Help with a DM74LS123N voltage output

[pierre_]

Dear Mike,

I can certainly make an error of testing, but I suspect that the problem is
something else since other part of the testing is ok.

What would you suggest ? I suspect the problem is coming from the input to the 123LS...

Thanks in advance.

 

[ericgibbs]

Hi Eric,

Sorry I dod not undestand what you mean here "Try the other LS123 in the ic, its a dual.."


Are you suggesting to test another LS123 component? output from the same component... I know my question is very naive...

pierre

hi,
If you look at the image in my post #15, you will see that there are two LS123 monostables in that plastic body.
Try your circuit using the second LS123 part.

What voltage do you measure on pin 13 of the LS123.???

 

[pierre_]

Hi Eric,

Based on the sheet I did find about the LS123 (see attached), ok I see what you are talking about on page 2, but then the output should but pin 5 right? not pin 13.. which would be out of the circuit right?
I'm going to make the test now.

pierre

 

[ericgibbs]

Hi Eric,

Based on the sheet I did find about the LS123 (see attached), ok I see what you are talking about on page 2, but then the output should but pin 5 right? not pin 13.. which would be out of the circuit right?
I'm going to make the test now.

pierre

hi,
Dont forget to move the timing resistor/cap etc over to the new pins, pin5 is the 'new' output...DONT connect the 1k and LED, measure it first when you have finished changing it.

You didnt say what voltage you had on pin 4 the 'old' /Q...?

 

[pierre_]

ok... will do. Back in a few minutes

 

[pierre_]

- You didnt say what voltage you had on pin 4 the 'old' /Q...?

I just did the measurment -5v.. with the same width as in pin 13....

Do you think the LED could make this ? on pin 13?

pierre

ps: for recording reason I need a +5 TTL

pierre

 

[pierre_]

I cannot believe it the red LED was the source of the pb.....

Many many thanks Eric, sorry Mike the measure was correct ...

What can I do if I want to still have a LED to light on the pulse....

Best,

pierre

 

[ericgibbs]

I cannot believe it the red LED was the source of the pb.....

Many many thanks Eric, sorry Mike the measure was correct ...

What can I do if I want to still have a LED to light on the pulse....

Best,

pierre

hi,
The reason for asking you to measure the pin4 voltage on the original circuit is because its the complementary output, I was interested to check that it was at close to +5v.

If you are using the 'new' half of the LS123 ic, then the 1K and series LED have to goto pin 5.

The RED led cannot cause the problem IF you have a 1K series resistor.