Hi,
Of course, what sequel said works better, as I tried it, but again the problem is resolution. Since putting 100k resistor in place of R3, and shorting R4 decreases resolution. I am already converting 50V to around 4V as Vin to LM331, so even if it changes 1 Hz frequency on this low resolution, the effect will be more. Because I have to change the frequency reading back to Voltage in Microcontroller.
One more problem is , when I built the same circuit (my original circuit), on separate board (using wires on universal board), it works very good, according to the specifications. It doesn't even change 5 Hz. But the same circuit on the prototype PCB board with other circuits doesn't seem to work, and starts behaving like I explained above. Sure I will also try with NPO timing capacitor. Can you guys help me to solve this? Why would they change the behaviour? Can it be the ground problem? I am also not putting decoupling capacitor, as my power supply has Electrolytic Capacitor of 1000 uF (at Vs pin-8 of LM331). I will try putting decoupling capacitor of 100nF at power supply, as I realized now.
Thank you very much.