Help with a DM74LS123N voltage output

[pierre_]

PLEASE I NEED HELP ....
Hello and thanks for reading the message...
I'm trying to get a +5volt pulse coming out from a DM74LS123N.
I could modify the witdth but I could not get a voltage higer than ~1.5v even
if the VCC is 5v.

Is there a way to modidy the output voltage and if so How?

Do you have any idea of what I'm doing wrong....

Thanks a bunch

pp

 

[ericgibbs]

PLEASE I NEED HELP ....
Hello and thanks for reading the message...
I'm trying to get a +5volt pulse coming out from a DM74LS123N.
I could modify the witdth but I could not get a voltage higer than ~1.5v even
if the VCC is 5v.

Is there a way to modidy the output voltage and if so How?

Do you have any idea of what I'm doing wrong....

Thanks a bunch

pp

hi,

How are you measuring the short from the mono output, also do you have anything connected to the output.?

 

[pierre_]

Hi,

ok I need to take a step back here.
What I'd like to do is a photosensor.. that would generate a 5v pulse volt
each time a light is presented.
So I do have a +5/-5v /0 voltage circuit. The output voltage is measured between the pin of the 123N and the ground. (same for the vcc in).

pierre

Could I send a pdf of the scheam in the forum?

 

[ericgibbs]

Hi,

ok I need to take a step back here.
What I'd like to do is a photosensor.. that would generate a 5v pulse volt
each time a light is presented.
So I do have a +5/-5v /0 voltage circuit. The output voltage is measured between the pin of the 123N and the ground. (same for the vcc in).

pierre

Could I send a pdf of the scheam in the forum?

hi pierre,
You can post a pdf using the 'manage attachments' button lower down the reply window.

I did ask some questions, an answer would be helpful.

 

[pierre_]

Schema of the photosensor

Sorry I could not make it better...

 

[pierre_]

I did ask some questions, an answer would be helpful.


Well then I don't get it Sorry

 

[pierre_]

Schema legend

The left component is the 123N and the right one is an UA 747 NM.

The circuit is working fine , sending pulses and I can easily modify the width of the pulse as well as the threshold to send this pulse... my problem is about the voltage of the pulse .. hope this help.

pierre

 

[ericgibbs]

Sorry I could not make it better...

hi,
Looked at the pdf's.

Whats the value of the cap on the LS123.?

What type of ic is the 14pin?

You should have decoupling caps on the +5V rail, sau 10uF and 100nF.

EDIT; just seen your 2 extra posts!!, so its a 741 opa.

What voltage levels do you get out from the 741, when the switch is pressed and not pressed.?

EDIT2: where are you measuring the low pulse output and how.????

 

[pierre_]

Thanks for your patience...

- Whats the value of the cap on the LS123.?

B8116

- What type of ic is the 14pin?

Sorry I do not understand what you mean... (again my electronic level is close to 0)

You should have decoupling caps on the +5V rail, sau 10uF and 100nF.

Sorry???

EDIT; just seen your 2 extra posts!!, so its a 741 opa.

What voltage levels do you get out from the 741, when the switch is pressed and not pressed.?


- EDIT2: where are you measuring the low pulse output and how.????

by measuring the pulse between the 0 and the pin 13

 

[ericgibbs]

Thanks for your patience...

- Whats the value of the cap on the LS123.?

B8116

- What type of ic is the 14pin?

Sorry I do not understand what you mean... (again my electronic level is close to 0)

You should have decoupling caps on the +5V rail, sau 10uF and 100nF.

Sorry???

EDIT; just seen your 2 extra posts!!, so its a 741 opa.

What voltage levels do you get out from the 741, when the switch is pressed and not pressed.?


- EDIT2: where are you measuring the low pulse output and how.????

by measuring the pulse between the 0 and the pin 13

hi pierre.
You are not answering my questions, with the answers I cannot help.


HOW are you measuring the output pulse.????? What is the measuring equipment.?

What voltage levels do you get out from the 741 output, when the switch is pressed and not pressed.?

 

[pierre_]

How...
with a osciloscope.. with a connection between pin 13 and the 0.

What voltage levels do you get out from the 741 output, when the switch is pressed and not pressed.?

If you are refering to the DM74LS123N the voltage pulse is about 1.5v

pierre

 

[ericgibbs]

hi,
Reason for asking how you are measuring is because some students try to measure pulses with a voltmeter. Its not possible to do this as the meter is too slow and the voltage reading is much lower than expected.

The number you gave me for the capacitor does not tell me the capacitance value.???

I asked what the voltages are at the output of the 741, because your diagram shows a +5V and -5V supply, I am concerned that you are driving the input of the LS123 with a negative going pulse, I see the clamp diodes, they should hold it at -0.7v.

 

[pierre_]

- I asked what the voltages are at the output of the 741, because your diagram shows a +5V and -5V supply, I am concerned that you are driving the input of the LS123 with a negative going pulse, I see the clamp diodes, they should hold it at -0.7v.

.. and you are right the clamp diodes hold at -0.7 v...
The reason why I'm very very confused is why is the triggerring properly done (such as adjustement of width) but the voltage so low...
Could that be related to the with a negative going pulse???
and if so what could I do to change it?

ps: I going back for testing tomorrow morning...

I really really do appreciate your help.

pierre

 

[ericgibbs]

- I asked what the voltages are at the output of the 741, because your diagram shows a +5V and -5V supply, I am concerned that you are driving the input of the LS123 with a negative going pulse, I see the clamp diodes, they should hold it at -0.7v.

.. and you are right the clamp diodes hold at -0.7 v...
The reason why I'm very very confused is why is the triggerring properly done (such as adjustement of width) but the voltage so low...
Could that be related to the with a negative going pulse???
and if so what could I do to change it?

ps: I going back for testing tomorrow morning...

I really really do appreciate your help.

pierre

hi,
I would expect about +3.5V at the LS123 output when its high.

Are you sure that it is a 1k resistor in series with the LED.?

Whats the colour of the LED.? look at this image.

 

[ericgibbs]

hi pierre,
I have just made your circuit on a breadboard and it works OK, approx +4.5V

Looking at the way you have drawn the diagram are you sure that you have the correct pin layout for the LS123.????

The image shows the TOP view.

 

[pierre_]

Good morning

I did remove the 1k resistor and to my big surprise the voltage did not change much...

The LED is red.

and the output voltage is definitively not 3.5v...

Best,

pierre

 

[pierre_]

Looking at the way you have drawn the diagram are you sure that you have the correct pin layout for the LS123.????


As far as Cext/Rext for the control of the width the pin mapping seems to be correct (since I can manipulate it).
I was wondering about the diodes part.. could that potentially affect it?

pierre

 

[ericgibbs]

Looking at the way you have drawn the diagram are you sure that you have the correct pin layout for the LS123.????


As far as Cext/Rext for the control of the width the pin mapping seems to be correct (since I can manipulate it).
I was wondering about the diodes part.. could that potentially affect it?

pierre

hi.
Ref your PM, if you only measure a 1.5V pulse on the output of the LS123 without the 1K connected I would suspect that the LS123 output has failed.!

Try the other LS123 in the ic, its a dual..

 

[pierre_]

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

 

[Mikebits]

hi.
Ref your PM, if you only measure a 1.5V pulse on the output of the LS123 without the 1K connected I would suspect that the LS123 output has failed.!

Try the other LS123 in the ic, its a dual..

Eric, I suspect his testing is in error. I doubt the chip has failed.