The datasheet shows a worst case swing of +-12V when the TL084 operates on a +-15V supply. So, worst case, it can't get within 3 volts of the rails. I'm surprised it even operates on a 5V supply.
See VOM on page 6
http://www.ti.com/lit/ds/symlink/tl081.pdf
Well yes, i operated the TL084 at the minimum supply voltage, should try it with higher voltage, but i think it will not change as much, probably i will change TL084 to something else, do you have any recommendation?, an rail to rail op amp that is pretty old, because electronic store here are suck, i can't order on digikey or mouser.
Op amp 3B has positive feedback (the feedback resistor is connected to the + input) so all it will ever do is switch between minimum and maximum output voltages. The note mentions hysteresis.
Op amp 2B doesn't appear to have any series resistors between the sensors and the pins. The gain could be quite high if the sensor impedance is ow.
Op amp 3A has a gain of around 100.
Op-amp 3B is supposed to be voltage comparator, that positive feedback resistor is experimental, means that if it oscillates horribly i will add it to create an hysteresis.
There are series input resistor on op-amp 2B, it's on sensor side, it was 100k x 4 summing resistor, you can see it on the article i've attached on main post, sorry i did'nt include the sensor's schematic.
Is'nt the gain of Op-Amp 3A is 1.01?, 1+(Rf/Rs), 1+(4.7k/470k), 1.01
That triangle generator doesn't look right. First you need to generate a square wave and that takes at least one op amp, three resistors and a capacitor. Then you need to create the triangle wave and that is nothing more than an integrator, a couple of resistors and ANOTHER capacitor.
http://www.ques10.com/p/22759/explain-triangular-wave-generator-using-opamp/
Well it should work, i have seen other tutorial article uses this triangle generator too
I am not sure what you mean by "jelly bean". You probably can't find a $0.03 op amp for this task. The one specified is roughly four dollars. You can do a quick search for what is available in your area and find op-amps for under a dollar that look like they would work. One example:
https://www.mouser.com/ProductDetail/Renesas-Intersil/EL5111TIWTZ-T7A?qs=sGAEpiMZZMtOXy69nW9rMwWeYYZE9XJK%2F%252BbJZK77xYE%3D
I can see how the triangle generator might work. The second op-amp acts as a comparator to flip the charge on the capacitor. The use of a parallel 470k and 20k resistor to get the specified 19k resistance (see the second schematic in your link) suggests that some tuning may be required.
Other things in the link article suggest that while this circuit works it requires some tuning. You can probably get the needed oscilloscope functionality out of one of the many free oscilloscope programs that use the computer sound card for input. They are crude, but will let you look at a lot more of what is happening.
I mean an easy to find op-amp that's pretty old but are a good op-amp, it was pretty hard to find a component that is pretty new or popular, even i can't find OPA4727 there.
I have try sound card oscilloscope on my laptop, but my laptop just can't detect it, it still uses internal microphone even when i connect the jack

What tuning?, i don't see any triangle wave generator tuning on my link

, it is just only the tuning of the line follower.
3. The Triangle Wave generator does'nt work, it just give plain 3.7V DC, i have pretty much checked any connection of the ic but it still does'nt work, the way i check the triangle wave generator is connecting them to speaker input (as i dont have an oscilloscope), and the speaker only sounds "dug" once when i connect or disconnect the triangle wave output to speaker's input. Pin 2 and 5 voltage shows 2.5V (5V Supply), pin 1 0.6V, pin 3 2.1V, pin 6 0.6V, pin 7 3.7V
The triangle wave generator will work with ideal components with the values given. However, not with the opamp you have chosen.
Change R2 to 10K and see what happens.
Here is the circuit drawn a different way...I prefer the signal flow...integrator to a comparator then fed back. Just my preference.
Sadly, but i will try to change R2 to 10k,
did your circuit will work with TL084?, but meh probably i will change my op-amp though.
You should build this circuit in LTSpice. Test each individual block alone. Verify everything. Then build the actual circuit.
LTSpice it too easy to use not to do this...especially for a purely analog circuit like this.
I have try to draw some part of it in LTSpice but i can't find TL084 there, and my LTSpice drawing skill is just bad, i have tried to draw the triangle wave generator and it takes me over 30 minute and it does'nt work

Indeed the way I read the data sheet is that the TL084 is not designed for single 5v supplies. It mentions dual 5v, but that would be twice what you have. The output swing drops off quite rapidly with loading <10k as well. The CA3140 (single supply down to 4v) might be suitable, but check in LTSpice.
Probably i will try to make the supply voltage higher, and see what happens, but i expect anything not much will happen though, In my schematic i was not loading the op-amp so much,
Well, i can find some CA3140 on some store there, but is it really worth it?, i have not checked the datasheet yet, and i don't think CA3140 are on LTSpice.
Edit : CA3140 is only single op-amp too.