Hi

I am trying to light up a LED dot matrix. I am using a Microchip uC, and extending the outputs with the 74HCT595. What I find though, is that the LEDS are very dim. (7 LEDS are driven by roughly 20mA).

Can someone please recommend how to get the LED's to glow brightly?
I cant source the high current 595 where I am.

Any suggestions would be most welcome.

Regards
a.

 

Assuming you're multiplexing the LED's?, 20mA shared between 7 leds is less than 3mA each! - you need to provide 140mA for the 7 leds to give an average of 20mA to each one.

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http://www.winpicprog.co.uk

 

Hi Nigel

Yes I am multiplexing the LED's. And yes there are 7 LED's per column that is approximately 3ma to each LED.

Could you suggest a way/ circuit that I could boost the current to each column?

Thank you
a.

 

A driver transistor!, either bipolar or FET.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

If you're using the 'HC595 to 'sink' current to a column (common cathode), you might consider switching to a Micrel MIC5841 IC instead... It's a serial to parallel 8-bit 500-ma sinking driver (example below)...

It might help if you tell us about your configuration... Is it several 7-segment displays or 5x7 or 8x8 matrices? Common cathode or command anode columns? Are you 'scanning' columns or rows? LED duty cycle?

Good luck with your project... Regards, Mike

Attached Images

moving_sign__20051023-1830_.jpg (154.1 KB, 913 views)

 

Here's an interesting thought:

LED's in parallel tend to light brighter than LED's in series. when you think about it, you can treat them like resistors.

resistors in series add up. resistors in parallel give lower values.

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-=: The best low-priced components to troubleshoot with are the speaker and the LED :=-

 

NO!!! - this is yet another STUPID suggestion, you can't treat LED's like resistors, and you shouldn't put them in parallel.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

Hi

I use the 74HCT595 to source current. The columns are the anode, and the PIC pins are the cathode.

I have 7x5 dot matrix; 12 units (will be expanded to about 20).

I would appreciate any suggestions as to how to increase the brigthness.

Regards
a.

 

As I said above, you've got to substantially increase the current, I would suggest driver transistors on both rows and columns (PNP to source, and NPN to sink) - a PIC won't handle the currents required either.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

Hi Nigel

Yes I think I understand. Can you actually suggest a chip that would do that for me (source 140mA)?

Thank you
a.

 

You might try looking at ULN series chips?, personally I'd probably use discrete transistors.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

Hi Nigel

Great thank you.

Would you have a part number for an 8 pint ULN device?

If I were to use discrete components I would need something like 100 of them.

Just in case could you recommend a part number for a discrete component as well?

Thank you
a.

 

They are not something I normally use, but have a look at http://www.allegromicro.com/control/prodline.htm under 'Power Source Drivers' and 'Power Sink Drivers'.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

I suspect you will want to 'scan' the seven columns for an overall 14.3% LED duty cycle... Overdrive the LEDs for adaquate brightness... I suggest using seven N-Channel MOSFET row drivers (or NPN Darlington, or....) capable of sinking the 100 LEDs in each row and P-Channel MOSFET (or PNP transistor, or UDN2981A) 'source' drivers for the columns...

I suspect you realize you'll need a 3 or 4 amp power supply to drive the 100 LED rows in your twenty 5x7 displays...

Good luck with your project... Regards, Mike

Attached Images

moving_sign_-_5x7_anode_columns.jpg (97.0 KB, 851 views)

 

Hi,

ULN2003A has 7 drivers
ULN2803A has 8 drivers

If you need 100 drivers you need to think about segmenting the driver system. You will not have enough time to generate a flickerfree display with enough light.

TOK