In SX Microcontrollers, SX/B Compiler and SX-Key Tool, johncouture wrote:

I am obviously missing a fundamental about transistors.
 

I have a four digit sign (i.e. a clock).  Each digit is seven segments. Each segment is 4 or 5 LED's.  Thus the max voltage drop is 8.4v.  My supply voltage is 12V.

 

Segments are controlled using RB to switch the cathode side of the segment.

 

Digits are controlled using RC to switch the anode side.

 

I can switch the segments ok (RB) by using 2N3904 (npn) transistors connected to the RB pins on the SX28.

 

I can't seem to figure out how to switch the anode side because I am switching 12V and need to protect the SX28 from that 12V.

 

Attached is a schematic of attempt # 3,281 (seemingly) of trying to get this to work.

 

My gut tells me I have to figure out how to "buffer".  I have at my disposal an unlimited number of PNP or NPN transistors along with a good supply of ULN2803 IC's (yea, I've tried them all but can't seem to get the right combination).  I've tried the Darlington Pair but, again, I think I'm missing something.

 

I was trying to avoid having to use 7407's (high current buffers) because I would have to order them and would like to use what is on hand.

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