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Topic: Schematic/Board design check (MAX7221 based board) (Read 3025 times) previous topic - next topic

xl97

Aug 27, 2013, 11:13 pm Last Edit: Aug 28, 2013, 04:32 am by xl97 Reason: 1
Hey gang-

I have a pretty straight forward pcb schematic ad board design I did...

Like to get some feedback/second pair of eyes before I send out for PCB's

4.35 in length .75 in height

MAX7221 & voltage regulator

Purpose:
to use in a prop that will be connected to an Arduino located elsewhere.

only 30 x 1206 leds  (couldnt imagine trying to fit 60 in here!)

so less than half of the chips' multiplexing 'functionality' is being used.. :)

if anyone sees any errors.. please let me know..

(I know the board design isnt 'pretty'..but it passed all design rules checks.) :)


thanks




mirith

How does it mount to the Prop? Screws?  Tape? Bubblegum?  Does it run along only 1 blade or is it across 2 of them?
Do you have a header for connecting and disconnecting the device?

And just out of curiousity, not as a critique, does everything mount to the prop, or do you have some form of rotating connector?

xl97

Quote

How does it mount to the Prop? Screws?  Tape? Bubblegum?  Does it run along only 1 blade or is it across 2 of them?
Do you have a header for connecting and disconnecting the device?

And just out of curiousity, not as a critique, does everything mount to the prop, or do you have some form of rotating connector?


* it mounts to the 'prop', as in it, presses up to a 'window' and is secured from behind (I believe there are some screws and some pieces/plate' to hold it up)

* not sure what you mean by 'does it run along one blade or two'?  its a single 'string' (matrix wired) to be used as a 'counter' (although Im sure other animations could be done.. ie: larson effect...or whatever you wanted)

*there is no header... there are 'pads' to direct solder wires to


* yes I believe everything will mount IN the prop.  this 'bargraph/counter' will be mounter in one section/area.....   there will also be another 'brain/arduino' in the main unit/area of the prop there will be some sort of wire connector used but it really irrelevant  at this point.. as it will be determined by what fits the actual prop/mating areas best.

thanks! 



anbody got some feedback on the schematic?  any errors?  :) the pcb passes the design rules check so I'll leave it up for a few more days before giving it a 'try' and ordering.

thanks!

Henry_Best

Are you talking about a propeller or a stage prop?  :~


Henry_Best


as in 'stage prop'....
As I thought, but others have taken 'prop' to mean propeller, hence the confusion.

mirith

Yep I was assuming an airplane propeller.  My apologizes.  The shape and the word prop made me think of a Persistence of Vision type device.
Its somewhat hard to tell if you've done anything terrible on the board, as you can only really see one side.  Possibly posting the eagle file/files would help, though I don't personally own eagle but others might be able to check. 

I still would recommend you include some form of connector, as while soldering directly to the board WORKS, it isn't the most robust design, and if this is a stage prop, I assume it will be moved around to some extent between scene changes, and movement and vibration can cause a lot more damage than you would expect, eventually leading to one of the wires breaking off. Though you can get away with either hot glue or epoxy as a strain relief, though I would then move the connections further away from the edge, or add more edge, just so you have something rigid to attach to that is not soldered wire.

The Schematic looks relatively well thought out.  I'm not an expert on the MAX7221 though.

CrossRoads

Looks okay to me. .brd could have been bigger for review. What size vias are you using?
Do you have ground planes as well?
Connector - you could use a right angle SMD connector so you pins to slip a connectorized cable onto.
For example
http://www.digikey.com/product-detail/en/S5B-PH-SM4-TB(LF)(SN)/455-1752-1-ND/926849
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

xl97


Looks okay to me. .brd could have been bigger for review. What size vias are you using?
Do you have ground planes as well?
Connector - you could use a right angle SMD connector so you pins to slip a connectorized cable onto.
For example
http://www.digikey.com/product-detail/en/S5B-PH-SM4-TB(LF)(SN)/455-1752-1-ND/926849


thanks Everyone/CrossRoads-

* I think my vias are as small as I can use use/get them.. (.012? off top of my head) (I have used them before with iTead)

* RE: ground planes?  hmm.. not sure?  I use the polygon tool to make a 'rectangle' over the top of bottom layers.. if thats what you means? if not..how do I go about doing so?  (reminds me to check if I named the polys now too)..

* I'll check out smd connector link..



thanks..






CrossRoads

12 mil vias,  that's what I use also, 10 mil signal traces.
Polygon, yes be sure to name them Gnd.

Will take a look when I get home.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

xl97

question:

I got to thinking about the current needed for this project..

I know the MAX chip multiplexes (which I understand means, it blinks very fast as to give the allusion of all leds being constantly 'lit'...  but I remember (maybe from the datasheet) about that I'll need to be able to supply enough current for at least 1 segment fully lit? or some other similar rule to use?

I think the SMD vRegulator I am using only lets through maybe 150-200mA tops its rated for?

(I am also working on another project for this with another MAX chip.. but using a 3-digit, 7-segment display/unit instead of a string of SMD LEDS.. so this answer will help me there as well)


thanks!

CrossRoads

I think it powers one digit at a time.
If there are less than 8 digits, you may have to lower the current because the ontime increases and could burn out the LEDs, I think that's what I read previoulsy.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

xl97

#12
Aug 28, 2013, 10:38 pm Last Edit: Aug 28, 2013, 10:40 pm by xl97 Reason: 1
hmm.. Im not grasping??
(still un-clear if the regulator needs to be changed out to a bigger one before getting pcb's made or not?)

there will be only 3 digits (in the 7-segment project) and 30 SMD leds in this project above..

I think the Iset resistor limits the current per segment......yes?  I think I have it set for around 23k for 20mA or so?

per SEGMENT?  (as in group of 8 led? or segment as in the 7-segments that make up the numeral?)

thanks!


CrossRoads

Read this, then reconsider  your Iset value:

Scan-Limit Register
The scan-limit register sets how many digits are displayed,
from 1 to 8. They are displayed in a multiplexed
manner with a typical display scan rate of 800Hz with 8
digits displayed. If fewer digits are displayed, the scan
rate is 8fOSC/N, where N is the number of digits
scanned. Since the number of scanned digits affects
the display brightness, the scan-limit register should
not be used to blank portions of the display (such as
leading zero suppression). Table 8 lists the scan-limit
register format.
If the scan-limit register is set for three digits or less,
individual digit drivers will dissipate excessive amounts
of power.
> Consequently, the value of the RSET resistor
must be adjusted according to the number of digits displayed,
to limit individual digit driver power dissipation.
Table 9 lists the number of digits displayed and the
corresponding maximum recommended segment current
when the digit drivers are used.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

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