Comparators and Schmitt triggers: the open-loop comparator, the difference between a comparator and an amplifier, positive feedback, hysteresis, and switching thresholds.

Outputs (up to 3 marks): with the sensor on the non-inverting input and the reference on the inverting input, the output goes high (near $9.0\ \text{V}$) when the sensor exceeds the reference and low (near $0\ \text{V}$) when it is below. At $1.0\ \text{V}$ (below $2.5\ \text{V}$) the output is low ($\approx 0\ \text{V}$); at $4.0\ \text{V}$ (above $2.5\ \text{V}$) the output is high ($\approx 9.0\ \text{V}$).

Sharp switching (up to 2 marks): the op-amp's open-loop gain is enormous, so even a tiny difference between the inputs drives the output fully to one rail or the other; there is no feedback to set an intermediate gain, so the output saturates and the transition is very steep.

Markers reward the low output below the reference, the high output above it, and the explanation that the huge open-loop gain saturates the output.

Positive feedback and the Schmitt trigger (up to 4 marks): a fraction of the output is fed back to the non-inverting input. This makes the switching threshold depend on the current output state, so there are two thresholds: a higher one to switch one way and a lower one to switch back. When the output is low the upper threshold applies; when it is high the lower threshold applies. The gap between them is the hysteresis.

Why hysteresis helps (up to 2 marks): a plain comparator with a slowly changing or noisy input near its single threshold chatters, switching rapidly back and forth as noise crosses the threshold. The Schmitt trigger's two separated thresholds mean that once it switches, the input must move a definite amount back before it switches again, so noise no longer causes multiple unwanted transitions; the output is one clean edge.

Markers reward feedback to the non-inverting input, two thresholds with a hysteresis gap, and the clean, chatter-free switching of a noisy or slow input.