CC vs CA 7-Segment displays

[zeke 693]

I think I made an ordering boo boo when I bought my 7-Segment displays. I got Common Cathode (20561) instead of Common Anode (10561). I rechecked the vendor website and it turns out that they only sell the CC version.

 

Here's a schematic:

 

All the circuits posted on the site so far are CA style. I'm feeling a little depressed now because I can't duplicate those circuits identically. :cry:

 

Can anyone inspired me on how to make use of these CC 7-segment displays?

 

<Embedded the picture instead of linking to it.>

[RobG 1,892]

Are you sure? Most of what I did is CC. I think you are fine.

To recap:

CC - for common side, we usually use TPIC6C595, ULN2003, ULN2803, NPN transistors.

CA - for common side, PNP transistors or n-channel MOSFETs like 2N7000.

[nexusone1984 32]

What are you using to drive the LED's?

 

My LED temperature project using the CD4511 which is made to drive a Common cathode LED display.

 

If you using something like a 74hc595, Just invert the data your sending.

 

Where one becomes zero and zero becomes one.

 

My 74hc595 output code:

 

j = ~data; //add the '~' which will apply a Not logic and reverses the data bits.

 

P1OUT &= ~BIT2; //ST_clock low

for(i=0; i < 8; i++)

{

P1OUT &= ~BIT1; //SH_clock low

if (j & 0x01)

{

P1OUT |= BIT0; set data high

}else{

P1OUT &= ~BIT0; set data low

}

P1OUT |= BIT1; //SH_clock high

j = j>>1;

}

P1OUT |= BIT2; //ST_clock low

[zeke 693]

Thanks Guys for the encouragement!

 

I thought I was hosed. All I could see in my mind is tieing the Anode to Vcc and the Cathode to Gnd via a current limiting resistor because that's the way I normally implement this.

 

I'm feeling a little better now since I now have a choice of using the hc595, the hc164 or the TLC5940. All of which I have on hand.

 

If only I could make a drawing like Mac does, then I could show you what I'd like to make with this stuff.

[RobG 1,892]

You can always get Eagle or ExpressPCB to draw your schematics

[Mac 67]

Hi Zeke,

 

You should be ok with those CC displays.

 

You've got a couple options for driving the columns (digits). On CC displays you can use active-hi NPN transistors or active-lo PNP (emitter follower) transistors as "sinking" drivers connected to ground. On CA displays you can use active-lo PNP transistors or active-hi NPN (emitter follower) transistors as "sourcing" drivers connected to VCC.

 

You need to be a little more selective about driving the anode segments on the CC displays. The 74HC595 can "source" or "sink" current so that's a good choice (except for its limited current capability). With CA displays you have a much wider choice of higher current cathode segment drivers including some very nice serial-to-parallel drivers (TPIC6C595, MIC5821, TLC5940, etc.). The only high current serial-to-parallel high side (sourcing) driver IC I've found for anode segments on CC displays is the Micrel MIC5891.

 

Good luck on your project(s)...

[nexusone1984 32]

Here is my CC 595 setup

 

* Revised

 

temp-led595.pdf

[SugarAddict 227]

 

SCL not high on the 74LS595's?

[RobG 1,892]

Since 74HC595's output sink/source current limit is ~35mA, I would (and I am in my projects) use TPIC6C595 in place of the second 595. Functionally, TPIC is almost the same as 595 but with much higher current limits. The only difference is that TPIC can only drain, not source the current, so it's perfect for CC displays.

 

Also, I have one suggestion. Since you have only 2 digits, instead of the second 595 you could use 2 NPN transistors and save a chip.

Mac suggested once a very clever way to drive 2 digits with 2 transistors and a single pin, basically the base of the second transistor is connected to the collector of the first one. This way if the first digit is on, the second is off and vice versa. You can connect the base of the first transistor directly to MCU or if you are using just 7 segments, to the 8th output of the first 595.

[nexusone1984 32]

 

SCL not high on the 74LS595's?

 

 

Opp's my bad... yes it is...

[nexusone1984 32]

I was slack no current limiting Resistors, and maxing out the pins selecting the digit taking all the current from a possible 8 pins being active.

 

I wonder what the current is ending up being based on my refresh rate?

 

I had it running for 24 hours, not killed it.

 

I would have(if I made a circuit board I would) put a transistor to sink the current on the common cathode side (digit select). Since each of the pins driving the segments only power one segment at a time normally so you have what 24 ma to maybe 35 ma(also would add current limiting resistors on it). I know cheep Engineering....

 

I should redraw it with above changes(Transistor/current limit resistors) , but just for test it's functional...

 

Since 74HC595's output sink/source current limit is ~35mA, I would (and I am in my projects) use TPIC6C595 in place of the second 595. Functionally, TPIC is almost the same as 595 but with much higher current limits. The only difference is that TPIC can only drain, not source the current, so it's perfect for CC displays.

[Mac 67]

Hi Zeke,

 

It just occurred to me that you might be trying to duplicate Rob's Charlieplexed display. If that's the case, replace the NPN column driver transistors, as depicted in the CA example drawing below, with PNP transistors and wire the emitters to the CC pins and the collectors to ground instead of VCC. The display driver will use the same segment data array and will setup the DIR register(s) exactly the same way but you need to invert the pattern written to the OUT register(s).

 

Cheerful regards, Mike

[zeke 693]

Your suggestion makes logical sense and I would like to prove that with a circuit.

 

Thanks Mac.

[Mac 67]

You're welcome Zeke.

 

Using PNP column drivers for CC displays does work. I tried it for my single chip Charlie Clock years ago but then switched to CA displays when I built the version 2 prototype because I found that PIC I/O pins directly driving the segments are a bit more efficient when sinking current instead of sourcing current which translates to slightly brighter displays.

 

Have fun. Regards, Mike

 

 

[bluehash 1,581]

Mac's diagrams are like e-porn for me.