ULN2803 with external BD137 transistor on each output would be a good start.
ULN2803 has 8 transistor outputs and is designed for use with TTL levels, which is what I assume you're using. Yes it can connect directly to your CMOS shift registers.What would the advantage be to using the 2803? Could I just use the transistors with the shift registers?
Yes. You will need to take into account the voltage drop of the high-side driver, the LED forward voltage and the ULN2803 "saturation" voltage.Edit: To calculate the resistors used, should I just use the values for a single LED, because the resistors would be on the columns, and only one led per column would be on at a time?
Okay, so I use a MOSFET to boost the current (Fairchild has the BD13716S that can handle 1.5A)Yes, what you've drawn would be correct except that the the ULN2803 can't handle 1.2A which is why WTP suggested using a BD137 on its output.
I think my shift registers can handle it, NXP's 74HC595N,112 can output 35mA per pin, with a 750mW max overall (using the DIP package). Would I take 20mA times the LED voltage of 2.5, or the output of 5 to get the wattage?Also, you might want a buffer on the column driver as the shift register won't necessarily be able to supply 20mA to all of its outputs at once (you might also like to increase the 20mA as each row is only driven for 1/16 of the time).
Yes, use a single MOSFET on each row and column.Okay, so I use a MOSFET to boost the current (Fairchild has the BD13716S that can handle 1.5A)
The maximum ground or supply current for the IC is specified at 70mA.I think my shift registers can handle it, NXP's 74HC595N,112 can output 35mA per pin, with a 750mW max overall (using the DIP package). Would I take 20mA times the LED voltage of 2.5, or the output of 5 to get the wattage?
They will be less bright.What would happen if I tried to run the LEDs under their normal current?
I don't think that will be as much of a problem, the LEDs I'll be using are super bright 800-1000 mcd ones, so having them be dimmer might actually be a good thing.They will be less bright.
No, it won't work as BD137 is NPN and the ULN2803 output is open collector.Does this look correct? I have the BD137's base attached to the output of the ULN2803, the collector to the matrix, and the emitter to the ground.
No, it won't work as BD137 is NPN and the ULN2803 output is open collector.
Just use a mosfet on each output of the '595.
Sounds good to me. The two sets of shift registers can be connected as a single string of shift registers as you'll need to update all registers for each column drive subcycle.Okay, so the cathodes of the LEDs are connected to the mosfets, which are connected to the ground, and the anodes are connected through the resistors to the 8 shift registers that provide power.
The mosfets are connected to the two shift registers driving the columns, and both sets of shift registers go to a chipkit.
Conventional current makes more sense when reading schematics due to the direction that the arrows are drawn in the symbols. It also seems more obvious that the current is flowing from a higher potential to a lower potential.Thanks for taking the time to explain this, I'm relatively new to all this (plus learning about conventional vs. actual current when I was young messed up my thought process). All of these light bulbs are going off in my head, and all of the crazy numbers on the datasheets now make sense.
Sounds good to me. The two sets of shift registers can be connected as a single string of shift registers as you'll need to update all registers for each column drive subcycle.
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