Driving a LED bar

[future]

Hello,

I want to build a digital meter, its output will be shown with a 20 led bar.

I don't want to waste many outputs with this, so a multiplex scheme/chip will be needed.

How would you drive a 20 led bar?

Thank you.

[Humberto]

You didn´t mention what PIC are you going to use, if it has a built in ADC or not.
If you don't want to use many pins, you will need a serial-in/paralell-out
IC with latched outputs and the code will depends on how do you want to
show the bar, a solid bar or a shifted dot.

Tell us more details regarding your hardware.

Humberto

[future]

Well, there is no harware yet.

It is a solid bar, it must be digitally driven.. a DAC + lm3914 is not enough.

The data comes via serial port, so ADC is not needed.

I want to use the PIC18F452 and many pins will be used for other things.

[Humberto]

Ok, we have more info now but not enough.

1) What kind of info is coming in the serial port ?
2) The magnitude you are going to show is linear or log ?
3) Signed or unsigned.

Once you know that, you will know also what is the max expected
value. Dividing the max expected value in 20 increments you get ONE step.
Then it's a matter of calculate how many steps represent each received value.

Humberto

[future]

The serial port routine is not the problem, neither the others...

What I am asking is from the PIC port to the 20 leds, certainly I could use 20 pins and drive the bar but they are not available.

I can write the mux routines, the routine to generate the 20bit pattern and everything else.

So the question translates to hardware architecture, ways to use less pins and don't be expensive.

[newguy]

CD74HC/HCT299 8-bit Universal Shift Register. Search for it on digikey.com. Cheap (about $.70 US), but you'll need quite a few pins on the PIC to drive 3 of them, even though you'll be able to mux their data lines together. The really nice thing about this chip is that it can shift right or left, ideal for creating an LED bargraph.

There is also a '194, 4-bit version of the chip.

If you want to use as few i/o pins as possible, then you can use the SN54/74164/LS164 series of serial in/parallel out shift registers. No choice of shift direction, but it shouldn't be a problem if the shifting is done quickly - the user shouldn't be able to see any flicker of the bargraph. They run about $.50 US or so.

[Humberto]

[Kenny]

[Charlie U]

You have asked about scanning/mutiplexing an array, so I'll try to handwave my way through a quick description. Since you want 20 LEDs, your array could be 4 by 5. This will require 9 pins on your processor. We will assume that the LEDs do not have integral resistors. This will be a requirement actually, since when scanning, the current must be higher that in a continuous application.

Here's the hardware description:

To do this properly, you will need a PNP pullup transistor such as a 2N3906 for each of the 4 columns, and an NPN pulldown transistor such as a 2N3904 for each of the 5 rows. To make visualization easier, let's scan the columns. To keep the description simple we will use a resistor with each LED. It is possible to simplify this by using a single resistor for each row, but let's keep this simpler for now. Let us also set up the resistor/LED series pair with the resistor connected to the Anode of the LED.
Now connect up the matrix. Connect the emitters of the PNPs to +5 and the emitters of the NPNs to ground. Connect the bases of each transistor to a processor pin through a resistor (something around 2k perhaps). When you initialize the processor, the PNP drive pins should idle high and the NPN drive pins should idle low. Now connect the open pin of the resisors in the first column to the collector of the first PNP transistor. Then the open pin of the resistors in the second column to the second transistor, etc. Next, connect the Cathodes of the LEDs in the first row to the collector of the first NPN transistor. Then connect the Cathodes of the LEDs in the second row to the collector of the second NPN transistor, etc.

To drive the matrix, just turn on (high) the pattern you want in the rows, then pulse (low) the column. After the column is turned off, turn on the pattern for the next column in the rows and pulse that column and so forth. You will need to experiment with the scanning frequency to reduce flicker. You may need to add some dead time between columns to reduce shadowing or ghosting of one column into the next. Also, you will need to look at the specs for your LEDs to see how much current is allowed when scanning. You may need to set the resistors such that about 4x the current is used since the LEDs are only on 1/4 of the time.

Hope this helps. Let us know how it turns out.

[Humberto]

Hi Charlie U,

You just wrote a bright and very descriptive method on how to drive a
LED´s array matrix, but future ask for this:

[Mark]

[Charlie U]

Thanks Mark, I did forget to mention putting them in a row, but at least to you and me it was fairly obvious that the physical orientation of the individual LEDs was a matter of user design. They could be put in a row to form the bar, or in a circle as a decoration with chasing patterns or in a true n by m matrix to form characters or other designs.

Humberto: Future wrote:

[Humberto]

Chalie U, Mark and others:

Surely there are many ways to do the same, the discusing key is how to
do it using only a few pins from the PIC.

Charlie U wrote:

[future]

I liked the 595 chip, they can be cascaded and are very cheap.

I'll use some 7-seg displays too.. look what I found.

http://www.piclist.com/techref/io/led/8x7s8pin.htm

[Mark]