Question on LED Matrix (again) :)

[Justin98TransAm]

Sorry, I know you guys are probably getting tired of me anyway, I asked in my other thread about 4 days ago, but no response, I think the thread is dead

anyway, here's the deal. I'm doing a 7x46 Segment LED matrix. my question is what type of Transistors should I use for the multiplexing, 2n2222's okay? or is there something else I should use? Voltage will be 2.5~3v at 25~30ma

 

[jrz126]

what are you using to control them?
How many transistors are you going to use?
If you are using +5V signals, I'd suggest using a ULN2803, it has 8 transistors build into a single 18 pin DIP
Thats what I'm using for the LEDs in my headliner
**broken link removed**
they are running at 3.5V and 20mA.

 

[Justin98TransAm]

I'm using two PIC 16F877's I'm still somewhat in the design/idea stage. I would assume I would need about 140 resistors

46 colums X2 transistors = 94
7 Rows X2 transistors = 14
94+14=108 transistors

is that correct?

I'm using it in a car so it will be a 12v native signal, I suppose I cold regulate it down to anything I wanted.

 

[jrz126]

Not quite sure what you are doing with these leds, but why do you need 2 transistors for each led?

I'm using 2 lm7805 voltage regulators on my leds. I know its a waste to do it this way, but I was able to keep the power dissipation down on my currentlimiting resistors so I was able to use the 1/8 watt. It also prevents the leds from dimming due to the voltage drop caused by my system.

 

[Justin98TransAm]

I don't need 2 Transistors per LED I need 2 transistors per row and column in the array/matrix. there are a total of 322 LED's in the Matrix. I'm basing it off of this..

http://www.armory.com/~spcecdt/electronics/LED_matrix/LED_matrix_schem.gif

 

[jrz126]

I dont understand why you need 2 transistors per row/column. Is it for higher current handling?

What exactly are you trying to make?

 

[Justin98TransAm]

they are on off switches basically. It will allow me to address each individual LED in the entire matrix while only using 53 outputs instead of having to controll each LED individually wich would require 322 outputs. Basically it's an LED sign that I can scroll text across or simple graphics. That's why I need to two PIC's

 

[jrz126]

I understand the need for the transistors, but I dont see why they are using 2 per row/column. Maybe a single transistor cant provide enough current for each row so its doubled up. If that is the case then you are going to have a problem with your rows, since they are 46 leds long. Worst case every led is on, at 25mA that equals 1.15 amps.

Or the CD4514 isn't able to provide enough current to the base of a single transistor so that enough current is able to flow through the Collector and emitter. Either way though, you are still going to have the high current problem. (A 2n3906 in the TO-92 package can only handle 200mA)
Maybe mosfets might be a better solution?

 

[audioguru]

I agree that the transistors will blow up because their max current is exceeded when the sign is displaying a big number 1 or letter I, when many LEDs in a column will be lighted. The original circuit was probably designed to light only a single or couple of LEDs at a time.
Since the display is multiplexed, a massive LED current must be used because they are dimmed due to their low duty-cycle.

 

[Justin98TransAm]

they aren't designed to be all on at a single time. the PIC's will control the matrix so it will be multiplexed, only one LED in a row or column will ever be on at any one given time.

 

[audioguru]

they aren't designed to be all on at a single time. The PIC's will control the matrix so it will be multiplexed, only one LED in a row or column will ever be on at any one given time.

Then the duty-cycle will be so low that you won't see anything! :shock:

Most screens make the raster by multiplexing only in a horizontal direction. Then the LEDs in a column are all lighted whenever required. Their duty-cycle is low so the peak LED current is very high. So high-current transistors are used and the LEDs must be able to withstand the very high peak current.

It sounds like you are trying to make a TV screen by stepping through many vertical columns horizontally, and many horizontal rows vertically.
Just think about how short a duration a single LED will be lighted after each scanning trip. Next to nuthin'. :lol:
Just think about how high a scanning frequency that you will need to avoid flickering, if the display can actually be seen. :lol:

 

[mstechca]

If you need extra brightness, look for LED's that have a high MCD rating. Dont bother buying cheapie LED's and driving them with excessive voltage, because you may have problems faster if you do that.

I think you should cut your power supply to 4.5V (3 AA or 3 AAA batteries) or 5V. Why? because then you can use any chip without trouble. I haven't used PIC's but my suggestion is that you should NOT give any IC more than 5V for Vcc unless the manual that applies to the chip states otherwise.

There are multiplexing IC's available, but I am not sure EXACTLY what you want to do with the LED's.

 

[audioguru]

Dont bother buying cheapie LED's and driving them with excessive voltage

Leds are driven with current, not voltage. A LED driven by a current-limiting resistor from a one-million volts supply will operate exactly the same as when it is driven by a current-limiting resistor from a 6V supply. :shock:

I think you should cut your power supply to 4.5V (3 AA or 3 AAA batteries) or 5V. Why? because then you can use any chip without trouble.

Justin has already decided to use an ordinary Cmos CD4514 "multiplexer" IC. Like all ordinary Cmos IC's, its max supply voltage is 18V or 22V. At only 5V, its output current is very low. :cry:

How are you going to get the very high current required from little battery cells? :?: Justin's circuit is driving 322 LEDs, not just a couple. :shock:

 

[Justin98TransAm]

Actually I'm not 100% sure about how any of it will function. I purchased 10,000MCD LEDs. Forward voltage is 2.5v and current draw is 25ma. The supply voltage is 12v Car Battery so amprage sholdn't be a problem. I should be able to regulate the voltage down to whatever I need. I'm just trying to create a 7X46 segment LED display It does however need to be fairly bright. the PIC's are so I can scroll text, graphics, and coordinate the two halfs of the matrix (it's actually two 7x23 segment arrays) so there will be one PIC for each and then they'll be connected together to pass on the info from one display to the next. clear as mud now?

 

[audioguru]

LEDs are made very bright by focusing the light beam in a narrow angle. You might find that even a wide angle (30 degrees) LED is still too narrow.

I assume that your display is 7 dots high by 23 dots wide, and you have two displays separately horizontally multiplexed and side-by-side.

You probably should sync the scrolling speed to the multiplex rate to avoid waves of dimming.

The column drivers must handle a total of 7 LEDs and the 2N2222 (metal case) is rated for an operating current of 500mA, so you can blast peak current pulses of 500/7= 71mA to the LEDs if they have this high peak current multiplex rating.
The 71mA is divided by at least 23 due to the multiplexing, so their average current is only 3mA. Will the LEDs be bright enough?

 

[mstechca]

How are you going to get the very high current required from little battery cells? :?: Justin's circuit is driving 322 LEDs, not just a couple. :shock:

Doesn't a high MCD value counteract the need for high current?

and as far as LED's are concerned, I think LED's that take less of a load are better. If used, they should have a resistor in series for protection.

 

[audioguru]

Doesn't a high MCD value counteract the need for high current?

How bright is "bright"?
LEDs aren't bright enough yet to light-up things like lightbulbs. So they focus thier low light in a very narrow angle to appear brighter. My 5000mcd, 30-degrees angle ultra-bright LEDs will blind you if you look directly at them at night. In the daytime, they are fairly bright on-axis, but can barely be seen a little to the side. In sunlight they wouldn't be noticed.

Your vision's response is logarithmic, so 10 times the brightness looks only twice as bright. That's why you can see in moonlight and in sunlight. Plus the AGC action of your iris that reduces very bright light.
I don't think that an average of only 3mA in an ultra-bright LED is bright enough.

 

[Justin98TransAm]

so, what I kind of hear you saying is, I'm going to loose brightness becasue I have to "sequesnce" the LED's so much? So how long would each LED have to be "light" to achieve it's max brightness? I was under the impression that LED's are almost and "Instant" on type of device. I don't know how fas the PIC can sequance across the entire matrix, but I would assume that would be my brightness limiting factor. Also the CD4514 isn't designed into my circuit, I had just pulled that scheme for the actual LED matrix itself. I'm thinking the PIC's will do all the controlling This is all still in very early stages of development, so would would you suggest?

 

[audioguru]

Hi Justin,
Your pics must "sequence" through the 23 columns of LEDs quick enough that they don't appear to flicker, about 50Hz at the slowest but much quicker would look best. Yes, the LEDs light and turn-off instantly.
Since each LED will be accessed for only 1/23 or less of the total time, its average current (and brightness) will also be only 1/23 or less of its peak current.

With your 2N2222 column driver transistors at their max current rating of 500mA, a lit column of 7 LEDs will each have a peak current of 71mA. Therefore the average LED current is only 71/23= 3mA, which is fairly dim.

LEDs that are made to be multiplexed have a peak current rating of 1A or more. Then with suitable high-current row and column transistors and a peak current for each LED of 460mA, their average current will be 20mA and they will appear bright.

To determine if your LEDs are suitable for your application, check their datasheet:
1) What is their angle?
2) What is their max peak current rating?
3) What is their max continuous current rating?
4) Is their brightness linear with increasing peak current, or does their brightness "flatten-out" (saturation) at high peak currents?
5) What is their typical forward voltage at a high peak current?

 

[mstechca]

so, what I kind of hear you saying is, I'm going to loose brightness becasue I have to "sequesnce" the LED's so much? So how long would each LED have to be "light" to achieve it's max brightness?

I think you are still lost.

An LED's brightness is not dependent on frequency. It is dependent mainly on how much current is injected into it, and the MCD rating of it.

As for your sequencing, you may get the brightness you require. HOWEVER, as a member said before, if you are treating the operation of the LED matrix the same as a TV screen, then what you need is a high frequency clock. You need to make the frequency high enough so that the character you want will appear. Don't go too high, or the LED will appear dim because of excessive frequency. As I said, it will appear dim, but what actually is happening is that the full brightness is seen for like a microsecond then no brightness. Mix this together, and you see a dim LED.

Maybe you should order an LCD.