Digital circuit confusion...

Thread Starter

MegaManV

Joined Jul 29, 2017
3
Hi everyone,

I am currently going through my first digital components class and were building a circuit that incorporates all that we have learned so far but I am struggling with how to bring everything together to make it work properly. We are designing a basic frequency display.

The circuit uses the following: a 555 timer, 7414 Schmitt trigger NOT gate, 4 749x counters, 1 7493 free count, 7408, 7400, 74121 multi-vibrator, 4 74192's, 2 7446 with 2 common cathode 7 segment displays and another 2 7448 with 2 common anode 7 segment displays.

We are drawing a logic diagram to illustrate the connections, which I am fairly capable of doing, however when it comes to setting up the finer details I am having a hard time understanding where certain connections go. Especially when setting up the reduction block and connecting everything together.


I have attached the basic diagram that we are building.

Anything helps! Thanks in advance!
 

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WBahn

Joined Mar 31, 2012
26,398
Try drawing your system as a block diagram in which each block has a clearly defined and stated purpose and the inputs/outputs of each are clearly defined and stated.

For instance, when you say you have a "reduction" block -- that is way too incomplete for someone to implement. Play role-reversal. If someone came up to you and all they did was ask you to make a "reduction block", would you know exactly what they wanted? So figure out what they would need to tell you in order for you to even know precisely what they want to achieve. The focus is on the interface -- they don't need to tell you anything about what is inside the block (that's YOUR job as a design engineer); they need to tell you what the input signals need to look like and what the output signals need to look like in response.
 

JoeJester

Joined Apr 26, 2005
4,390
What was the title of the diagram or what did the professor write or say about the display when the thing was operating normally.

In other words ... What was the problem statement.
 

Thread Starter

MegaManV

Joined Jul 29, 2017
3
Thanks for the replies, I have been trying to view it from different perspectives to try and understand how to design the circuit. My problem arises when it comes to connecting multiple chips together, such as the 749x's to create the reduction block in order to get the desired frequency.

The professor didnt give a problem statement exactly. The goal is to draw a logic diagram showing how everything is supposed to work and then during class, we build the circuit to prove our logic diagram.

The only thing he said that it needs to do is the 555 timer needs to be set to 750hz and at 3 test points measure 1hz, .25hz and .50 hz.

It is a beginners class to understand the basics but im having a hard time determining connections for chips and what pins will not be used to get the circuit to function properly.

Thanks again for the responses, hope this helps clear up my struggle.
 

absf

Joined Dec 29, 2010
1,949
Are you trying to make a frequency counter to display the frequency of the output of the 555 oscillator?

In that case, I think your block diagram doesn't make sense. The divided down frequencies from the 7493 which is so low is send to 4 stages of 74192 counters (BCD decade up/down counters). They are further divided the <1hz frequencies down to 0.xxxx Hz which is unrealistic for freq. displaying.

Here is your block diagram which I have edited to make it clear and adding in comments for the various functions.

image1A.JPG

Can you explain the purpose of the 7408 and 7400 gates? How does the 8Hz-8KHz clock signal from the Sig. Gen. reacts with the frequency you wanted to display?

Allen
 
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Thread Starter

MegaManV

Joined Jul 29, 2017
3
The block diagram was taken directly off the white board where he drew it. He wants us to translate it into a logic diagram and then come in and wire it all up and make it work.

The monostable purpose: send a DC pulse once every four seconds to clear the 74192 details, its what he had written above the 74121 on the bottom right of the page, it might be hard to read but that is what it says and how he told us it was to be used.

The F1, F2, F3 reading i believe he plans on measuring with the o'scope and not with the 7 segment displays. The displays i think he says would be reading the signal generator frequency but to be honest, im not 100% sure on that.

the purpose of the 7400 and 7408 isnt qutie clear to be yet, i know its to invert the signal but i am not certain. Still learning a lot of how digital works. Hope this helps clear more up, probably not, but its the best i can do.
 

JoeJester

Joined Apr 26, 2005
4,390
Your making a frequency counter.

To clear up those two chips, look at the notes absf provided. You should see the purpose of them upon further review.
 

absf

Joined Dec 29, 2010
1,949
Here's a simulation of the 2 gates with the sig gen set to 8Hz.

CONFUSION.PNG
That's what I thought how this circuit works

Allen
 
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