Schmitt trigger using op-amp

Inverting schmitt trigger based on op-amp (comparator). 44KOhm feedback resistor controls hysteresis range. Voltage devider resisters controls threshold hi and lo voltage. The real op-amp or comparator like lm324 has Vhi = VCC - 1.5V, so real out for 3V3 VCC will be 3.3-1.5 = 1.8V. Hysteresis calculates easily. 1. First you need to set center point for reference (IN+) using resitor voltage devider. In this circuit it set with 22k and 10k resistors to ~1V. 2. Then you apply a feed back resitance that will work as parallel resistor circuit applying its value to high (VCC) or low (GND) divider side depending on Op-Amp out level, so it will shift divider voltage creating a hysteresis. Calculation: 1. First calc high side parallel resistor: Req = (22 * 44) / (22 + 44) =14.67K 2. Then low side: Req = (10*44)/(10+44) = 8.15k 3. Then calc devider voltage with Req for high out: 10/(14.67+10)*3.3 = 1.3V 4. Then for low out: 8.15/(22+8.15)*3.3 = 0.89V You need to know formula for voltage divider: Vout = R2 / (R1 + R2) * Vin, R1 - high side And formula for parallel resistor connection: R = (Rx * Ry) / (Rx + Ry). TI Design of Hysteresis Comparator https://www.ti.com/lit/ug/tidu020a/tidu020a.pdf

published 1 year ago