The threshold is a minimum parts count design, using only a single opamp to compare the input signal to an onboard reference that is set via the potentiometer control. U1 is configured as a non-inverting comparator with approximately 100mV of hysteresis provided by R1 and R3. VR1 provides the comparator's reference. C1 is a bypass capacitor for U1 that sources current to the opamp during fast switching events.
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