Best way to control AL8871Q dimming without microcontroller?
Apart from switching current-sense resistors for adjusting the output level as suggested by Kartman and Jasen, another good option is to use the CTRL pin since its intended use is brightness adjustment.
As stated in the datasheet, setting this pin's voltage between 0.125V and 1.25V will give a brightness level of 10% to 100%.
The REF pin outputs a regulated 1.25V reference voltage, and this voltage along with divider resistors can be used:



simulate this circuit &ndash; Schematic created using CircuitLab
With the circuit above, the brightness level will be 50% when the SW1 switch is open. When it's closed, the brightness will be the full level.
Brightness level based on the resistor values can be calculated as follows:
$$
\mathrm{
D_{percent} = \frac{R2}{R1+R2} \cdot 100
}
$$
For example, selecting \$\mathrm{R1 = 3\cdot R2}\$ will give 25% of full brightness when the SW1 is open. R1 can be set to zero with SW1 for full brightness.
NOTE: Since the max output current of the REF pin is 1mA, the divider resistors should be in kiloOhms range.