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Aurelien
Posts: 5
Hi every body,

I want to make a radio/nav for cessna with mobiflght.

I have a question about the Max7219 controller. https://www.sparkfun.com/datasheets/Components/General/COM-09622-MAX7219-MAX7221.pdf

I think I will buy some 7 segments display with five digits https://fr.aliexpress.com/store/product/Factory-wholesale-5-Bit-Common-cathode-0-23-0-23in-Digital-Tube-0-23-inch-Red/121968_32622712546.html?spm=a2g0w.12010612.8148356.47.b2de7a6btddxnX
Do you know how many Max7219's would I need to control 40 digits? (5digit/frequence X 8)

Thank you
2019-02-02 18:06
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pizman82
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From: ETSI, Germany
Posts: 2464
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Hi

At first the Displays looks good. ... Important here is: It must be common Cathode .... AND It should have a good PIN System ( 13 in that case is perfect)

About Max Chips.
If you plan to build a own PCB you can simply buy Max Chips and you can solder it (including the whole wirework in lines on the PCB )

If you plan to simply connect those MaxChips via 5 wires to the Mega and via wires to the Displays then i recommend "MaxTubes" ( with removeable Diggits)
Something like this .... https://www.amazon.de/8-Digit-Display-MAX7219-Digital-Control/dp/B00P9277BE
Buy those kind (green pcb) that alow to remove the 2x4 Blocks .... Then you can simply connect your 1x5 Block on it with wires and some solder work.


About number of Max Chips.

In Mobiflight a Config is located alwaysto ONE Max7219 Chip..... If you use a 5 diggit Block for example on digit 1,2,3,4,5 then all is fine and you have no Problems.
If you like to use the 3 empty Diggits now with another 5 digit Blcok you got a problem.
Example:
Display A (5 diggits) is set to MAX1 pin 1,2,3,4,5 all is fine !
Display B (5 diggits) is set to MAX1 Pin 6,7,8 AND Max 2 Pin 1,2, THEN thsi is difficult maby
Display C (5 diggits) is set to MAX 2 Pin 3,4,5,6,7 all is fine again.

You see.... Its much more easy and recommend to set a Display only to ONE Max Chip and not "crossover" between two chips.

*****

Summary.
I personaly recommend to use ONE Max for ONE Display logic.( much more tidy, logical and clean )
If you realy care about the now empty 3 pins on each MaxChip i recommend to use 3 Diggit Outputs ( like CRS, HDG and so on) for this.
(Whatever a Max is realy cheap.... i would not care about that unused pins)
Finaly i NOT recommend to split Displays over 2 Max Chips. Its possible BUT you need 2 Configs and you need some advanced logicsto split them !

In Easy words.... If you need 8 Blocks with 5 diggits i would simply buy also 8 Max Chips!
Good Luck !
2019-02-02 23:47
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Aurelien
Posts: 5
thank you for your complete answer! So I will use one Max7219 for 5 digit. :thumbup:

Your idea to use Maxtubes with amovible digit is very interesting, but my 5 display will have 13 pins and the maxtube have 12 pins. I'm not sure I can solder a pin more... :blush:

I prefere make my own PCB in that case. But, I often see on different assemblies the use of transistor in the circuit; Do you know if I will have to use this ?!
2019-02-03 10:28
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pizman82
Moderator
From: ETSI, Germany
Posts: 2464
Supporter
Yes..
With the Transistor Question Stephan is the guy who can help you ( Pretty sure he reply within next hours)

About the 13th pin....

Here the logic is easy !
A Display got always the same Pin Number logic.... 7x Segemnt Pin ( For the seven segements) 1x DP (For the Decimal Point) AND Cathode Pins with same number of like Diggits in use.

For example a 5 Digit Display must have 13 Pins ( 7 Seg + 1 DP + 5 Cathodes = 13 )

The MaxTubes are Designed for 2x4 Blocks.... So it have 2x12 Pins ( 4 Diggits = 7 Segment + 1DP + 4Cathode = 12)

If you like to wire a 5 Digit Display to those tubes you will use all 12 Pins from Left Block ( That include 7x Segment, 1x DP and 4 Cathode (For Digit 1,2,3,4 )
In case you have 5 diggits you must now use from Right Block also 1 Pin ( The Cathode of Digit 5)

Simply remember.... the Segment and DP Pins on each Block (Left/Right) are combined and the same .... Only the Cathode Pins are individual and must be used correctly.!
Good Luck !
2019-02-03 14:08
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StephanHo
From: EDDG, Germany
Posts: 1260
Supporter
Hi Aurelien,

please send an eMail to stephanho ( a t ) t-online.de

then I will send you a pdf for the modification. A Transistor in my modification is not required. But you can make your own PCB, but then attach it to the existing IC sockets. See my tutorial.

@pizman: if you want change from 8 to 5 digits you have to use the right block for the first 4 digits and from the left block only the one for the 5th common cathode.
The count is 4321 4321 or 8 7 6 5 4 3 2 1 for the digits (valence).
Grüße,
Stephan
2019-02-03 14:24
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pizman82
Moderator
From: ETSI, Germany
Posts: 2464
Supporter
iconStephanHo:



@pizman: if you want change from 8 to 5 digits you have to use the right block for the first 4 digits



Not sure.... I just got remember the Mobiflight GUI.... There the Displays numbered fom LEFT to RIGHT.
If i choose Diggit 1,2,3,4,5 then its the Left Block (4 Digits) and the "DIG1 Pin" Of the Rigth block i thought.

Whatever maby i´m wrong.... Logic is the same. For a 5 Digit Display you need one Block 12 Pins and 1 Pin of the other ( The segment pin near to the 4 Block of other side)
Good Luck !
2019-02-03 22:36
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Js4100
Posts: 1
Hi Aurelien

I prefer to connect each 5 digits display (Common Cathode only!) to a separate max7219 (8digits). So, for common Bendix-type radio i use 4 max7219 PCB
Here is an example layout for Bendix Kx-155/165


regards

Konstantin
2019-04-03 11:05
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pizman82
Moderator
From: ETSI, Germany
Posts: 2464
Supporter
Thats also my recommendation !

Sure....If you need two Displays with 4 Diggits you "can" use ONE Max and wire each on a side. .... Same with 2x3 or "1x4 + 1x3"
(Here every Side is simply ONE display.)

With a 5 Digit Display you can theoretical use the other 3 Diggits for another 1x3 Display.... But then you need to use One Wire from thi side to the Other Display (not tidy)

Finaly i high recommend to NOT split Displays over two Max Chips ( For example if you need 3x5 Displays you can use 5 On Max 1 .... 5 On Max 2 And another 5 split over the two Max ( 2 on A 3 on B for example)
Here you need finaly TWO Configs for that Display and a Logic to show correct Value (Modulo% Function) .

Summary.... I said it thousend times here.... a Max is cheap. Number of Max is theoretical not limited in Mobiflight. The most easy and tidy system is to use ONE MaxChip for ONE Display.... whatever it has 2,3,4,5,6 Diggits. Then every Config got its OWN Device and every Display got ONE Wire-Block . so in hard and Software 100% Logical !
Good Luck !
2019-04-03 15:16
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bheadley58
Posts: 2
Consider using four (4) 7-segment displays for the active NAV/COM displays, with an additional (first) 7-segment display hardwired to display "1".

This would probably require a custom breadboard or PCB made to get the display to get the digit spacing correct.

In this fashion, a single MAX7219 chip could theoretically be used to display both the active & standby frequencies.
[Last edited by bheadley58, 2019-06-20 06:02]
2019-06-20 05:50
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pizman82
Moderator
From: ETSI, Germany
Posts: 2464
Supporter
What do you mean with breadboard ??

In your example the One 4 Digit Block will be connected to One side of Maxchip .... the Other (Standby) will connected to the Other side.
So right.... you controll ACT and STB with one MaxChip ( Whatever you just show the last 4 Diggits each)

About the "1" at front.
In that case you will buy (two) 1 diggit Displays with same optic (size color) like the 4 diggit Block.
then you glue them on the Left of the Blocks.

The wirework is "splitted" . The 4 Diggit block is connected with 12 wires to the max as you know. ..... The 1 Diggit Block is connected indipendend directly to a power suply (5v MEGA) OR it is seen like a LED and use a Output Pin (LED) on the MEGA if you like to set it ON/OFF in case of Battery status or so on.

Technical you have here 1 GRD connected to the GRD of Diggit..... And also a Power line ( 5v or Pin) that is connected to all needed segments together you will light for the "1" on the display.
Look up!!!! The Segments can´t handle 5v 20 mA from Pin or power suply ..... You need a resistor that is calculated for the Input Power and the final number of Segments you light.
Check the Datasheet of you Display to find out what power requires your segment-led got.

***********************
Again last note like above.....
I recommend to not make those worksteps. Simply buy additional Max Chips ( Below 2$ from China) and don´t care about unused posible Diggits. Here the save of time got a higher value as the save of maby 30$ in a full cockpit !
Good Luck !
2019-06-20 16:33
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