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How can I temporarily disable either this decoder or counter? Answered

http://pdf1.alldatasheet.com/datasheet-pdf/view/12613/ONSEMI/74LS168.html     -counter
http://pdf1.alldatasheet.com/datasheet-pdf/view/12657/ONSEMI/74LS42.html        -decoder

I'm using these two in a circuit to make a dot move left and right across a row on an led matrix. I only want the led to light up on the last row. Another counter-decoder system activates each row. How can I make the dot light up only on the last row by deactivating the counter or decoder for the left-right system. By "deactivating" I mean that I want the decoder to produce all active high signals on the other rows.


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Best Answer 9 years ago

You'd have to put a value of greater than 9 on the inputs of the '42 for all the outputs to be off.  A value of 9 will light only the last row (or 7 if your matrix is 8 wide).  Beware of that '42 truth table - A0 is on the left, not the more usual right.
(I used to do a lot of TTL logic design - Nowadays I use microcontrollers #;¬)