Split a voltage: a schematic puzzle
Two resistors turn 9 V into the 3 V a sensor input can accept.
The two resistors form one path from 9 V to ground, so the same current flows through both. Each one drops a share of the 9 V in proportion to its size: Vout = 9 V × 4.7 ÷ (10 + 4.7) ≈ 2.9 V. It is a see-saw: make the bottom resistor bigger and Vout rises; make the top one bigger and it falls. Anything you connect to Vout steals current from the bottom resistor, so keep loads light or the numbers drift.
10 kΩ resistor: The top of the see-saw. It drops the larger share, about 6.1 V, and together with the bottom resistor limits the total current to a gentle 0.6 mA.
4.7 kΩ resistor: The bottom of the see-saw. Its share of the voltage is the output: 4.7 ÷ 14.7 of 9 V, about 2.9 V.