temperature switch control problem ?

[Mohammed Odeh]

I am having a unknown issue while working on my temperature switch project
actually its a small project :
I am using lm35 sensor connecting it to amplifier 741 with a gain of 5 ( my source suply 5V ) , and the connecting from 741 to comparator LM311 ( to V+ ) , and for (V-) of the LM 311 I design small circuit from 15 K and variable resistor 25 K .
the 15 K is connected to source 5v and then to 25 variable resistor ,and the 25 second terminal to the ground .
LM311 is to compare V+ with V- , if V+ > V- that mean ON other wise off .
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the problem is on the LM311 part :
for example I am getting 1.36 v on V+ from LM35 and 1.73 from V- ,this mean Off because V- > V+
but actually it is ON until the defference become more like V+ = 1,36 and V- = 2 v .
can any one help me with this issue
Note i try connecting another 741 op amp as a buffer after the first 741 but nothing work .

 

[u7karsh]

I guess in order to cascade ur amp and comparator stage, you need to use decoupling caps.. also try to simulate this in any of the freely available SPICE simulators and then verify on hardware..

 

[ahsan_i_h]

For 5v supply (5v and gnd) you should use single supply opamp like LM358, LM324 etc. You should not use 741. For LM311 issue, check the operation of LM311 in a bread board using a potentiometer and two resistors as voltage divider. You first need to make sure that LM311 is working properly.
Upload schematic, otherwise it is difficult to understand.

 

[kripacharya]

Also make sure the output of LM311 has a pullup to Vcc. Without this it will not work.

 

[Mohammed Odeh]

i forgot to say that i have -5 v also for the op amp
i will upload the design hope that will help

- - - Updated - - -

but i am working with dc only ?

 

[ahsan_i_h]

but i am working with dc only ?

You can work with DC only. No problem.

Also upload the LM311 test circuit. Show how you have connected load (LED or voltmeter) at the output of LM311. Connect 0.1uF(104) ceremic disc capacitor, between pin 8 (positive supply) and pin 1 (gnd), also between pin 4 (negetive supply) and pin 1 (gnd). For this project you do not need offset adjustment, so it is better to Connect pin 5 (balance) and pin 6 (balance/strobe) together.