BCD Decoder

Thread Starter

KCHARROIS

Joined Jun 29, 2012
311
Hello,

I'm looking into building an 8x8 matrix and would like to activate each column one at a time. The pic I'd like to use only has 20 pins and I would only like to use three pins to control the columns. Now to do this I have bcd decoder (74HC/HCT42). But this decoder turns on 7 pins and turns 1 OFF and I'd like to the opposite??? Can I use the pin that is currently OFF as ground?

Thanks
 

thatoneguy

Joined Feb 19, 2009
6,359
Do you have access to 1:8 or 1:10(decade) multiplexer ICs?

If you are using a microcontroller, the MUX and shift register logic chips make things simple for port expansion.

This was posted just a couple days ago The OP did an excellent job with description, write up and code. Perhaps that may help you? Continue in this thread, rather than bumping the completed projects with this question.
 
Last edited:

MMcLaren

Joined Feb 14, 2010
859
You might consider using a 74HC238 instead. A 74HC595 might be a good candidate, as well as a 74HC164, but none of these devices can source much current.
 

Thread Starter

KCHARROIS

Joined Jun 29, 2012
311
Thats the thing I was looking at the car game microcontroller from Brad but I dont understand how he used the 7442? I think the decade counter will work better.

Thanks
 

crutschow

Joined Mar 14, 2008
34,201
...................... Now to do this I have bcd decoder (74HC/HCT42). But this decoder turns on 7 pins and turns 1 OFF and I'd like to the opposite??? Can I use the pin that is currently OFF as ground?
The pin can be considered as a ground but it can sink only a maximum of 4mA. For more than that you will have to add a buffer IC or transistor.
 

MMcLaren

Joined Feb 14, 2010
859
Thats the thing I was looking at the car game microcontroller from Brad but I dont understand how he used the 7442? I think the decade counter will work better.

Thanks
He was able to use the 7442 as a sinking driver because he was driving the cathodes on his common anode row display.
 
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