I have built a 8x8 LED Matrix with 74HC595 for row scanning and 8 pins from a TLC5940 for cols PWM. This setup works great.
I want to extend this to use high powered LEDs (while still remaining in a Matrix!!) meaning the pins on the 595 need to control something like Darlington PNP-s (to source drive the LEDs)
I have a bunch of BC516s and I changed my code so the row scanning has LOW for the active row and HIGH for all the others. (reversed the original)
I have wired up the following diagram (simplified) but the Darlington does absolutely nothing. I can't get any output. I have searched for days for an answer, if someone could explain what is going on I'd be grateful!
Not too sure what the 74HC595 is bringing to the party here, is it for multiplexing?
What voltage is Vin, it needs to be only 5V or you will not be able to turn the darlington driver off.
What current have you set the TLC5940 to?
Will the LED light up if you just connect the anode to +ve?
The 595 switches the rows in the Matrix while 5940 does the PWM on that row. The matrix will be extended to an RGB 8x8 matrix where I will use 2 TLC-s (3x8 PINS for RGB PWM and the 595 will switch through the rows)
Perhaps to simplify the issue is this: Is there something special about the BC516 as a source driver? Lets take the TLC out of the picture. Lets say I just want to source current for a LED with a 595 and BC516 as a PNP Darlington and sink the current to GND from the LED> How would I go about doing that, and will it work with the BC516?
I tried to make another schematic with what I am testing now. I have cut one of the rows to the matrix with the Darlington in to source the current and nothing happens ever, seams like the BC516 does nothing at all.
Well it will drop 1.4V across it when it is on. Maybe there is not enough volts left to turn on the LED, but you should see something. Generally I use an p-channel FET to supply the power, like in this project which is in effect a 8x8 RGB matrix. http://www.thebox.myzen.co.uk/Hardware/Hexome.html
osvathrobi:
I have wired up the following diagram (simplified) but the Darlington does absolutely nothing. I can't get any output. I have searched for days for an answer, if someone could explain what is going on I'd be grateful!
A transistor doesn't conduct when the base is at some particular voltage (eg. 5V), it conducts when the difference between base and emitter is greater than some value. If you put the transistor before the LED in the circuit this will probably never happen.
With PNP transistors you usually put them after the load (nearer to ground than the LED). If you want to put them before the load you use NPN.
