Light out, light back

Light becomes current, current becomes volts

A phototransistor is a transistor whose base is driven by light instead of a wire. Infrared landing on it lets current flow from its collector to its emitter; the more light, the more current. In the dark it carries almost none: Vishay gives 10 nA typical.

On this board the collector is SIGNAL and the emitter is GND. Between SIGNAL and VCC sits the 4.7 kΩ pull-up. So whatever current the phototransistor takes has to come through that resistor, and every 0.1 mA drops SIGNAL by 0.47 V. That is the whole circuit.

Pick a surface. The emitter's current never changes. The phototransistor's current, and so SIGNAL, follows what comes back. The voltages are this book's model, not a measurement; the order they come in is the point.

A floor, and a ceiling

SIGNAL cannot go above VCC: with nothing coming back, the pull-up holds it there. It cannot go below a few tenths of a volt either, where the phototransistor is fully on and conducts as hard as it can. It takes about 0.64 mA to pull SIGNAL from 3.3 V to that floor. In this book's model white paper at the best distance sends back about 0.44 mA, which leaves SIGNAL near 1.2 V: well down, short of the floor. A shinier surface, or a sensor that couples better than the model assumes, can reach it.

In between is where the reading means something: grey card, a white surface farther away, the edge of a black line half under the sensor.

High means dark or far

A high reading has two causes the sensor cannot tell apart: a dark surface, or no surface at all. That matters for a robot about to drive off a table. Over the edge, the reading jumps high, exactly as it would over a black line. The next chapter takes that further: the reading also changes with distance, so it is not a measure of either colour or distance alone.