I am trying to drive 12cm 7-segment LEDs that are common cathode.
I have wired it up with 2cm LED displays which Arduino can power directly.
Here is the datasheet for the displays
Here is the scheme for the small displays and the big ones have the same pin layout (I forgot to draw the resistors..):
I tried using a couple of different transistors to get 12 volts to drive the big one. I got it powered up but the segment I tested didn't want to work properly.
I hooked it up like this: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1240103773
I used 2N7000 and BC549.
I am thinking that either I wired it wrong, used the wrong resistors or that it isn't as simple as I thought.
I found some information about adding a ULN2803 to the shift registers I have. Would this work better? A lot easier with wiring.
Where exactly are your transistors?
You have to source 40 or 60mA from each of the shift register's pins, which is too much, so you need 7 or 8 high side drivers.
You have to sink max 7*60 = 420mA at the cathode (low) side per digit!
If you do not use current sources the resistors for each segment should be around 12 V (supply?) - 8.7V (4x forward voltage) - 0.8V (2x collector-emmitter diodes) = 2.5V /60 mA = 47 Ohms.
The ULN chips would be fine as digit drivers, but note they have a max current which is most likely much less than 8x420 = 3.5 amps
How strong is your power supply BTW?
Common anodes would suit your application better...
Sorry guys,
I wasn't very clear, haven't slept too much due to this
Really appreciate your help.
The small 2 cm displays are working properly with the circuit I posted.
I want to run the bigger ones (the datasheet ones) but I don't have a circuit drawn on that. But I have 6 in series not four as in the circuit scheme.
Quote:
but the segment I tested didn't want to work properly.
What does THAT mean? You have our sympathy. But if you want our help, you must actually reveal what happens.
I have a test sketch and that segment G (middle horizontal one) just lights up when displaying "1" and "7".
If it was connected to the wrong pin it still doesn't make any sense.
The data sheet says the the segments only want 20mA, so apparently the individual LEDs inside are only 10mA each. But the data sheet did confirm that each segment needs 7.4V I don't see how it is doing anything at only 5V. It is an LED miracle! Shocked
It is no miracle. It does nothing on 5V. I am using an external 12v power supply.
The power supply I have is 12 V 1,7A.
Are those ULN2803 useless for this then? I looked at a lot other ones that just sinks the current but I thought this one should work.
So my LED displays is 7x20=140mA
and the ULN2803 is rated 500mA collector current.
I have a test sketch and that segment G (middle horizontal one) just lights up when displaying "1" and "7".
If it was connected to the wrong pin it still doesn't make any sense.
It is connected up wrong or your software is light up the wrong segments - take your choice it is one of them, depending on your point of view. It only doesn't make sense to you because you think you have done it right but reality is giving you the hint that you have not.
If you start diving your shift registers with 12V then you need to condition your arduino outputs to be able to drive them at this otherwise you will blow up your arduino.
