A few PCBs later...

A project log for Back To The Future Time Circuit Clock

Back To The Future Time Circuit Clock with clock, alarm and FM radio receiver.

atherosatheros 04/14/2014 at 19:090 Comments

What's new?

During the week I had little time to work and no access to a workshop, so all I could do was ordering the components and planning. I was also busy with eagle designing all the PCBs needed by this project. During the weekend, I had an occasion to etch my board designs at my brother in law's workshop (meet my teammate, BWA). While I'm still very far from having all the boards designs needed, there is some progress: I now have some basic skills at designing boards with Eagle CAD and I already learned by mistakes a few valuable lessons:

Here are some photos I did of the boards I attempted to make this weekend:

This is a 4 display module and driver before I decided to use a single shift register for two 7 segment characters.

 Display driver with half the number of shift registers, resistors and pins :) SMD components pads are on the other side.

And finally a working year display module.

The modules

I've create a GitHub repository to hold all the designs and code:

I'll try to keep all the schematics, board designs and code up to date there.

Bellow is a list of components and modules I need to create to finish this project.

Display module

There will be 3 or 4 display modules in each of the 3 clock rows:

There might be 4 displays in one of the rows if I decide to reuse a prototype module I already made for a year display. We'll see how much time will I have and decide then.

Display driver module

This will generally contain shift registers and will have display boards solder to them. I already worked on one of the boards, however I run into problems regarding very small vias. Right now I will try to make such module using prototyping board and through-hole components to see how small can I make it. If it will turn out well, I'll use this approach for all driver modules and the main controller board, as the only tool needed is a simple soldering iron. That would allow me to work at home and will rise my chances of completing the project before the deadline.

Display controllers

As I lack experience with electronics, I try to keep each of the modules as simple as possible, so I don't waste too much time replacing a module if I make some mistakes in them. This rises the number of PCBs and components required, but at least I can divide problems into more manageable chunks. I guess it's my software developer side, but I can't help it.

These will be driven by AVRs (arduino mini or nano?) and will control an entire row of LED displays.

Main controller

I didn't design the main controller yet, but I already have some ideas about it.

It's brain will be some arduino board. This controller will do the glue logic between input buttons, radio module, RTC and display controllers. All communication will be done through I2C.

RealTime Clock

I will use PCF8583 i2c RTC chip with a battery backed RAM. I'll use it's ram to store additional configuration, like programmed radio stations and alarm date. I don't know how this chip compares to other I2c RTCs out there, but I already used it in my previous project and I still have a bunch of unused chips on my desk (buying extra components when doing some project is a good idea!)

The realtime clock circuit will lie on the Main controller board.

Radio module

This one is a bit of a challenge. I found 3 chips with I2c interface: TEA5767, QN8025 and SI4703.

TEA5767 with small breakout modules is available everywhere I looked. It comes in very small breakout board with low pin count. It's main problem is it doesn't support RDS/RDBS

QN8025 is similar to TEA5767 with the addition of RDS. It seems however it's not available anywhere anymore. My local supplier has it in his catalog, but nothing on stock. It comes in a very similar small breakout board like TEA5767

The last chip is SI4703. It seems to be popular, but more expensive than others (3 to 4 times more expensive). It has RDS support and comes in two types of modules: breakout module and module with small amplifier for headphones. Unfortunatly, the only modules I found in Europe (aka close enough to be sure I'll get it before contest deadline) are the later ones. So I won't be able to hook them to the speakers I planned to have.

This is a big problem, I should have ordered the SI4703 modules a long time ago... My plan is now to order all the module types I find and see what I can do with it. If I don't get a module with RDS support, I'll have to modify my plans regarding the last display row of my clock.

What next

I'll try to make a working display driver module. If that goes well, I'll make all the 7 segment displays and drivers this week. Then I'll work on 14 segment displays for months and RDS display.

In the mean time, I'll try to assemble all the remaining pieces of electronics I need and start thinking about the case.

I hope I still have enough time...

Whish me luck!