Problem with MUX - Page 2 - Electronic Circuits Projects Diagrams Free

ruzfactor · 7th May 2008, 07:03 PM

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 · 7th May 2008, 07:53 PM

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 · 7th May 2008, 08:39 PM

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

audioguru · 7th May 2008, 08:47 PM

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 · 8th May 2008, 08:22 AM

Quote:

Originally Posted by 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 ?

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.

ruzfactor · 8th May 2008, 01:06 PM

I will try to get one LM393. In the mean time I have think of something. Why cant I use a CMOS Inverter solve the problem?
One more thing, I have a LM324.

ruzfactor · 8th May 2008, 01:24 PM

I have added a 74HC04 in the circuit and the MUX now works fine. But Will it be a problem later?

audioguru · 8th May 2008, 01:39 PM

Quote:

Originally Posted by ruzfactor

I have added a 74HC04 in the circuit and the MUX now works fine. But Will it be a problem later?

The input threshold voltages of a 74HC04 are very close to the output high and output low voltage at the output of the lousy old 741 opamps and the circuit might fail by itself or fail if the 74HC04 or an opamp are replaced.

ericgibbs · 8th May 2008, 01:43 PM

Quote:

Originally Posted by ruzfactor

I have added a 74HC04 in the circuit and the MUX now works fine. But Will it be a problem later?

hi.
Have a look at these two links and see why I am concerned about the logic levels.
Remember you are going to be controlling electrically 'noisy' motors close to the logic, so what do you think is best...

http://www.interfacebus.com/voltage_threshold.html

http://www.interfacebus.com/voltage_LV_threshold.html

ericgibbs · 8th May 2008, 01:51 PM

Quote:

Originally Posted by ruzfactor

I will try to get one LM393. In the mean time I have think of something. Why cant I use a CMOS Inverter solve the problem?
One more thing, I have a LM324.

hi,
The LM324 quad OPA has a Vout low of about 200mV, so it would be a better choice than the obsolete 741..

audioguru · 8th May 2008, 02:24 PM

Quote:

Originally Posted by ericgibbs

hi,
The LM324 quad OPA has a Vout low of about 200mV, so it would be a better choice than the obsolete 741..

Its output voltage is 0.2V only when its output current is very low like the input of a Cmos IC. If it tries to drive the high current of a TTL gate then its output low voltage is very close to the TTL threshold voltage of 0.8V.

ericgibbs · 8th May 2008, 03:59 PM

hi ruzfactor,

As a quick check I have built your circuit using the LM324 and 74LS151.

Used a number of different LM324 and 74LS151 ic's.

Highest values at the output of the LM324, +0.65V low and 3.7V high.

If you add a 2K2 pulldown resistor on the outputs of the LM324 you will get
a low of +0.5V and a high of 3.6V, which is good enough to drive the LS151

If you cannot get the LM393, try the LM324 it will be much better than the 741.

I would strongly recommend that you try to source a LM393 as it will give a more reliable operation of the circuit.

ruzfactor · 10th May 2008, 10:32 AM

Hi ericgibbs
I have got a LM393. Let's see if it works. And there's a small mistake in the circuit schematic. The inverting input should be the reference voltage.

ruzfactor · 10th May 2008, 04:14 PM

It worked! LM393 gives almost zero output and it gives +2.45V on high at O/P.

ericgibbs · 10th May 2008, 04:26 PM

Quote:

Originally Posted by ruzfactor

It worked! LM393 gives almost zero output and it gives +2.45V on high at O/P.

hi,
Thanks for letting us know.

Keep us posted on the project..

EDIT:
hi ruzfactor.
Be sure to fit a 3K3 resistor from the outputs of the LM393 upto +5V..

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7th May 2008, 07:03 PM	(permalink)
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 ? __________________ I'm here to learn from the EXPERTS!

7th May 2008, 07:53 PM	(permalink)
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?

7th May 2008, 08:39 PM	(permalink)
audioguru	Happy Birthday to you! The lousy old 741 opamp is 40 years old this year. May it rest in pieces. __________________ Uncle $crooge

7th May 2008, 08:47 PM	(permalink)
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. __________________ Uncle $crooge

8th May 2008, 01:06 PM	(permalink)
ruzfactor	I will try to get one LM393. In the mean time I have think of something. Why cant I use a CMOS Inverter solve the problem? One more thing, I have a LM324. __________________ I'm here to learn from the EXPERTS!

8th May 2008, 01:24 PM	(permalink)
ruzfactor	I have added a 74HC04 in the circuit and the MUX now works fine. But Will it be a problem later? __________________ I'm here to learn from the EXPERTS!

8th May 2008, 03:59 PM	(permalink)
ericgibbs	hi ruzfactor, As a quick check I have built your circuit using the LM324 and 74LS151. Used a number of different LM324 and 74LS151 ic's. Highest values at the output of the LM324, +0.65V low and 3.7V high. If you add a 2K2 pulldown resistor on the outputs of the LM324 you will get a low of +0.5V and a high of 3.6V, which is good enough to drive the LS151 If you cannot get the LM393, try the LM324 it will be much better than the 741. I would strongly recommend that you try to source a LM393 as it will give a more reliable operation of the circuit. __________________ Eric "Good enough is Perfect" PIC tutorials: Gramo's: www.digital-diy.net/ Bill's: www.blueroomelectronics.com/

10th May 2008, 10:32 AM	(permalink)
ruzfactor	Hi ericgibbs I have got a LM393. Let's see if it works. And there's a small mistake in the circuit schematic. The inverting input should be the reference voltage. __________________ I'm here to learn from the EXPERTS!

10th May 2008, 04:14 PM	(permalink)
ruzfactor	It worked! LM393 gives almost zero output and it gives +2.45V on high at O/P. __________________ I'm here to learn from the EXPERTS!