Frankly, I'm overwhelmed with this circuit that I have to start building in lab tomorrow. Last week we built an octal decoder that takes a 3-bit binary input from 000 to 111 and displays a unique character of our choosing on an LED display (common anode). It was all combinational logic and I understand it fairly well. Now we're expanding on the circuit and I don't understand a lot of the circuitry or how it works. I will briefly describe the assignment and then try to explain what I do and don't understand. Here is the schematic:
Note that n=3 here.
The assignment:
We have to design and build a circuit that will display 9 bits of data on 3 separate 7 segment (common anode) displays. A DIP switch controls the three inputs of the decoder built last week. The three separate digit devices have corresponding segments wired together (e.g. a to a, b to b, etc.). The output only appears on one of the displays at a time, but if they are scanned fast enough they should all appear lit. For the multiplexing I have to use 7403 open collector logic. For each digit, we must design a transistor to function as a "high end digit driver" in order to source/sink appropriate current to light the digit adequately. We must design a counter circuit that produces the digit drive signals sequentially from right to left.
My Understanding:
Multiplexing
My prof described multiplexers essentially as electrical switches; they choose which signals to transmit. I think my biggest misunderstanding is rooted in what purpose the multiplexer serves in this circuit. Is the multiplexer represented in the schematic where it says "buffers"? Is that common? Originally, when we were using one display with the decoder, we didn't need a multiplexer. If the ultimate goal is to display the output on two additional displays, why couldn't we just wire the all the a's, b's, c's, etc. together? He also mentioned demultiplexing, but it's not clear if that's necessary here.
High End Digit Driver
The purpose of the transistor drivers are to supply/sink current to/from the LEDs, but it's unclear to what it's connected to electrically in terms of the emitter, collector, and base. If the drivers are simply sourcing or sinking current, why are they connected to the multiplexer?
Counter Circuit
I've built a counter circuit once before using a 555 timer. This is the schematic:
I believe I could use this circuit and tinker with the R values until I get a desirable output. However, what is considered a desirable output here? The procedure just says the counter circuit has to produce the digit drive signals - this seems vague.
No comments:
Post a Comment