Heres an example with a piezo siren, but its the same analogy for the segment displays



GND would be applied to Pin 8 on the ULN2003
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Pin 9 on the ULN2003 is connected to the positive supply for inductive loads such as motors and relays

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The datasheet of ULN2003 shows that there is a 2.7 khm: resistor at the input. So I can use it without a resistor right? And the output logic of the patterns have to be inverted.

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You still need segment current limiting resistors.

A '1' on an input turns on its corresponding output which sinks current to the LED segment turning it on.

Stick with common anode displays and use the ULN2003 or ULN2803 to drive the segments.

 

That’s correct, a digital "High" (5V) at the input will supply an earth to the output (like a switch to earth), a logic low (0V) will turn off this earth ("open circuit") thus turning the segment/device off

How can you use the ULN2003/ULN2803 for a common anode segment? They switch earths, not supply voltages - the common +ve supply pin is purely for inductive loads (notice every output has built in diodes to protect from back EMF)

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PIC Micro's - Your mind is the limit

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Not sure what you mean by a common anode segment but common anode displays have the anode of each segment tied together and supplied or switched at a positive voltage. The other end of each segment requires a sinking driver, like the '2003. A simple example is shown in the post just previous to your last post. You connect the ULN2003 diode connection to the same positive supply that you're using for the common anodes.

I assumed you were describing using a ULN2003 as a sinking driver for a multiplexed common cathode display, using it to replace the sinking NPN drivers in the "common cathode" drawing below. Yes, no?

Attached Images

Mux CA & CC.PNG (43.6 KB, 6 views)

 

Oh god, you're correct. I'm muddled, the resistors are needed, they are the load of the collector.

He meant using both PNP and the ULN2003. PNP to supply the common anode while ULN2003 used for the patterns of the 7-segment display.

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That would be complete overkill wouldnt it? Just use the ULN2003 to switch the common cathode for each display - the pic can drive each segment easily, but a resistor for each would still be required to limit the current for each segment.


common anode segment display



Common Cathode

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PIC Micro's - Your mind is the limit

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But if you only have common anode displays, and the additional components already, then go with the transistor switching..

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Because previously I complained that the 7-segment display is not bright enough. If I use common cathode, the current sourced by the PIC is enough?

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Yes, I agree, it is "overkill" but it provides you with a full brightness display. If it takes 10-ma per segment at 100% duty cycle for full brightness you're going to need 40-ma per segment pulses at 25% duty cycle for full brightness (and a source driver capable of 40-ma for 7 or 8 segments = 280..320-ma). Driving the segments directly from the PIC pins as you suggest just isn't going to cut it if you need full brightness. Sorry.

 

No, you'll still have the same problem. In fact, in my experiments these last few years I've found that common anode displays are slightly brighter than common cathode displays when you're directly driving the segments from PIC pins. It seems the PIC pins can sink slightly more current than than they can source. Or, there's a slightly higher voltage drop when sourcing current, or somthing like that.

 

Ya, that's why I choose to have the PNP with common anode and the driver.

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PIC's can throw out up to 25mA per output.

I try and aim for around 20mA, and it’s a clear display with that much current

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Mine is also bright enough if I use only one 7-segment display. When the four 7-segment display being multiplexed, they becomes dimmer.

Any idea if I want to turn 4 LED's on at the same time with PIC with the brightest? But I'm not going to use any transistor or driver, just the LED's, resistors and the output port of the PIC.
I would sink the current from the PIC.
There are 3 extra pins not being used, I plan to use all of them for four LED's. But how should I connect them? Definitely not in series right? I will use parallel for all.

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Sure, you'll get full brightness at 100% duty cycle with 20-ma. Four displays at 25% duty cycle will see about 5-ma average current. That's quite a bit dimmer than full brightness but fortunately these LED segments throw off quite a bit of light at even a miniscule 2 or 3 milliamps average current.

If you a one to four digit display you're fine. If you've got more than four digits and you can get by with a lower brightness display then by all means, go for it...

Here's a very fuzzy picture of a 4-digit common cathode stick display. The common cathodes have transistor drivers and the segments (anodes) are directly driven. This board is actually using the Charlieplexing type of multiplexing where 8 pins are used to drive the columns (common cathode transistors) and the segments (anodes) instead of 12 pins. It's quite a bit dimmer than a full brightness display. It throws off plenty of light but you can't see it very well in a sunlit room and you can't see it at all when it's in direct sunlight.

Attached Images

Stick Display.JPG (59.9 KB, 5 views)