Count down using 555 timer and 7 segment display

Thread Starter

lewl lewl

Joined Feb 1, 2018
10
I need help. Our requirements is to make our 7 segment display count up and down, we have a circuit but it only counts up, how can we modify it to count down? All answers will be highly appreciated.
 

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dl324

Joined Mar 30, 2015
13,554
Welcome to AAC!
All answers will be highly appreciated.
Is this homework? Even if it isn't, most members draw the line at doing more work then the requester.

Are there any restrictions on the components you can use to implement this circuit?
 

Thread Starter

lewl lewl

Joined Feb 1, 2018
10
Welcome to AAC!

Is this homework? Even if it isn't, most members draw the line at doing more work then the requester.

Are there any restrictions on the components you can use to implement this circuit?
There are restrictions with the components. We need to use those components
 

dl324

Joined Mar 30, 2015
13,554
There are restrictions with the components. We need to use those components
I'd say it would be difficult with an up counter with preset capability, and impossible with 7490.

Since it isn't very educational to assign problems that can't be solved for school work, there must be something you've misunderstood.
 

Thread Starter

lewl lewl

Joined Feb 1, 2018
10
I'd say it would be difficult with an up counter with preset capability, and impossible with 7490.

Since it isn't very educational to assign problems that can't be solved for school work, there must be something you've misunderstood.
Is it really impossible? He said we can modify our circuit, and use inverter or add more logic gates.
 

crutschow

Joined Mar 14, 2008
28,215
Here's a table of the up and down binary numbers and decimal equivalents.
I see no obvious way, other than a lookup table in ROM, to change the up count to an equivalent down count.

upload_2018-2-3_22-19-37.png
 

eetech00

Joined Jun 8, 2013
2,658
Think about this before you run off half cocked. You have a BCD up counter.

What do you get when you invert 0000? Is that a valid BCD digit? What do you get when you invert 0001?
Yes...if U2 were a binary counter, you could simply invert all outputs. But its a BCD counter.
Looking at the truth table, you might be able control the up/dn count displayed by controlling the Q3 output of U2.
 
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