LED multiplexing tutorial. - Page 2 - Electronic Circuits Projects Diagrams Free

williB · 16th September 2006, 10:47 AM

I agree that there is no need that i can see to blank the display before updating it , did you see what it looked like without blanking it ?
maybe its worth a try?
nice tutorial

**Nigel Goodwin** · 16th September 2006, 10:49 AM

Quote:

Originally Posted by williB

I agree that there is no need that i can see to blank the display before updating it , did you see what it looked like without blanking it ?
maybe its worth a try?

No, I've not tried it, I seriously doubt it makes any visible difference - but it was the 'nice' way to do it

Mike, K8LH · 16th September 2006, 11:05 AM

Quote:

Originally Posted by Nigel Goodwin

Well, this has sparked some lively debate!

As for LED brightness, the scrolling numbers tutorial is clearly visible across the full length of the workshop - about 40 feet.

I agree. There is nothing wrong with your design considering the scope of your tutorial. My Charlieplexed display experiments showed you could get 'decent' brightness by directly driving the displays.

But I did discover a significant increase in brightness by simply adding a column or row driver transistor (which ever way you're scanning) and thought I'd pass along that observation.

The four transistors in the circuit below really make a big difference (sorry about the fuzzy picture).

Mike

<added>

I should also mention that there are some very low current (3-ma/segment) 7-segment displays from Fairchild and Agilent that provide very good brightness levels when directly driven in a multiplexed design. I wonder if there are "low current" discrete LEDs available for your 8x8 matrix?

Mike, K8LH · 16th September 2006, 11:36 AM

Quote:

Originally Posted by bbarney

1 just got some dual color 8x8 common cathode led matrix's and some STP16C596 16-Bit, constant current LED sink driver chips and some uln2803a my problem is how to wire all this together to a 18f2550 or a 18f4550.

I believe you're going to need "source" drivers for the LED anodes. The STP16C596 and the ULN2803A are both "sink" drivers.

If you can exchange your matrix displays for common anode versions then you could use the STP16C596 16-bit "sink" driver for the LED cathodes along with a UDN2981A "source" driver (instead of the ULN2803A) to drive the common anodes.

Mike

**Nigel Goodwin** · 16th September 2006, 12:55 PM

Quick update - I've added a short video clip of the scrolling display in action, about 20 seconds worth. Interestingly you get a nice interaction between the LED strobing and the camera, which gives a 'twinkling' effect to the LED's.

BTW, if anyone is interested?, I 'stole' the bit patterns from the Hitachi type LCD modules - I'm currently doing the letters as well.

williB · 16th September 2006, 02:05 PM

Nigel ya might as well link to it directly
i just saw it , its pretty cool !!
http://www.winpicprog.co.uk/video/led1.wmv

bbarney · 16th September 2006, 05:18 PM

Quote:

Originally Posted by Mike, K8LH

I believe you're going to need "source" drivers for the LED anodes. The STP16C596 and the ULN2803A are both "sink" drivers.

If you can exchange your matrix displays for common anode versions then you could use the STP16C596 16-bit "sink" driver for the LED cathodes along with a UDN2981A "source" driver (instead of the ULN2803A) to drive the common anodes.

Mike

Mike
Unfortunatley I'am stuck with the cc matrix's.so my stp16c596 is no good what would you suggest use two 74hc595 in place of it and still use the 2803a?

philba · 16th September 2006, 05:26 PM

Quote:

Originally Posted by Nigel Goodwin

Well, this has sparked some lively debate!

OK, I'll explain further.

My original plan was to use source and sink drivers in order to push more current through the LED's - and the original circuit I started drawing incorporated that - but I thought it was getting a little away from the 'simple' philosophy of the tutorials.

While doing mental calculations of suitable resistor values I realised it's quite plausible using just the PIC to drive the displays, so I dropped the drivers and increased the values of the current limiting resistors to 150 ohms.

Due to the modular nature of my tutorials, a further thought was to optionally add driver boards between the PIC and the display - I was thinking of two seperate boards, a 'sink' board and a 'source' board - both of which could also be used for other purposes (again reinforcing the modular nature of the boards).

Amen!

Quote:

BTW, I've just assembled a quick program that puts all the LED's ON, and measured the battery current - it was only 62mA? - which seems rather low?, but the LED's seem plenty bright enough. This was on my meter at work, I wonder is the current ranges aren't reading properly? - it very rarely gets used on current, I'll measure again with my meter at home.

that does seem low. Are you certain the duty cycle for each row is 1/8 (minus a little for blanking during row change)? based on your numbers, it should be about double that. with some voltage drop from the pic, I'd expect more like 100-110 mA.

**Nigel Goodwin** · 16th September 2006, 08:46 PM

Quote:

Originally Posted by philba

that does seem low. Are you certain the duty cycle for each row is 1/8 (minus a little for blanking during row change)? based on your numbers, it should be about double that. with some voltage drop from the pic, I'd expect more like 100-110 mA.

Just measured it again, at home this time, still 62mA?.

Certainly the multiplexing was 1/8th the other day when I scoped it, I'll try it again and see what's going on!. Might not be till Monday now though.

**Nigel Goodwin** · 18th September 2006, 10:34 AM

OK, I've added two more sections to the tutorial, the first adds full text capability, and the second uses that to scroll a text message across the display.

**Nigel Goodwin** · 18th September 2006, 12:55 PM

Quote:

Originally Posted by philba

that does seem low. Are you certain the duty cycle for each row is 1/8 (minus a little for blanking during row change)? based on your numbers, it should be about double that. with some voltage drop from the pic, I'd expect more like 100-110 mA.

OK, I've got the scope round it - with all 64 LED's lit, the PIC pins are only pulling the bottom of the LED's down to 1.65V above 0V, there's 1.75V dropped across the LED's, and 1.6V across the 150 ohm resistors. So this is about 10mA peak through the LED's.

As you say, adding sink drivers would easily increase that, but it's bright enough as it is.

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16th September 2006, 10:47 AM	(permalink)
williB	I agree that there is no need that i can see to blank the display before updating it , did you see what it looked like without blanking it ? maybe its worth a try? nice tutorial

16th September 2006, 12:55 PM	(permalink)
Nigel Goodwin	Quick update - I've added a short video clip of the scrolling display in action, about 20 seconds worth. Interestingly you get a nice interaction between the LED strobing and the camera, which gives a 'twinkling' effect to the LED's. BTW, if anyone is interested?, I 'stole' the bit patterns from the Hitachi type LCD modules - I'm currently doing the letters as well. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

16th September 2006, 02:05 PM	(permalink)
williB	Nigel ya might as well link to it directly i just saw it , its pretty cool !! http://www.winpicprog.co.uk/video/led1.wmv

18th September 2006, 10:34 AM	(permalink)
Nigel Goodwin	OK, I've added two more sections to the tutorial, the first adds full text capability, and the second uses that to scroll a text message across the display. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk