Branching out

I mentioned in the last post that I will be trying out the display modules. I found it easier to break away from the original TfaC Git Hub project and create a new one dedicated to the research and development for the display module. The new one is simply ClockDisplayModule.

The first consideration I needed was what 7-Segment displays. The original have a digit height of about 19mm. It is not possible to obtain this with the resources I have. So there is my first compromise. A near fit is about 20.32mm or a 15.24mm. In order to accommodate the usage of either of these, since neither is ruled out, I decided to create two branches on the ClockDisplayModule: largeDisplay and smallDisplay for the two 7-Segment sizes respectively.

driverThe first thing I needed was the circuit diagram for connecting the MAX7221.

I decided to model most of the connections as global labels to avoid clutter on the diagram. It also meant I could separate out the various components of the module in terms of the array of 7-Segment displays, the indicator LEDs and the driver itself.

The data sheet for the driver recommends an electrolytic 10μF and a 0.1μF near the V+. The rest is pretty much as per the application notes on the data sheet. At the moment I have selected a 25KΩ for the Rset which controls the current to the LED segments. This may be also subject to review based on the final 7-Segment selected. I will be providing the working for the final calculation.


The 7-Segment displays have been modelled using the bus feature of KiCad. The common cathode of the displays goes back to the “digit select” on the MAX7221.

The indicators for the display will be implemented using discrete LEDs rather than any decimal point on the 7-segment display units. Mainly because they are not in a position that I consider useful.

I have not seen the MAX7221 driving discrete LEDs along with any 7-Segment display, but I am keen to try this out. I prefer to use discrete LEDs rather than any decimal point on the 7-Segment units as a matter of position. I believe that I will have to consider the ratings of the LEDs and possibly match them with the display segments to avoid any issues loading the MAX7221 inappropriately.

indicatorsMy participation in the Contextual Electronics course has given me the tip to place some zero Ohm resistors in series with the indicator LEDs. The MAX7221 is capable of limiting the current to the LEDs it is driving through the Rset (R101). However if I can not match the indicator LEDs correctly, I at least have the option of swapping the zero Ohm resistors with something more appropriate.

What’s Next?

The next step will be to lay this all out onto a board. I already have the measurements I will use – that is the same size as the existing display panel – 3.5″x1.3″.


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