Electronics > Projects, Designs, and Technical Stuff
Allegedly a TTL clock (build log)
<< < (4/4)
harnon:
Oh wow I think you might be right. I must have looked at that 20 times and not noticed.
harnon:
I finished the breadboarding about a week ago and the oscillator circuit and debouncing worked fine. I was using a 74HC123 to debounce but in the end I found a relatively large capacitor to ground did just as good a job.

I managed to steal a DMM to measure current from work (my old DMM only ever measures 0 current), and the breadboard circuit uses about 23mA - much lower than I'd thought but in line with what Mr. Hess said previously :)

I ordered the PCBs and received them today. There is a large main board and twelve "digits" of the clock dial. I reflowed a test digit and it worked absolutely perfectly. The LEDs are quite visible even under the lights. They're going to be facing towards the timber back so I got Mrs Harnon to hold our daughter's toy hammer for the right ambience. This is the "hour hand" lit up.

I then tried reflowing the main board. There were some immediate issues. I've reflowed boards up to about 5x10cm just using a cheap $30 electrical skillet. So far I've done it by feel but I also happened to get a new :-DMM in the mail today which has temperature measurement. The larger board really struggled to get up to temperature and reflowed inconsistently. Some parts were not reflowed while others on the edge were reflowed. Looking at it a bit closer showed that the centre of the board wasn't touching the skillet. I haven't had a whole lot of luck using sand on the skillet in the past but I gave it a go. Unfortunately the board (after a long period of time) only got up to 220C. If I had a hot air rework gun I think it would only take a gentle wave, but unfortunately I don't.

When I took the board out of the skillet with some needle nose pliers it also chipped the solder mask, enough that the board is a write off for this project. So a few issues to work out, but making progress.

I'm going to take some blank boards over to "my carpenter" to help mock up the timber base and check it looks like we think it will when the bits are physically laid out.
harnon:
Raising this build log from the dead!! I'm stuck at home at the minute with more free time than I'm used to, so I've been able to finally pick up this project.

Since I last posted (months ago!) I managed to build the main board using my cheapo reflow technique and a dodgy hot air gun from eBay. It took a couple of goes because there were solder bridges on the SC74 surface mount components that were a little difficult to fix.

The next big issue was then cleaning the board, because it has a nice black matt finish that really shows up the flux. I was able to use the fume hood at work and tried some cleaning techniques on a board with solder bridges. It was very difficult to remove the marks left by acetone, but a copious mix of ethanol / IPA / deionised water, scrubbing with a toothbrush and dabbing dry with kim wipes seemed to do the trick.

My Dad finished the carpentry and you can see the built up clock in the images below.

I wasn't game to plug the board in until I had a power supply with current limit built in as I was too scared I'd blow the board up. I received one just the other day and now the clock is sitting beside me disassembled and ticking slowly away.

I'm not 100% happy with the way the digits are connected to the main board. I've built a bunch of dupont connectors and have cables running from the main board to the digits, but the connections aren't very good and might need some hot glue to stay there. It works, but I think might be really flaky. I think I'll also get some clear perspex laser cut for the back so that the cables can't be pulled by little hands.
 
Renate:
It's pretty late to mention this, but...
U5 is not necessary.
Just shift everything on U7 down 3 taps.
Navigation
Message Index
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod