i m sorry its my fault..
neways thanks a lot for ur help. . i appreciate

 

My pleasure!

Brian

__________________

---------------------------------
Electronics Test Development Engineer
---------------------------------

 

hi brian
i hav made the circuit and its working very nicely now whenever i push any switch the corresponding output of the dmux goes low. . .
now there is one very simple problem left
the output remain low only till i keep pressing the button . .i want to use a latch,which can hold the output till it is manually resetted. . .
can u suggest anyway or an IC for this thing . .

thanks in advance


note:using push button is mandatory

 

Hah - I told you that would be a problem didn't I!

I'll have to have a think about that one. I've got Welsh classes tonight, so it's going to be tomorrow before I can have a look at it. In the meantime, why don't you try coming up with some suggestions of your own? I'm happy to give advice on anything you can come up with, as are the others on the board I'm sure.

Brian

__________________

---------------------------------
Electronics Test Development Engineer
---------------------------------

 

how about connecting a (s-r latch) IC74ls279 at the output.
datasheet:
http://rabbit.eng.miami.edu/info/datasheets/74LS279.pdf

we get low signal at the corresponding output pin of the input applied. . so when the pin gets low the output of this IC (SR latch) will be high and remains high till we manually reset the IC. . .

i think this will work

 

mayankguru,

Your suggestion of using an SR latch is a sound one, however I am a little confused as to where you suggest to put it. My suggestion is to use the latch in between the output of your multiplexer and the input to the NAND gate which stops the counter from counting. Then, when a key is pressed the output from the multiplexer goes low, causing the SR to latch low. This causes the counter to permanently stop counting, and all you need to do then is decode the data on the multiplexer's inputs which correspond to which key was pressed. I've changed the setup of your demultiplexer and connected it as a decoder instead, so that it will suit this purpose.

When I simulated it in multisim I found that sometimes the decoder would give an erronous output - in other words the output of the decoder did not correspond to the key that was pressed. I think that's just a bug in the simulation, it probably wouldn't happen in real life. The error only occured every now and then - all other times the output of the decoder corresponded perfectly to which key was pressed, and I checked this for all of the keys.

What do you think?

Brian

Attached Images

Electro-Tech2.gif (25.1 KB, 6 views)

__________________

---------------------------------
Electronics Test Development Engineer
---------------------------------

 

By the way, you need to be mindful of your clock frequency here. It needs to be fast enough that the buttons are scanned incredibly quickly, and pressing any of the buttons results in a perceived immediate reaction. But the clock must not be so fast that during the propogation delay of the SR latch and the NAND gate it moves on to the next count!!!

Brian

__________________

---------------------------------
Electronics Test Development Engineer
---------------------------------

 

i think there is some misunderstanding . .
the circuit is not to show
"who pressed the key first"
but to know
"which keys are pressed"

i.e if i press key 1 the latch at the output of the DMUX will hold the corresponding output low untill we manually reset the latch.and we dont need to stop the clock. . the clock will b stop just for a spur of moment that is when the push button is pressed once it is released the clock will start counting again. .and i m using latch at the DMUX output so that the led will keep glowing even after releasing the push button..

actually this circuit after testing will be divided in two parts

part one will consist of:
KEYS
MUX
NAND gates
COUNTER
555 timer


part two will have
DMUX
LATCH
LED's

i might sound sillly but really hav a question in my mind
can we use different 5v supply for each part?

 

Hmmm. I think you've chosen the wrong circuit altogether. If you look at the original circuit you fetched off the internet, it's called a "first event detector". In other words, it's designed to tell you which event occured first. I critisised that circuit because there was no latching system employed, and that meant it only told you which event occured first for as long as the event occured - not a good circuit.

I've added a latch into the circuit now so it does what it says on the tin, but now you're saying all you need is a circuit which tells you which buttons have been pressed. There's no need to know which one was pressed first. That's a different circuit altogether!

Complete redesign required.

Brian

__________________

---------------------------------
Electronics Test Development Engineer
---------------------------------

 

All you need here mayank guru, is your row of buttons each connected to a latch of their own. Tie all the reset lines together. When a button is pressed it will be latched on, and the LED will for that button will glow continuously. As other buttons are pressed, the same will happen to them. The LED panel will continue to represent which buttons have been pressed until you press your reset button, which will clear all of the SR latches simultaneously.

Job done.

Brian

__________________

---------------------------------
Electronics Test Development Engineer
---------------------------------

 

Hang on, I just remembered you saying that it was a requirement to reduce the number of cables between the Mux and Demux so that the buttons and the display panels could be in remote locations without having to run lots of cables. In that case, the mux/demux circuit still serves a purpose. Leave the event detector as it was (don't use my latch), but you'll need to latch each of the outputs of the demux and tie all the reset lines together. Easy modification, and it'll do what you want.

Brian

__________________

---------------------------------
Electronics Test Development Engineer
---------------------------------

 

right it will work like this,
but what about using different power supplies(+5volts) at each part (part1 mux and part 2 DMUX)
can i use different power suppllies or not??

 

I see what you mean - the mux and demux will be in seperate locations and therefore will need seperate supplies. Yes you should be able to use seperate supplies without a problem. In a worst case, you would need to tie the grounds together so that the TTL logic levels are references from the same potential, but I don't think that will be an issue...

Brian

__________________

---------------------------------
Electronics Test Development Engineer
---------------------------------

 

ok i will try that and will let u updated with the circuit. .

 

Yep let us know how it goes!

Brian

__________________

---------------------------------
Electronics Test Development Engineer
---------------------------------