Falling Threshold Switches Low - Electronic Circuits V & VI: Front-End Conditioning and Op-Amp Feedback Step by Step

A Schmitt input with HIGH history holds that state through the middle band and switches LOW only after the input falls below the lower threshold. The rows use an ideal memory-and-threshold model; real switching points move with input offset, resistor tolerance, supply variation, noise, propagation delay, rail limits, and measurement uncertainty.

The same Schmitt input uses the lower threshold to leave a prior HIGH state. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Inside the band, the prior state holds

The middle input lies between the two thresholds, so prior HIGH stays HIGH.

2\ \text{V} < \tfrac{5}{2}\ \text{V} < 3\ \text{V}\Rightarrow \text{hold prior}

Dropping below the lower threshold switches LOW

The input is 1 volt, below the lower threshold of 2 volts.

1\ \text{V} \le 2\ \text{V}\Rightarrow \text{LOW}