Thanks for purchasing the 74LS90 Binary Coded Decimal Clock kit! Here you’ll find step by step assembly instructions. If anything is missing from your kit or you have any questions, please send an email to ian@boltind.com. I’ve got plenty of extra parts and I’m happy to help troubleshoot your clock.
9/16 update:
The clock was not advancing the hours correctly. The parasitic capacitance and/or the propagation time of the newer 74LS90 chips that I got this time seems to be quite different. The solution is to cut a trace, and solder a diode and a capacitor on the bottom of the PCB. I’ll be sending an additional bag of parts in the near future. HERE is a video where I demonstrate what needs to be done.
Here is a screenshot of the CAD render that shows where to cut the trace to make way for the diode.
The red multi meter probe is pointing to the via that needs to be cut out. You could also cut the trace.
Putting Your Clock Together
If you are looking for the open source design of this clock, you can find it HERE.
- C4 and C5 need to be 2.2nF instead of the 10nF caps that were originally supplied with the kit.
- The clock doesn’t need to be assembled in a particular order, as long as everything gets put in the correct place. I recommend starting with the shortest components and moving up to the tallest components. This makes everything lay nice and flat against the PCB while you’re soldering.
- 60/40 lead based solder is recommended. You can use lead free solder, but be sure to use a soldering iron with the correct temperature capability for your solder. It is really easy to get a lower temperature soldering iron intended for lead based solder, and it’s also very easy to get lead free solder intended for a higher temperature soldering iron. I’ve seen a number of people ruin their kits with this combination of high temperature solder and low temperature soldering irons.
- Always solder in a well ventilated area, and use a fan to blow the smoke away from you. If you’re using lead based solder, be careful to avoid ingesting it. Don’t solder on your kitchen table and wash your hands afterwards!
- The side of the 32.768Khz quartz crystal oscillator MUST be glued to the PCB for accurate timekeeping.
- Each kit has 4 washers to allow enough space between the front and back for the clock to fit into our base. Midway through making the kits I ran out of M3 screws and I had to order more. Unfortunately, the second batch of screws were too short. They’ll work without the washers, but it probably won’t fit into the 3d printed base. If you need some longer screws, please don’t hesitate to send me an email.
- You’ll need a USB-C to USB-A cable to power your clock. Unfortunately the clock is not USB-C PD compliant. Ideally it would have two 5.1KΩ resistors from each channel configuration pin to ground. This means you can’t use a proper USB-C PD charger. The upside is that I was able to find a USB-C plug in a through hole package, but this USB-C plug only has pins for the power.
Step 3: Solder the LED’s. Be careful of their polarity! The long leads all go to the left. There’s also a notch in the side of each LED that aligns with with the flat spot in the silkscreen on the PCB. Note that it’s a little tricky to get the LED’s straight. One technique is to solder one side of each LED, trim the leads, then press the LED down while holding the clock with one hand while you re-heat the one solder pad. You’ll feel the LED snap into place, nice and straight.
IMPORTANT: The side of the Quartz crystal oscillator must be glued to the PCB. If it’s not glued down, the clock won’t keep time very accurately. A tiny drop of super glue will do the job.
Step 6: You’ll need to solder one of the 10nF capacitors that was supposed to be for C4 and C5 to the bottom of the board. This will prevent the minutes from resetting to 2 instead of 1. I do apologize that there’s not a spot on the PCB for this, but it caught me bu surprise. Below is a picture of the bottom of the board that shows where to solder this cap. It needs to go from pin 5 to 7 on U5. If you already soldered C4 and C5 and you don’t have any extra caps, then there’s another solution. You can make two capacitors by strategically placing two big blobs of solder on the top of the PCB. See the picture and video below that show how to do this. This isn’t the best looking solution. If you’d rather use an real capacitor, then send me an email to ian@boltind.com and I’ll get you another capacitor.
In this video I’ll explain the bug with some of the 2 bits staying on, and how to fix it.
Troubleshooting:
If your clock doesn’t work quite right, send an email to ian@boltind.com with a high resolution picture of the front and back of the main PCB, or a video of the issue. I’ll be happy to help troubleshoot!
Here is a 3d printable stand to hold your clock:
