Need some help in my school work...

[audioguru]

When it doesn't have a load, the output of a lousy old 741 opamp goes as low as 1.2V to 2.0V above its negative supply voltage (your circuit's 0V).
It goes as high as 1.2V to 2.0V less than its positive supply voltage.

The curves on the datasheet show how much the output voltage swing becomes less with load current.

The schematic of the 741 opamp in the datasheet shows why:
Its output transistors are emitter-followers with a 0.7V loss and their driving parts also have a voltage loss.

The output of a modern Cmos opamp goes to 0V and to +5V in your circuit. It is called, "rail to rail".

 

[ericgibbs]

Thank a lot to u guy, u all saved my life...Today the teacher come and check on the circuit and the teacher say it was good job...But the output of the 741 will be taken off as its going to the input of the mircocontrollor..At the same time the teacher gave me some problem or question..1 of them im still dont really know why...Teacher ask that when the force sensor (Ain) is lower than the Vref the output didnt went to 0..Also when its go higher then Vref its not going to 5V...I was stun when teacher as me that question..When lower than the Vref, for the 3V detect..The voltage at the output was 1.84 and the 3.5V detect was 1.54..When higher than the Vref, for the 3V detect the voltage go to 2.01V..and the 3.5V detect go to 4.85V (for the 3.5V i think should be ok)...Do u guys know why?..Also i using 2 different IC...1 is HA17741 and the other 1 is UA741CN....

hi,
You were told of this has being a problem with the 741.

audioguru has now explained in more detail.

 

[woon_h88]

which mean the circuit has no problem but the 741...just asking...hope u guy dont angry...is there anyway to fix it?? so as my teacher say unstable voltage may cause the micro controller not to work properly...

 

[ericgibbs]

which mean the circuit is have a problem itself...just asking...hope u guy dont angry...is there anyway to fix it?? so as my teacher say unstable voltage may cause the micro controller not to work properly...

hi,
Im not angry in away way.

When operating the 741 on a single supply voltage, in your case +5V
We told you that the 741 is so old it cannot change its voltage output to zero volts,, it stops about +1v to +2V., no lower.

Also it cannot change its ouput to +5V,,the highest will be +3.5V approx.

To cure your circuit problem use a modern OPA, I use the CA3140 and CA3240 opa's, they cost about 50pence.

 

[woon_h88]

Hmm...so i just need to replace the 2 741 to CA3140 or CA3240 right?..or is it i have to redesign the circuit for the 2 new model IC? do this 2 op amp work almost the same way as 741?

 

[audioguru]

To cure your circuit problem use a modern OPA, I use the CA3140 and CA3240 opa's, they cost about 50pence.

The CA3140 is also pretty old. It has Cmos input transistors and ordinary output transistors. Its minimum supply is 4V.
Its output high voltage is 3V to 4V in your circuit, the same as the 741 opamp. Its output low is almost 0V.

Most semiconductor manufacturers make opamps that have rail-to-rail outputs but I have never used one.

 

[Sceadwian]

Even though they're capable of rail to rail operation their specs are quiet a bit lower if you do so. That's why it's usually simplest to just use a split supply and give it enough voltage headroom so you don't bump into the rails.

 

[woon_h88]

op amp sure is a hard topic...just 1 8-legged stuff can kill me...-.-''' I will tell teacher about it , see what the teacher will do...hoping the teacher wont say to rebuild a circuit...X.X Thank for ur info and spend some time with me...Thank...

 

[audioguru]

Even though they're capable of rail to rail operation their specs are quiet a bit lower if you do so. That's why it's usually simplest to just use a split supply and give it enough voltage headroom so you don't bump into the rails.

The datasheet for the old CA3140 opamp says its max output high voltage is only 3.0V when its supply is 5V.
Maybe you are thinking of the older all-Cmos CA3130? Its output is rail-to-rail. With a 5V supply its max input voltage is only 2.8V. It needs an external compensation capacitor.

 

[ericgibbs]

The CA3140 is also pretty old. It has Cmos input transistors and ordinary output transistors. Its minimum supply is 4V.
Its output high voltage is 3V to 4V in your circuit, the same as the 741 opamp. Its output low is almost 0V.

Most semiconductor manufacturers make opamps that have rail-to-rail outputs but I have never used one.

hi agu,
On his original circuit the requirement was to light an LED from one 741 when the input was greater than Vx and and another 741 to light an LED when the input was greater than Vy... the cct I posted does that.
Tried on a project pcb.

I did say when its not supposed to be lit, the LED would be not off, but dim.

There was no mention of a PIC input.? or a requirement to swing 0V thru +5V.

The use of a CA3140 even though its 'elderly' will give enough swing on the outputs to enable a high/low signal to be sensed by the PIC.

Normally when I suggest using the CA3140, I point out that the Vsupply should be at least +8V in order that the output can swing to +5V.

 

[Sceadwian]

Oh I didn't mean that specifc opamp audioguru, I mean all rail to rail opamps in general, they don't fill out their freq/gain specs out when near the rails. In order to get 'full' performance out of any opamp (even rail to rail) requires supply voltages outside the range of the signal in/out limits. Rail to rails just require less of an overhead voltage for full spec operation.

 

[woon_h88]

Sorry...still under same project so didnt open a new thread... my teacher told me that there a thing call schmitt trigger, its can help the output of the 741 to work more better...i search lot of site and found that there too much schmitt trigger, even op-amp itself also can be used as a schmitt trigger...Do anyone know how to use a schmitt trigger on the circuit??

 

[ericgibbs]

Sorry...still under same project so didnt open a new thread... my teacher told me that there a thing call schmitt trigger, its can help the output of the 741 to work more better...i search lot of site and found that there too much schmitt trigger, even op-amp itself also can be used as a schmitt trigger...Do anyone know how to use a schmitt trigger on the circuit??

hi,
Which type of schmitt triggers do you have available.?

 

[woon_h88]

currently only got using 741 as a schmitt trigger...because school term not yet start, cant really get other type of schmitt trigger...>.<

 

[ericgibbs]

currently only got using 741 as a schmitt trigger...because school term not yet start, cant really get other type of schmitt trigger...>.<

hi woon,
Its the 741 performance thats causing the problem, as we keep saying it will not work well from a 5V supply.
Also the outputs will not go less than about +1.8V and no higher than about +3V.

The circuit you are using at the moment isnt a schmitt trigger, the 741 is wired as a comparator.

Do you have two small 9Volt batteries that you could use.??

 

[woon_h88]

currently in school...there power supply to use... Then what type of schmitt trigger is suitable to use together with a comparator??

 

[ericgibbs]

currently in school...there power supply to use... Then what type of schmitt trigger is suitable to use together with a comparator??

Do you have a low power npn transistors.?

 

[woon_h88]

nope..>.< cant find any....

 

[ericgibbs]

nope..>.< cant find any....

I would like to help, but having no components etc, its impossible.

What do you have.??

 

[woon_h88]

found some transistor but dont really know what their function..>.<....have to plan out something...if teacher aprove, then will order the component...>.< what's ur idea?..mind telling?..O..also the reason on the schmitt trigger is to stable the logic so the mircocontroller wont gone "crazy" till to the jumping voltage coming out from 741 output....