7 segment PCB layout suggestion

[yoga2509]

hi, i recently want to build a 737 MCP for a cheap price. i want to build my own 7 segment module as the 7 segment module in the market are available only with two 4 digits 7-segment. so i decide to build a PCB which can support only 3 or any number of digits that i need. i need some suggestion and correction for the PCB layout that i have made with the software EAGLE with my basic knowledge about PCB layout, and i just learned about this stuff for about a week. i want to make this 7 segments LEDs to be controlled by MAX7219 IC and then connect them via arduino and set it all again with mobiflight. please help me to make this work, this is my first time building a pcb layout, so its a very big challenge for me, but i want to learn it so bad. if you guys can help me and this could work, i will share this pcb layout for you guys to make your own. thanks for the help...!

[Buddym]

Thanks for sharing! Nice work. Have you found anyone to make the boards for you yet? I have used PCB4U.com and PCBNET.com, both with great results. One thing I suggest is get the silk screening! Even with a small parts count on the BOM, it helps during assembly. Also, I don't see any mounting holes on the PCB, is that intentional? Last question, had you considered going with thru-hole construction for the discreet components? It would make assembly easier for folks without the skills to do surface mount,

[tiburon]

A few suggestions : The most used LED displays are the ones with the pins on the side instead of top and bottom like the Kingsbright SC36-11YWA.
I agree with Buddym, SMD's are a pain to solder.
Also, it is possible to daisy-chain the MAX 7219 so you should duplicate the connection pins to the interface card
Otherwise, great job !

Martin

[BushPilotWannabe]

Two digit, seven segment displays sometimes come up on eBay with twelve pins for two digits. The space between the groups of pins may help routing the traces and give more room around the vias connecting traces on both layers.

Soldering two "wire" SMD components is easy, even between contacts of two layer perforated project boards. It's the removal of the SMD without a hot air reflow gun that takes time. You need PCBs with clean solder mastwo rows of ks around the pads to control bridging between contacts of multi pin components. The heat of the soldering iron draws excess solder to the iron and produces a clean finish. Solder paste is made for assembly line soldering. The paste must be forced onto the solder pad, it does not spread easily. Best to apply a very thin film of flux to the contact and carry a small ball of solder on the iron to the component sitting on the pad. Using the smallest amount of both materials will produce the best result. This is assuming that you get used to working with your nose resting on an illuminated magnifying glass.

Hugh

EDIT

What size displays are you drawing? Two digit 0.5" modules may not help. There can be nine pins top and bottom, not the six pin rows on the 2 digit and 3 digit 0.28" modules for my project.

[BushPilotWannabe]

How about this. SPI interface, I2C interface if pins A4 and A5 are reassigned. Six 0.5 single digit displays. The SevSeg library will run additional code with proper use of interupts. Four spare pins of which two are analog input only. Lots of room on the board to accomodate switching transistors if required.

[yoga2509]

Thanks for sharing! Nice work. Have you found anyone to make the boards for you yet? I have used PCB4U.com and PCBNET.com, both with great results. One thing I suggest is get the silk screening! Even with a small parts count on the BOM, it helps during assembly. Also, I don't see any mounting holes on the PCB, is that intentional? Last question, had you considered going with thru-hole construction for the discreet components? It would make assembly easier for folks without the skills to do surface mount,

Well actually i planned to make board with a pcb maker in my country which is outside the US, so i can minimize the budget for the build. As i said on the first post, this is my first time building a pcb layout and i have no idea that i need to make that hole lol. is it really necessary to make a thru-hole construction? Because its kinda complicated for me, but thanks for the sugesstion.

A few suggestions : The most used LED displays are the ones with the pins on the side instead of top and bottom like the Kingsbright SC36-11YWA.
I agree with Buddym, SMD's are a pain to solder.
Also, it is possible to daisy-chain the MAX 7219 so you should duplicate the connection pins to the interface card
Otherwise, great job !

Martin

So you recommend me to use the 7 segments with the pins on the side of the led? I also have recognized that almost every builder on the internet that i found use those kinds of leds but i never thought that it would be that hard if i build the module with the upper-bottom pin kind of 7 segments led. I bought almost all the parts from aliexpress and the cheapest that i could find was that kind of leds on the picture, so i think i will search for that kind of leds for now. Thanks for the suggestion

Two digit, seven segment displays sometimes come up on eBay with twelve pins for two digits. The space between the groups of pins may help routing the traces and give more room around the vias connecting traces on both layers.

Soldering two "wire" SMD components is easy, even between contacts of two layer perforated project boards. It's the removal of the SMD without a hot air reflow gun that takes time. You need PCBs with clean solder mastwo rows of ks around the pads to control bridging between contacts of multi pin components. The heat of the soldering iron draws excess solder to the iron and produces a clean finish. Solder paste is made for assembly line soldering. The paste must be forced onto the solder pad, it does not spread easily. Best to apply a very thin film of flux to the contact and carry a small ball of solder on the iron to the component sitting on the pad. Using the smallest amount of both materials will produce the best result. This is assuming that you get used to working with your nose resting on an illuminated magnifying glass.

Hugh

EDIT

What size displays are you drawing? Two digit 0.5" modules may not help. There can be nine pins top and bottom, not the six pin rows on the 2 digit and 3 digit 0.28" modules for my project.

I also recognized the 2 digits 7 segments leds that available on aliexpress have 10 pins on the upside and bottom side. Wow i never thought making pcbs is this hard and complex. Do you recommend to build a single layer or double layered layout? I planned to use 0.56" leds as the 0.36" may be too small for the MCP, or is it? Because i never seen an 7 segments led before in any size so i cant compare between each size.

[yoga2509]

How about this. SPI interface, I2C interface if pins A4 and A5 are reassigned. Six 0.5 single digit displays. The SevSeg library will run additional code with proper use of interupts. Four spare pins of which two are analog input only. Lots of room on the board to accomodate switching transistors if required.

Hi, thanks for helping me again, i never thought this topic would be complicated and get this much attention from you guys. Well actually i connect those leds pins to the MAX7219 chip then connect it to the Arduino mega. So the layout would be different than yours, can you help me design the pcb? That would be great but its up to you. And i want to ask about the resistors and capacitors position on the pcb, is it right? Because i don't quite understand about electronics and all i know is every negative pole of the leds must be connected to a resistor before eventually connected to ground. And the capacitor must be connected both to ground and the power source.

[BushPilotWannabe]

Yes, do it yourself electronics has a steep learning curve. Not that you need a two year tech course, but you cannot separate the basic electronics from the construction, from the micro controller or dedicated circuit board code, from the integrated circuit components like the MAX7219, from just parts acquisition from circuit design. It is wrapped up in one small ball and it all comes at once. That's just the electronics then there is the theory of flight, simulator program and computer set up, and the ever present pressure to jump ahead into the big airliners. Small steps, grasshopper, as you are doing with the MCP. Get one project finished and you can see the beginning and the end at the same time, then see how the details in the middle come together. Future projects will then have less beginning and more middle to start with and you can say, "maybe this will work?"

Sorry to make this sound like a poor attempt at inspired instruction, but I always hope that it will help viewers take their first steps into hobby electronics with a flight simulator objective. I must confess to a bias towards simple analog controls and the theory that a MEGA 328 microcontroller on a cheap Pro Mini clone board can do or replace anything on the market.

About the resistors and capacitors. A current restrictor must be placed somewhere in the circuit from VCC through the LED to GND. The MAX7219 does this internally with only one external resistor for reference, where the Pro Mini requires one resistor between each digit pin on the MCU and the common pin on the associated display LED digit.
The Pro Mini has a complete filtering system on board and does not need external capacitors required by the a discrete component such as the MAX7219. If you have not purchased your drivers yet, I would recommend the MAX 7221 if you can get them for the same price. The reason for this is available on the web. It is a question of MCU board resources and time to transfer data through four or five display drivers.

The Pro Mini using SevSeg library can assign the output pins which really helped me drop all traces from the MCU straight down to the traces connecting similar segments. This drawing is two sided. A single sided board should be possible but will probably be larger and will require exposed jumpers. Custom PCBs are not expensive considering the quality put into the product but they are not cheap either. I am sure a compromise can be found. I will look at a rough drawing using MAX7219s and a couple different chips. The final design will depend on your choice of 7 segment modules. I don't mind helping, it gives me a chance to look into things without spending the money

Hugh

[BushPilotWannabe]

You have a good grasp of Eagle. I have found with KiCad that it is easier to shut the check for errors as you lay out the PCB button off in tight spaces and start in the middle and work outwards, experimenting until something works. Unless the trace is intact from contact to contact, the program may reject the placement of a trace part way to the destination or refuse to connect the first half of a trace to the second half just layed down. This might just be KiCad. Turn the error checking back on after everything looks good and redraw the couple of traces that fail.

This should help you with the intermediate layout. Triple check all connections. Al my drawings are tagged REV - second to last. Like there is no connection on the lower board between the 5V power header and the MAX7219 +5V pin. No problem, move the trace to the left over and put in a .060x.025" via.

The image on top has the MAX7219 mounted on the back of the board. Both boards have the capacitors and resistor behind the board.

Hugh

[gkkes]

I cant help with pcb build and Eagle, Gerber etc as I'm going through the same learning curve process myself.
I would suggest however doing a search for phpjj
Peter's work on the MCP is revolutionary, particularly PMDG interface and CDU.
Good Luck