). You can eleminate the jacks, if you solder the interface cables directly to circuitry. The audio transformer are for isolation and not impedance matching. Use 600:600 line xformers, or whathave you, but pay attention to power handling capability. This will imit the amount of audio from the radio or sound card that the transformer core can handle before saturating. Running the core to saturation is a bad thing, lots of spurious signals will be generated resulting in a trashy signal or transmit or bad reception. Remember that the radio's speaker output is being used here, so there's lots of audio power available. The simple fix is to turn down the volume!

The specified RS reed relay is good for this job as it's easy to energize. It requires very little current to pull in, so a laptop or USB-2-Serial adapter should accomodate it easily. Most designs I've seen around like to use an optocoupler to acheive the PTT function. Well I tried that and had problems with about half the rigs I tried it on. It just flat wouldn't key the radio. I switched to this relay and it's worked fine on every radio since. I wouldn't key my SB-200 with it, but it will suffice for mobiles well enough.

The LED and resistor in series with the relay coil is really to provide a little voltage dropping as most RS-232 outputs are 9-12 volts and the relay is designed for 5 VDC. The amount of current required to energize the relay is about the same as whats needed to lite an LED, so there ya go. Voltage dropping and indication in the same part. The LED can be any color, theres no standard, but be aware that the resistor may have to be changed to accomodate something different than green.

Bill of Materials:

Qty Description Part # Ref Desg#
3 3.5 mm Stereo Jack Any J1,2,4
2 3.5 mm Mono Jack Any J3,5
1 Reed Relay, 5V SPST RS 275-232Any K1
1 LED, T1-3/4, Green Any LED1
2 Transformer, Audio Isolation Any T1,2
1 Poly Project Box Any  
3 3.5 mm Stereo M-M Cables, ~3 ft Any J1,2,4
2 3.5 mm Mono M-M Cables, ~3 ft Any J1,2,4
Table 1 - Bill Of Materials

Here is a layout suggestion looking at the top of the project box. Holes for the connectors and the LED can be started with a hot soldering iron and then filed to fit. Wiring is point to point with the transformers wired directly to the jacks.

Figure 2 - Project Box Outline

Part two will include some assembly photos and the hookup and use of the interface.

Stay tuned....73's DE ~Steve>



This application and data entered thereinto is the property of KB4OID.
Click here for our privacy and copyright statements.