On an ESP8266 I try to analogRead(A0) my soil moisture sensor using a voltage divider but I get unexpected low values.
The MT3608 increases the esp's output voltage from 3.3V to 5.5V which is the recommended voltage for my capacitive soil moisture sensor v1.2 (different to the picture below I use v1.2, not v1.0). When there is no voltage divider in the game and the sensor is dry, the sensor outputs ~4.5V (yellow wire). The output is around ~2.3V when the sensor is completely wet.
For my voltage divider I use a 150 Ohm (R1) and a 39 Ohm (R2) resistor. Given ~4.5V Vin on the yellow wire this should provide me with ~0.9V Vout on the green wire, because Vout=Vin*(R2/(R1+R2)). A voltage <=1V I could safely attach to the esp8266 A0 analog pin. However, using the voltage devider I measure only 8,3mV instead of the expected 0.9V! It drops to 7.7mV when Vin is 2.3V (wet sensor).
What am I doing wrong? Any advise is highly appreciated!
For testing I currently use a board which has a build-in internal voltage divider that allows for up to 3.3V input, however in the final design I need to run everything using an ESP12-E without such an internal divider.