Design a schmitt trigger using a 311 comparator with threshold at +1.0v and +1.5

Question

Design a schmitt trigger using a 311 comparator with threshold at +1.0v and +1.5v. Use a 1.0 K pullup resistor to +5 volts and assume the 311 is powered by +-15 volt supplies.

Okay, so when it says powered by +- 15 volts does that mean point A and B are +-15 volts or does that mean Vcc is 15 volts and ground should be - 15V? Also, does the value of the 1.0k resistor really matter since the entire V drop will occur across it anyway?

So I assumed A and B were +- 15 volts and Vcc = 5 volts, I chose R2 = 30k and R1 = 10k so that it's biased at 1.25 volts. then i chose R3 = 40k so that when V->V+ (V- is where the inputs applied.) that the threshold is increased by .25 to 1.5 volts. But then that means it will decrease it by .75 right? because the 30k and 40k will be in parallel when V+>V-? Am I even doing this right? Please help! Thanks!

Explanation / Answer

Yes, +15 to -15 Volts is what they are looking for. You are correct about the value of the 1K Pullup. The only difference it makes is to what you are driving - You can calculate the max current through the resistor by taking into account VCC and the voltage drop across the open collector transistor in the LM311. Regarding the hysteresis: The best way to design something like this is by looking at LM311 datasheets. I bet if you go to digikey.com, pull up all the LM311's, and look at each manufacturer's datasheet, you are bound to find one with this exact application on it. If not, this guide is useful: http://www.physics.utah.edu/~bergman/3610-6610/notes/Lecture05.pdf Good Luck.

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