74LS90 headache ! please help ! Thanks !

[ichigo]

Hi i was given this assignment by my teacher to find the replacement of 74LS90 the question is given :

Do You Know The Substitute?

Joseph was given the task to build the electronics prototype of a single digit counter. The lead-time given to him was three weeks. Happily, he designed the electronics circuit of a single digit counter as shown in figure 1. However, he is now in a dilemma as he is facing some problem with this circuit.

“I had a big problem with the electronics prototype of the single digit counter which I am supposed to design and build.” said Joseph.

“What’s the problem, the circuit design looks good?” asked John.

“I could not buy all the components in time to build the circuit.” replied Joseph.

“Have you tried the different suppliers and multi-channel distributors? Some of them have website whereby you could conveniently do an on-line search for components and the lead-time for delivery” asked John.

“Yes, I have tried that. Some of the components, 74LS90 and 74LS47 in my design have either long lead time or are already obsolete. I am given only three weeks to build the prototype circuit.” answered Joseph.

“Have you tried looking for the substitute or compatible parts? You may need to re-design part of the circuit as well.” said John.

Imagine that you were Joseph, what alternatives would you try to overcome this problem? Proceed to re-design the single digit electronics counter circuit. Ensure that all the components used in your design are available with short lead-time in order to meet the requirement of the three weeks schedule.


**broken link removed**

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I searched the internet for relevent resources but it doesn't seems to link with the question . Can any kind soul help me out ? thxs !

 

[Sebi]

Check this chips: 4511,4518,4029.
https://www.electro-tech-online.com/custompdfs/2005/07/MC14511B-DPDF.pdf
https://www.electro-tech-online.com/custompdfs/2005/07/MC14518B-DPDF.pdf
https://www.electro-tech-online.com/custompdfs/2005/07/MC14029B-DPDF.pdf

 

[Dean Huster]

The original circuit is flawed and any subsequent circuit using a similar input will be flawed. You cannot use a "naked" pushbutton switch as a reliable clock input, as it is not "debounced". Each press of the switch, regardless of how hard or how softly you strike it, will produce multiple closures and multiple clock edges that will cause to counter to advance by any number of steps other than "one".

Dean

 

[Pyroandrew]

The original circuit is flawed and any subsequent circuit using a similar input will be flawed. You cannot use a "naked" pushbutton switch as a reliable clock input, as it is not "debounced". Each press of the switch, regardless of how hard or how softly you strike it, will produce multiple closures and multiple clock edges that will cause to counter to advance by any number of steps other than "one".

Dean

The question is not "is the circuit any good".
The questions is; find equivilant parts for the componants listed.

 

[Pyroandrew]

part of the question says "proceed to redesign the circuit", how far can you go on this? I'd say use a pic if you have the knowlege and facilities, they are available with no lead time and it will give you a much smaller design.

If you are looking for drop in parts, the 7447 is available with no lead time from Rapid electronics in the LS, HC and standard versions.

Alternatevely you could use the following;
4510 in place of the 74LS90
4511 in place of the 74LS47
These are both available from Rapid with no lead time.

Not too sure if these are true drop in parts for the 74 series ones in the design, but they are replacements.

 

[JimB]

For 74ls90 try 74xxx390. This is a dual decade counter.

For 74ls47 try 74xxx247.

xxx = whatever logic family is available.

JimB

 

[k7elp60]

I aggree with Dean the triggering of the counter needs to be debounced.
A 74LS132 (quad 2 input nand schmitt trigger) will take care of that.

https://www.futurlec.com/IC74LS00Series.shtml
Has in stock all the IC's and they do not have a minium order.

For a replacement set of IC's try 4000 series cmos, 4510 as a counter, 4543 decoder driver, and 4093 as the debounce circuit.

Yea the 4543 is listed as a LCD driver, but it will also drive CA and CC LED displays. Since the original circuit used CA the 4511 won't work with out external transistor.

https://www.futurlec.com/IC4000Seriesb.shtml
Has in stock all the IC's also