I'd like to ask you a question about my circuit to automatically turn off battery charger. It works now fine except when voltage is close to the switch of treshold, coil of relay sounds noisy so I know that I need some histeresis but have now idea how to implement it there.
Thank you
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Hystersis is positive feedback. You may know you can get negative feedback by connecting the amplifier output with a resistor to the inverting input. So you can get positive feedback by connecting a resistor to the positive input. Sometimes a small capacitor across the feedback resistor, perhaps 100 pF or so, is useful to speed up the transition.
If the signal coming into the +ve input was from a voltage source, then this wouldn't work. However, the input is from an effective resistance of a little below 1 kΩ to a couple of kΩ depending on the position of the potentiometer. That variability will make the size of the hysteresis window dependent on the pot position.
Andy's suggestion of 100 kΩ is a good place to start for the feedback resistor, and will give you a hysteresis of one percent or two of the opamp output swing.
