Warning: I am a complete newbie in Electrical engineering and only an expert in programming. Please dumb it down for me. I've only done a handful of tutorials with the micro-controller (no shift register/transistor experience).

I would like some opinions on my schematic in the link below. Note, that it is not done at all and I am looking for opinions on whether the 2 SiPo Shift registers/resistors/NPN transistors/LED setup makes sense to anyone?

The top shift register controls which columns of LEDs should be toggled on and the bottom shift register controls which rows should be on. The intersection of what the top and bottom shift registers say should be on will actually turn those specified LEDs on.

Please ignore power and micro-controller setup.

So the question would be, am I doing it right? Concerns?

 

I think you need a resistor in series with the base of each npn transistor but I'm not completely familiar with the 74HC family.

 

I think you need a resistor in series with the base of each npn transistor but I'm not completely familiar with the 74HC family.

Thanks for the post. Yes, I forgot to mention that the micro controller and the shift registers I am acutally using are different. So output volts are different. But yes, I think you are right. In most cases I would need a resistor.

 

You might look into using a ULN2803 driver IC in place of the 8 transistors, it's a 16 pin DIP IC with 8 darlington transistors it also includes the base resistors configured to be driven from 5 Volts. It can handle 500 ma.

 

There is also the 2N7000 mosfet. Many ways to skin the cat.

 

If I read the part number correctly you have a 74NC595 shift register to drive the LEDs. That is spec'd for ±35mA of source/sink current (you are sourcing) but with a 1K series resistor you are just sending a fraction of that to the LEDs. So far that looks OK.

However, you have all the LEDs of a column in parallel. That means the brightness will vary depending on how many LEDs are on.

So you'll need a lower value resistor and one resistor per LED.

 

You might look into using a ULN2803 driver IC in place of the 8 transistors, it's a 16 pin DIP IC with 8 darlington transistors it also includes the base resistors configured to be driven from 5 Volts. It can handle 500 ma.

Can you explain to me how the 8 Darlington Transitors would be connected in the circuit? I did a little bit of research and it seems like Darlington transistors are meant to amplify current. I want to allow or cut off current on certain circuits based on the shift register 2's output. So I must be missing something if both of you are agreeing.

 

I've updated the schematic here.

Note that only 1 of the columns will be lit up at any given time. So Shift Register 1 will only have 1 output high at any given time. Shift register 2 will output high any LED row in that column that should be lit.

 

…
Note that only 1 of the columns will be lit up at any given time. So Shift Register 1 will only have 1 output high at any given time. Shift register 2 will output high any LED row in that column that should be lit.

Ok, only one column will be lit at any time. But how many rows could be lit at the same time?

 

Ok, only one column will be lit at any time. But how many rows could be lit at the same time?

Up to 8 rows. So basically only 8 LEDs can be lit at any time. I am hoping the mico-controller is fast enough so that it gives the illusion that up to 64 LEDs can be on without flickering.

In other words...

The microcontroller attempts light any needed LEDs only in column 1, then only in column 2, then only in column 3.... etc.