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About the circuit
The circuit consists of just to chips, two push buttons, 11 resistors and one 8×8 bi-colour display. All these components can be bought for under $10. How it works: The 8×8 bi-colour display is multiplexed i.e all the anodes are connected together in rows- 8 rows of led anodes, then all the red cathodes are connected together in columns - 8 columns of red led cathodes, and finally, all the green cathodes are connected together in columns - 8 columns of green led cathodes. So all up we have 8+8+8 = 24 pins for our 8×8 led display. Since it is multiplexed, we cannot just turn on all columns at one time in order to draw a picture - we need to activate each column of led cathodes one column at a time, and then grab 8 bits of data from the microcontroller to be displayed in that column. We hold that data on for a split second, then we activate the next column in sequence - grab 8 bits of data and then hold that info on the screen for a split second.
We do this untill all 8 columns have been activated and displayed their data, if we do this fast enough the screen will appear to give us a steady image! (sort of like how your television works…) This is where the 7442 comes in handy, we input a 3-bit number (from 000 to 111) to activate just one column of led cathodes at any one time, so the first column will be activated at 000, then the second at 001 - then all the way upto the last column at 111. NOTE: I could have used a 1 0f 8 decoder instead ( i think they are 74138) but i just didnt have any, thats why i used the 7442) For full details of how the circuit works - just have a read of the assembly code = )
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