I'm having a problem with MUX in my circuit. It's a strange problem. When I use MM74C151, circuit works as it should be. But when I use SN74LS151 replacing the MM74C151, circuit doesn't work as before. What could be the problem? I don't think there's any problem in SN74LS151 because I have three of those.

Plz help!

__________________
I'm here to learn from the EXPERTS!

 

hi,
It could be a logic level threshold or fanout problem.
Please post you circuit diagram.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

May be the VCC of SN74LS151 doesn't support 5.9V. Coz I'm using a 7806. If I use 7805 another problem arises with my 6V relays which won't switch on at 4.9V. What should I do??

__________________
I'm here to learn from the EXPERTS!

 

hi,
The 74LS151 is designed for a +5V supply.
The easiest solution would be to use the 7805 for the logic supply.

Add a low cost npn transistor and resistor to drive the relay, supply the relay with voltage that is input into the 7805.

Is it possible that you could post a circuit, so that we can suggest a solution.?

What power supply voltages do you have on the project.?

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

74LS151 has maximum rating of 5.5V. But I had used 5.9V. Could this destroy my 74LS151?

__________________
I'm here to learn from the EXPERTS!

 

hi,
The absolute supply rating is +7V, so it should be OK.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

Can't solve the problem. Here's my schematic. In my schematic where it says 5V, I have used +5.9V.And when I use MM74C151 it works fine but 74LS151 doesn't do what it should! hELP ME OUT!

Attached Images

Circuit.JPG (102.4 KB, 14 views)

__________________
I'm here to learn from the EXPERTS!

 

hi,
I dont see any schematic.

I see it now.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

hi,
I would say its the 741's thats the problem, powering them with a +5V rail, I suspect the outputs of the 741's do not goto thru the switching thresholds of the LS151's.

Can you measure the 741 outputs [high and low] and tell us what they are.?

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

A lousy old 741 opamp is not supposed to work when its supply is as low as only 5.9V.
If it does work, its output low voltage is about +1.2V.
The maximum allowed input low voltage for an old TTL IC like a 74LS151 is +0.8V so of course it won't work when fed from a 741 opamp.

The maximum allowed input low voltage for the Cmos 74C151 is +1.77V so of course it works.

__________________
Uncle $crooge

 

741 O/P: High- 5.53V
Low- 2.24V

Should I use CMOS IC?

__________________
I'm here to learn from the EXPERTS!

 

hi,
That lower voltage is the problem, its not a logic low for the 74LS.

Using a CMOS ic could also give random problems.

What other OPA's do you have.?

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

Don't have any other op-amps. Which one do you recommend? This circuit is for a Line Follower which I have built. Do you think CMOS ICs would also give same problem after sometime?

__________________
I'm here to learn from the EXPERTS!

 

Do you recommend any change in the schematic? Is there any other version of 741?

__________________
I'm here to learn from the EXPERTS!

 

hi,
Do you have access to the LM393 dual compartor.?
This would replace the 741 OPA compartors, use a 4K7 on the outputs of the LM393, a 1mA sink current should give a saturation voltage of < 0.5V.

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/