Problem with MUX

[ruzfactor]

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!

 

[ericgibbs]

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!

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

 

[ruzfactor]

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??

 

[ericgibbs]

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??

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.?

 

[ruzfactor]

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

 

[ericgibbs]

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

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

 

[ruzfactor]

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!

 

[ericgibbs]

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!

hi,
I dont see any schematic.

I see it now.

 

[ericgibbs]

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!

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.?

 

[audioguru]

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.

 

[ruzfactor]

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

Should I use CMOS IC?

 

[ericgibbs]

741 O/P: High- 5.53V
Low- 2.24V
Should I use CMOS IC?

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.?

 

[ruzfactor]

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?

 

[ruzfactor]

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

 

[ericgibbs]

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

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.

 

[ruzfactor]

I have also grounded the another offset null pin on both 741. Now the O/P low is about +2.04V. Is there any way to minimize this ?

 

[HATHA]

Hi all
is it possible to use 741 as in the schematic without any feedback to reduce it gain? doesn't the noise effect to the out put of the 741?

 

[audioguru]

Happy Birthday to you!
The lousy old 741 opamp is 40 years old this year. May it rest in pieces.

 

[audioguru]

The very simple circuit should have a high value resistor added for hysteresis as positive feedback so the circuit switches quickly at its threshold instead of amplifying noise or oscillating.

 

[ericgibbs]

I have also grounded the another offset null pin on both 741. Now the O/P low is about +2.04V. Is there any way to minimize this ?

hi ruzfactor,
We seem to be going around in circles, the 741 is NOT suitable for this application.
Whats the point in 'fudging' the circuit to make it 'sort of work'.

A LM393 dual comparator will take up half the space on the pcb as 2 * 741 do, also it will consume less power from the battery pack.

Read thru these attachments.