A resistor “resists” the flow of current.  Voltage in a circuit with a resistor can be likened to water pressure.  For a given amount of electric current, more voltage (pressure) is lost across a larger resistor than a smaller resistor that has the same amount of current passing through it.  If you instead keep the resistance constant and vary the current, you can measure a larger voltage (pressure drop) across the same resistor with more current, or less voltage with less current.

The micro:bit module’s analog inputs are invisible to the phototransistor circuit.  So, a circuit plugged into the A/D2  socket on the cyber:bot board is monitored by the micro:bit, but the micro:bit has no affect on the circuit.

Take a look at the circuit below.  With 3.3 volts (3.3 V) at the top and GND (0 V) at the bottom of the circuit, 3.3 V of electrical pressure (voltage) makes the supply of electrons in the cyber:bot’s batteries want to flow through it.

The reason the voltage at A/D2 (V_AD2)  changes with light is because the phototransistor lets more current pass when more light shines on it, or less current pass with less light.  That current, which is labeled I in this circuit, also has to pass through the resistor.  When more current passes through a resistor, the voltage across it will be higher.  When less current passes, the voltage will be lower. Since one end of the resistor is tied to GND = 0 V, the voltage at the V_AD2 end goes up with more current and down with less current.

If you replace the 2 kΩ resistor with a 1 kΩ resistor, V_AD2 will see smaller values for the same currents.  It will take twice as much current to get V_AD2 to the same voltage level, which means the shadow will have to be twice as dark to reach the 0.1 V level, which is the default voltage in the script halt_under_shadow that makes the cyber:bot stop.

So, a smaller resistor in series with the phototransistor makes the circuit less sensitive to light.  If you instead replace the 2 kΩ resistor with a 10 kΩ resistor, V_AD2 will be 5 times larger with the same current, and it’ll only take 1/5th the light to generate 1/5th the current to get V_AD2 past the 0.1 V level.  So, a larger resistor makes the circuit more sensitive to light.

Connected in Series  — When two or more elements are connected end-to-end, they are connected in series.  The phototransistor and resistor in this circuit are connected in series.