Large LED display driving?

[kentken]

I am driving a CA 2.25 display that runs on 8v and sinked with a ULN2003.
What is the best way to use PNP transistors on two PortA pins to drive the display?

 

[Nigel Goodwin]

I am driving a CA 2.25 display that runs on 8v and sinked with a ULN2003.
What is the best way to use PNP transistors on two PortA pins to drive the display?

Assuming you're multiplexing just two digits, you can use a single pin and two PNP transistors - check my seven segment tutorial.

 

[kentken]

I tryed that before, and it didnt work right. I will try it again.
When the base gets pulled to 8v, will this efect the Pic pin? If so, what can I do to protect it?

thanks
Kent

 

[Nigel Goodwin]

I tryed that before, and it didnt work right. I will try it again.
When the base gets pulled to 8v, will this efect the Pic pin? If so, what can I do to protect it?

It won't harm the PIC, the series resistor limits the current, and the protection diodes in the PIC prevent the pin going high. However, with an 8V supply you need some way to allow the transistor to turn off, the easiest way would be to feed it from the open-collector output pin, RA4. Or you could use an external NPN transistor to give an open-collector feed, you could also use a suitable resistor between the base and emitter of Q1 to prevent Q1 turning on when the PIC pin is at 5V.

Or you could simply feed the display from 5V - why do you want 8V?.

 

[kentken]

The forward voltage for the display is 8v, it has 4 or 5 leds in each segment.
I will try using RA4.

Thanks
Kent

 

[kentken]

Hi again Nigel, I've been moving, so no time to play.
I Hooked it up like your tut with one transistor working off the other, and hooked to RA4. The ony thing is that the supplies to each of the digits are different, the sencond tranistor is lower voltage and amp.
Could I hook each transistor to a seperate pin and pull the base up to the 8v, with out hurting the Pic.

What resistor should be used between the Pic and Transistor?

Another idea is to drive each digit seperate. Hooking the segments to seperate ports in the pic, and direct power to the displays, sink through the pic. Work? Problems?

Thanks for the Help
Kent

 

[Nigel Goodwin]

Hi again Nigel, I've been moving, so no time to play.
I Hooked it up like your tut with one transistor working off the other, and hooked to RA4. The ony thing is that the supplies to each of the digits are different, the sencond tranistor is lower voltage and amp.
Could I hook each transistor to a seperate pin and pull the base up to the 8v, with out hurting the Pic.

What resistor should be used between the Pic and Transistor?

Another idea is to drive each digit seperate. Hooking the segments to seperate ports in the pic, and direct power to the displays, sink through the pic. Work? Problems?

Thanks for the Help
Kent

I don't quite understand what you mean, try posting the circuit of that section.

 

[kentken]

The more that I thought of that idea, the more I found that it wouldnt work right.

What I came up with is ising RA0-RA1 through a 1k res to the bases and a 100 ohm pull up.

I then found that the current was only 20 ma, so I took out the ULN2003 and just used the Pic to sink the current.

This all semes to work great and the display is nice and bright.

If you have any suggestions, please let me know

Thanks
Kent