CD4026B display driver

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manie

Senior Member
My PC board is getting very crowded. It measures 125mm x 75mm and is populated by:

1 3x ULN2803's
2 3x CD4026B's
3 1x Picaxe 08M
4 62x 1/4W resistors
5 1x 2screw terminal
6 1x 3 screw terminal
7 1x Serial stereo jack
8 1x 4 digit display (2x 2digit HP's)
9 1x 12 keypad
10 1x 16way male ribbon cable connector

I just can't seem to fit in 3 traces/connections/jumpers to the last 08M pin to be able to reset display to zero....

Can you suggest a software solution ?
Manie
 
E

eclectic

Moderator
manie

Firstly, can you provide a schematic of the board, and
the present layout?

I'm sure that there are experts in PCboard Origami on the forum.


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E

eclectic

Moderator
Thanks Andrew.
62 1/4W resistors take up a LOT of space.

And the next thought.

Depending on Price / Postal time / Presbyopia

Go small.

Using SOIC / SMD components ?

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M

manie

Senior Member
Using SOIC / SMD components ?
Click to expand...
Would love to one day with the proper equipment... can hardly read the "BC327" on the tranny now ! I'm afraid, at present I don't think I'll be able to work with SMD's etc. Resistor networks could help however. Values like 220R, 270R available in networks ?

Board layout attached
 

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E

eclectic

Moderator
M

manie

Senior Member
Moved some stuff a little and used COPIOUS jumper connections, 08M can now reset display drivers.

Eclectic: Thanks, 330R should be OK as well, goes between CD4026B BCD outputs and ULN2803 inputs. Must get some stock.

In which case would one use the Commoned Networks, all resistors connected to Pin 1 ?
Manie
 
E

eclectic

Moderator
manie said:
Moved some stuff a little and used COPIOUS jumper connections, 08M can now reset display drivers.

Eclectic: Thanks, 330R should be OK as well, goes between CD4026B BCD outputs and ULN2803 inputs. Must get some stock.

In which case would one use the Commoned Networks, all resistors connected to Pin 1 ?
Manie
Click to expand...
Errrrm don't know.

Have a look at the Tech-Spec
http://www.rapidonline.com/Electronic-Components/Resistors-Potentiometer/Resistor-Arrays/SIL-Resistor-networks/29581#techspec

and
http://www.rapidonline.com/netalogue/specs/63-0010e.pdf

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lbenson

lbenson

Senior Member
"Bussed" resistor arrays are suitable for pullup/downs, e.g., a 5-pin 4K7 array with pin 1 tied to +5V could provide the pullup resistors for 4 DS18B20s. "Isolated" resistor arrays have, like normal resistors, two connections per resistor, e.g., pin 1 & 2 for one resistor, 3 & 4 for another, so you could have both pullups and pulldowns in the same array, and series resistors as well.
 
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Andrew Cowan

Senior Member
Bussed resistors are not limited to pullup/downs. I recently used a set when I had 10 LEDs, all wanting protective resistors, connected between 0V and the LED. Much neater to use a bussed 100 ohm resistor network.

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E

eclectic

Moderator
manie said:
Eclectic: Thanks, 330R should be OK as well, goes between CD4026B BCD outputs and ULN2803 inputs. Must get some stock.
Click to expand...
@manie
I've re-read the quote above.

Have a look at page 2 of the 2803 datasheet. 3rd diagram.
http://www.rev-ed.co.uk/docs/ULN2803A.pdf

There is already an internal 2k7 resistor for each input.

So, unless I've made another mistake, you don't need to add resistors
between 4026 – ULN2803.


Also, I can't tell from the diagram in post # 5 , but
are the 2803's for driving the LED displays?
Are they not bright enough from direct connection to the 4026?
(Via resistors of course.)

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M

manie

Senior Member
Thanks guys: I worked out what the use of commoned resistors would be straight after asking a STUPID question...

After looking at the ULN data sheet AGAIN, I saw the 2k7 input resistor. You are right again...

Could not see in the 4026 datasheet whether one could drive LED's directly. Their application example shows open collector buffer between LED and 4026. Spec sheet shows what looks to be +-1 (+- one) mA only. Is that enough ?? (At 5V supply)
Manie
 
E

eclectic

Moderator
manie said:
Thanks guys: I worked out what the use of commoned resistors would be straight after asking a STUPID question...

After looking at the ULN data sheet AGAIN, I saw the 2k7 input resistor. You are right again...

Could not see in the 4026 datasheet whether one could drive LED's directly. Their application example shows open collector buffer between LED and 4026. Spec sheet shows what looks to be +-1 (+- one) mA only. Is that enough ?? (At 5V supply)
Manie
Click to expand...
manie.
If you are happy to use the ULN2803, then fine.
You can easily supply say 15mA to each segment.
Warning:- Most are max 20mA/segment, but check the Datasheet first.

In the meantime, VERY carefully, use resistors and a DMM,
then check the segment brightness, at say 2mA.

It will depend on where your display will need to be read.
In dull foggy England, 2mA may be OK.
In sunny SA?

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M

manie

Senior Member
Fortunately inside a softly(dimly) lit annexe room to the chik-house ! So could use low current. Spec sheet (attached) shows 2V drop at 20mA so I'm thinking 10-15 mA will be ok. Now I have to get busy stripping of components not required anymore on the board, example" At least 24 1/4W R's can go for starters.... GOOD STUFF !
 

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fernando_g

fernando_g

Senior Member
manie said:
Moved some stuff a little and used COPIOUS jumper connections, 08M can now reset display drivers.

Manie
Click to expand...
Unless you have a multilayer board, jumpers are a simple and time proven solution to get a signal from point "A" to "B" while simultneously crossing "C", "D", "E" and "F".

My two yen on your layout...the ground (common) traces look awfully thin. Make them as wide as possible. Additionally I can't see any decoupling caps (but without the benefit of a schematic don't know for sure). Sprinkle 0.1uf ceramics liberally across your layout....bare minimum one per IC package.
 
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manie

Senior Member
Fernando: Thanks for that, I will squeeze a bit more space out and include some 0.1's at each IC. The ULN2803's are gone, taking Eclectics advice and see 4026 driving segments directly via 220R's = 9mA/Segment at 5V. Basically a new layout/artwork I'm doing.
Manie
 
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MartinM57

Moderator
Depending on how you are making the board, it can be good practice to minimise the amount of copper you remove - on a single sided board try to "fill-in" all the unused areas with copper at 0v (or if you can't get a connection to a particular area, leave it unconnected)

Looks like you are removing at least 90% of the copper - I would aim for removing 10% (i.e just the gaps to stop all your signal and +V traces shorting to the 0v lands, if you see what I mean...)
 
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