I assume it measures OK before connecting to the Bluetooth board?
Everything is soldered to the board. Please see the pic above.
A voltage divider will always work by itself if the voltage-input is correct and if the resistor values are correct. It's based on physics and laws of nature...
agree with u
If so, the input on the Bluetooth board has too-low of an impedance and it's "loading" the voltage divider. (A lower resistance in parallel with R2 will lower the voltage.)
That could happen if it's not really an "input" pin, or maybe if the Bluetooth module isn't powered-up.
That is an input-pin, right? Not the 3.3V power input?
It's input pin not powering pin with 3.3V tolerance.
It's the pin PIO4 on HM-10 Bluetooth module actually. By selecting the right mode, it turns this pin as an INPUT mode. I tested my circuit on that BL module before I soldered it, but I used an STM32 3.3V output pin to test the circuit instead of ATtiny84 with voltage divider.