Sunday 10 March 2019

shift register - How does LED matrix circuit work in the following example?


The circuit below shows how to build an LED matrix with shift register multiplexing.


From what I understand, R1-R8 is the column vector setting the LEDs high or low and the shift register will pick one row at a time.


What is the output of the shift register Q0-Q7?


In order to light up bottom left LED, I Would need to set R1 3.3V and Q0 grounded (or 0V I guess?) whilst Q1 - Q7 must be floating. However, in this circuit, I don't see how to float the Q1-Q7.


Or are Q1-Q7 actually 3.3V so that all other LED in the first row have zero potential with the Q1-Q7?


enter image description here In the example below, it's more understandable as the shift register is switching the transistors, therefore, multiplexing the rows to the ground.


A - How does the first circuit work?


B - What are advantages and disadvantages of the first and the second circuit?


enter image description here





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