Display Multiplexing

[be80be]

Everything I use is in the "drawing" toolbar... Have a look...

Mike, K8LH That's is cool as can be.

 

[cne]

I just came across a circuit on the SparkFun website. It is the controller for a giant clock. It uses a 4511 driver for each digit, along with a ULN2003 driver. The PIC latches the data into each digit using the latch pin. This seems better because there is no multiplexing involved.

SparkFun Electronics - Wall Clock Controller - PIC 16F877A

 

[Gayan Soyza]

I'd away with CD4511 designs because it has odd looking digits on "6" & "9" .

See the digits 6 & 9 in the picture.

 

[cne]

I agree. The CD4543 looks better.

 

[cne]

I guess with Mike's design it uses much less power.

 

[blueroomelectronics]

I guess with Mike's design it uses much less power.

How do you figure?

 

[cne]

Because it uses multiplexing. With the cd4511 design all of the digits are constantly on, therefore creating a 210mA draw per digit.

210mA x15 digits= 3.15A

With Mike's Design:

210mA x3 digits on at a time= .63A

 

[blueroomelectronics]

Remember they will be dimmer when they're muxed.

 

[Mike - K8LH]

I guess with Mike's design it uses much less power.

Well, let's see. If I want full brightness or about 20-ma "average" current per segment with that 1/5th (20%) duty cycle design then I need 100-ma "peak" current per segment. Multiply that by eight segments and I've got 800-ma current per digit in each 5 digit group and 2400-ma total current for all three groups. Compare that to 15 displays lighted at 20-ma per segment at 100% duty cycle... also 2400-ma.

Son-of-a-gun, lighting a display to full brightness takes the same amount of current whether it's multiplexed or not (grin).

 

[Gayan Soyza]

Yep.

There are LED's that spec says they need 20mA current (standard).But you can pump more current to it like 200mA pulsed current then it can light up more brighter.The oval LED's that I use has this feature.It can go up to 200mA pulsed current.

If you pump more current to a normal LED by exceeding to its maximum current limits it will decrease its brightness after couple of months.So you need to find the correct LED to your application.

 

[cne]

OK, maybe my understanding of multiplexing is wrong. I thought only on display is on at a time, drawing the same amount of current as one digit.

 

[Mike - K8LH]

OK, maybe my understanding of multiplexing is wrong. I thought only on display is on at a time, drawing the same amount of current as one digit.

Yes, only one display is "on" in each group but if you want the same amount of brightness as the non-mux'd display, you don't use the same amount of current as one digit.

The design I posted uses a 1/5th (20%) duty cycle and so you would want to provide "peak" current of five times the value you desire. If you want the same brightness that you would get with 20-ma per segment on a non-mux'd display, you need to provide 100-ma per segment "peak" current on the mux'd design to get the same brightness.

 

[be80be]

Most any led can peak 120ma flashing

 

[cne]

So I finally decided to use a CD4543 driver for each digit. The PIC lactches each driver one after another while outputing the bit for each digit onto the bcd bus.

The code is as follows,

1. Put data for digit one on the bus (ie. 9 or 1001)
2. Turn digit one 4543 latch pin high
3. Turn digit one 4543 latch pin low
4. Short Delay (1ms)
5. Repeat above for the rest of the digits

I have a problem though...

When the PIC outputs a number (ie. Dig 1=2, Dig 2=9), sometimes one digit goes blank, or does not change it's value from before. The digits seem to be missing data.

Cameron

 

[blueroomelectronics]

You might be latching too fast for the 4543, add a short delay before and after the toggle.