Construction Project 2008

RX/TX Computer Interface.

(Originally by Rahul Srivastava, VU3WJM)

These days it's very easy to dig deep into your pocket and pull out £100 and buy an off the shelf black box type interface to use on the "DIGI MODES".  However it's much more rewarding to build your own for a few pounds.

This interface can be used on one of the many DIGI Modes such as PSK31, MFSK16, MT16, Hellschriber, SSTV, RTTY, AMTOR, PACTOR, PACKET, CW, Echolink and EQSO. Not forgetting the later WINDRM or FDMDV Digital Audio over analogue radio modes.

The original design by VU3WJM can be found on the following webpage http://www.hamradioindia.org/circuits/digivu.php

Step 1.  Making the PCB.

• Wear eye protection and protective gloves.

• Carry out the etching in workshop/shed or garage and wear old clothing.

• Avoid splashing the Ferric Chloride solution, it stains clothes and wallpaper!!!! YOU HAVE BEEN WARNED.

Equipment Required

1. Photosensitive PCB (Positive) material 60mm x 80mm.

2. Transparency of the PCB overlay track.

3. UV Light box.

4. Laser Printer.

5. Ferric Chloride Hexahydrate Granules.

6. Universal Developer.

7. Container for developing the PCB.

8. Container for etching the PCB.

9. 0.5Lt Water.

10. PCB Drill.

11. Eye protection goggles.

12. Latex protective gloves.

Photosensitive PCB (Maplin Pt No:JP55K)

Universal Developer (RS Pt No:690-849)

Ferric Chloride Granules (Bought on e-bay)

Transparency of the PCB Overlay

Right Click image above and "Save As"

When printed the two holes shown below

should be 70mm between centres

(Check as different printers vary)

1. Download the PCB overlay.

2. Print the overlay image of the PCB tracks onto a transparency sheet of the type used on overhead projectors.  You will get much better results using a laser printer. Also print in monochrome and not greyscale or colour.  When you look at the black tracks they should be solid black in colour.

3. Place the Overlay in the Light box and then place the Photosensitive PCB on top of the Transparency.

4. Expose to UV light (Time depends on UV light source).

5. Remove the Photosensitive PCB from the light box and you should see a fait outline of the tracks on the PCB.

6. Dissolve the universal developer powder in warm water (50°C).  Use 25g of universal developer powder to 1/2 litre of water.  Allow to cool down to about 22°C before use.

7. Pour your developer solution into a suitable container and placing the UV exposed PCB into the dissolved developer solution.

8. Gently rock the container to create a small waves in the solution.  You should see the tracks starting to appear.  Leave in the developer and keep rocking the container gently until all of the tracks appear clear.

9. Remove the developed PCB and rinse under running water.

10. Dissolve the Ferric Chloride Granules in warm water (50°C) in a suitable container.

11. Allow the dissolved Ferric Chloride solution to cool to about 22°C before use.

12. Place the developed PCB into the Ferric Chloride solution and again gently rock the container from side to side, it can take between 10 to 20 minutes for the etching process to complete. 

13. Once the PCB has been etched and you can clearly see the tracks of the PCB, remove it from the solution and rinse the board off under a running tap.

14. Use a small PCB drill to drill the component mounting holes.

Homebrew PCB

Components Required

Step 2.  Soldering components to the PCB.

The PCB below shows where all of the components should be installed.  The component layout is very straight forward.  Just take care to install the electrolytic capacitor the correct way around and also the two 1N4148 diodes

Your board should look something like this below.....

Here is the circuit diagram