change hysteresis in schmitt trigger circiut

[Zeinabenaz]

hi
someone can help me to analysis this circuit?
when Rpot is 1k and I start to reduce pot value the relay change condition((ON)) when Rpot= 275ohm and when I increase Rpot relay comeback unchange mode((off)) in 500ohm ,this operation it's okay to me but if I put Rpot = 470 ohm and start to reduce the Rpot the relay is on mode :-?: why???
and why does designer put Re2,Rb1,Rb2 ??? when I pick up Re2 my hysteresis change??

 

[KlausST]

Hi,

is this for learning or did you really build it?
Wha not using a simple comparator?

***
Your circuit and the threshod depends on supply voltage. This is not given.
The thresholds depend on almost every single resistor value (more or less)

Changing the total value of Rpot won´t change the thresholds. You will always get the thresholds at 275/500 Ohms.
But for sure when your OFF threshold is at 500 Ohms, and your Rpot is only 470 Ohms .. you will never reach the 500 Ohms threshold. --> it will stay ON.

Imagine you have a trunk with 2m in length... and you want to build a bridge across a stream that is 2.50m wide....

Klaus

 

[Zeinabenaz]

hi
I think I could not say my question correctly
my supply voltage is 14v
when I reduce Rpot form 1k to zero my relay change state in Rpot=275 ohm and when increase Rpot form 275 to high relay is off in Rpot=500 ohm but when I reduce 470 ohm to low relay is ON , I think the diode is ON -->Q1 off -->relay ON
so when I reduce Rpot form 1k to zero ,the relay must be ON in 470 ohm but relay on in 275 ohm,why??
I want this circuit have a hysteresis between 275 to 500 ohm ,
thanks

 

[KlausST]

Hi,

Try to decrease R4. Don`t go below 1k.

Klaus

 

[crutschow]

Below is the LTspice simulation of the modified circuit.
Changing R3 to 65Ω, R7 to 3.24kΩ and removing Re2 gave pot resistance trigger points of 500Ω and 282Ω.
(Note that the value for R1 is a non-standard resistor value.)

But, as has been noted, this is a complex circuit, with difficult to adjust threshold points, which could have been done very simply with a single comparator such as an LM339.

 

[Zeinabenaz]

Hi,

Try to decrease R4. Don`t go below 1k.

Klaus

hi dear Klaus
I can not understand why you say I must change R4 value , this resistant value depend to UTP ?

 

[Zeinabenaz]

Below is the LTspice simulation of the modified circuit.
Changing R3 to 65Ω, R7 to 3.24kΩ and removing Re2 gave pot resistance trigger points of 500Ω and 282Ω.
(Note that the value for R1 is a non-standard resistor value.)

But, as has been noted, this is a complex circuit, with difficult to adjust threshold points, which could have been done very simply with a single comparator such as an LM339.

View attachment 141014

hi
Please explain more about the reasons for the changes you gave?
thk

 

[andre_luis]

Please explain more about the reasons for the changes you gave?

He could answer "due more simplicity" if the option is with discrete components, but I would ask istead: Could you exaplain why did you used the circuit you posted ?

 

[KlausST]

Hi,

hi dear Klaus
I can not understand why you say I must change R4 value

Because R4 influences the hysteresis..

Klaus

 

[Zeinabenaz]

He could answer "due more simplicity" if the option is with discrete components, but I would ask istead: Could you exaplain why did you used the circuit you posted ?

Hi
I use this circuit for detection a sensor .my sensor is like a variable resistor
but I'm not designer of this circuit ,I must solve just this problem and it is a attractive for myself why I have this problem
I hope to solve the problem instead of change the design

thanks for your collaboration

 

[crutschow]

hi
Please explain more about the reasons for the changes you gave?
thk

I made to changes based upon my understanding of the circuit and which resistors affected both the hysteresis and trigger points (there are several interacting resistors, which makes the design difficult to analyze).
I then adjusted those in the simulation until I got the results shown.
I did not do a formal analysis of the circuit.