Designing a 10x40 LED display

[micr0man]

Hello everyone,

I am designing an LED matrix which will be a 10 row 40 column display. The 10 row is a preference

The 400 5mm red LED's were brought in bulk in a sale and no datasheet was given so I am guessing a forward voltage of 2 volts and I would like to run them at 15ma.

For the display i shall be running a row scan so will use some high power N channel MOSFET's. for the columns i shall be using some shift registers powering some PNP transistor arrays.

I am in the design process thinking about how i will do things so i have not thought about specific part numbers but the micro controller i will use will be either the PIC18F4620 or the PIC18F4220

My question is this,

How is the series resistor's ohmage and wattage calculated for each column. I understand that the LED will only be lit 1/10 of the time so I can increase the current but is it by 10 times.

I have also been told that using a constant current source would be better to be used because otherwise i may get unevenness it the brightness of the LED's. Is this true and would it have much of a benefit. I can see it being much more expensive considering i am keeping a very low budget

Thank you very much for reading,

David

 

[MrDEB]

Here is a link of a project that has more LEDs but the idea of driving them would be the same
**broken link removed**
The coffee table Pong Game is perhaps the biggest example.
Basically they all use a ULN2803 to drive the rows/colums

 

[ericgibbs]

hi David,

For the display i shall be running a row scan so will use some high power N channel MOSFET's. for the columns i shall be using some shift registers powering some PNP transistor arrays.

Is the Row scan the high drive to the Rows, with Shift Reg/npn transistors sinking the Columns.?

Hi David,
From our Chat, is this what you have in mind.

 

[MikeMl]

What is the duty cycle at any given LED? 15mA with a duty cycle of 1/40 or even 1/10 using no-name, discount LEDs will not likely be very bright.

I'm thinking you will need ultra-bright LEDs rated to be driven at 50 to 60mA to get daylight visibility.

 

[3v0]

You don't have to guess about Vforward. Hook an LED to a reasonable resistor and measure the voltage drop across the LED leads.

The resistor sets/limits the current but the voltage drop for the LED will stay very nearly the same.

 

[micr0man]

Thank you for all your replies:

@MrDEb, Thank you for the link. I shall have a look at it now

@ericgibbs, I was originally thinking of the row MOSFET's sinking the current but it does not matter at all. i suppose it may be better because devices such as the ULN2003 can allow increased current loads while they are cheap

@MikeMI it is a 1/10 current. I do not mind about the reduced brightness as it is for indoor's. I have not yet received the LED's so I am waiting for my delivery to come but i have been told 'the LED's have a slightly muddy colour'

@3v0 As i said in the above reply i have not yet received the LED's so am unable to test them. One thing i am worried about is it will be created in different batches causing lots of variation in brightness etc

Again thank you for all the replies,

David