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Hi ,
In my flowcode projects a regular RED 3-digit 7-segment display with current resistors works at the correct brightness.
With displays of other colours it is dim , and even by changing the current resistors I am unable to increase brightness, or the on time.
I am using PIC16F628's, with internal OSC and regulated at 5V. The displays are common Anode.

Any clues are appreciated.

THX.

Hi

You probably need to check the forward voltage of the LED displays that you are having problems with. Most likely it is higher than for the Red one.
If you are driving the displays directly from the PIC then you need to have a look at the data sheet to check the likely voltage drop for the pins that are sourcing and sinking current.
If the total for all the voltage drops is nearing 5v then you will not be able to get the current you require for greater brightness.

Leigh

Might also be worth lowering the display refresh frequency if you are multiplexing several displays as this should allow the displays to be brighter.

Hello,
Tried lowering the Refresh Freq. to a point of flicker and still the same .
I Feel its what LeighM says, The green displays have a higher forward voltage, I guess they are maxed out.

In the display routines , each display is turned on at a time (refresh freq) with the value to be shown. Is it possible to switch on in sequence one segment at a time for a given number for example then switch to the next digit and so on, this way only one led is on ,hence it can draw the required current ? Just a thought.

Thanks Guys for the ideas.

Hello,

Hmm it may be possible to only light up a single segment at a time but all the additional multiplexing will likely dim your display further rather then making it brighter.

What about adding transistors to allow you to drive the display without the limitation of the uC Voltage and Current. You would need PNP type transistors for your anode pins and NPN type transistors for your Cathode pins.

You could then run the display using 12V with current limiting resistors to avoid damage to the display.

There is a tutorial on this here.
http://www.matrixmultimedia.com/article.php?a=52