Homemade PCB: Part Two – A Fresh Start
Things always happen for a reason… Since the last post I have thought more about the requirements for the power supply. Though 1 amp may be sufficient now, it may not be in the future. It will be powering multiple pcbs and stuff on the bread-boards. So I have replaced the 7805 with a LM338 which can go upto 5 amps. This has changed the size and layout of the homemade PCB but only in it’s length. The PCB is now 100mm long but still about 45mm in width.
Second time around went a lot smoother having a good quality pre-sensitized board and a decent drill. I won’t bore you with pictures of the process as that was covered last time in part one. The major difference in this part is the “silk screen”. Last time I tried the laser print and acetone method which didn’t work for me. I’ve since discovered the print has to be used as soon as possible. Otherwise the plastic in the “ink” bonds so well to the paper it won’t transfer, my print was over a day old.
So I thought I would try an alternate method, printing onto sticky-backed clear film and sticking that to the pcb. I already had some of the clear film I got on eBay a while back. In Kicad, I made an svg of the silk-screen layer. Then I took it into Gimp and changed some of the fonts for the text and toned down the color. Finally, I printed it on a clear sheet.
It’s best to leave it for an hour or so to let the ink dry properly as it’s on plastic. After a couple of hours I cut out the sheet roughly a cm border around the print so that I could trim it once on the pcb. Note that I cut out a section where the heatsink will go as I don’t really want the smell of melting plastic every time I turn on the power supply 😉 Once the film was on the pcb I used a craft knife to trim the edges and poke all the holes for the components.
Next up – populate the pcb with the components. As usual, I start with the shortest components like the resistors and diodes. Then I work my way though to the tallest in this case the 4700uF electrolytic capacitor. I use a “third hand” to hold the pcb upside down at approximately 45 degrees. I also used Blue tack to keep the components in place until they were soldered.
The last step is testing, does this thing actually work? Well.. I’m happy to say it does work. When I bread boarded the circuit it was giving out 4.96 volts. This is in spec for most if not all TTL, CMOS and NMOS chips that I will be using. Now that it is on a PCB it is giving 5.03 Volts, which I don’t mind. It is still in spec and there is always some voltage drop over the leads. I would rather it be over than under.
I adapted the circuit from one found here. I have made the following changes:
- Instead of a transformer I am using a 12 Volt wall plug.
- There are two output connectors, one for TLC-MBC and the other for the bread boards.
- At the front there are a pair of pins covered by the red jumper, this is so I can easily measure amperage.
- On the far side, those five pins are gound points – always handy to have.
Homemade PCB Conclusion
As usual, the Kicad project files for this will be up on Github shortly. This first homemade PCB has been a fun and learning project and I am happy with the result. I would like to try the laser-print and acetone method again with a “fresher” print as it would be cheaper. The sticky-backed film is about a £1 per sheet. One advantage of the film is that, if needed, the silk screen can be printed in color whereas other methods can only do black.
I would also like to try alternate ways of protecting the copper traces, such as the green spray. Finally, at some point I would like to get a can of photo-resist spray so I can make my own boards. I know the spray is not cheap, but in the long run it will be cheaper than buying the pre-sensitized boards.