I'm relatively new to electronics and this is not my usual field of work.
For a bigger project I need to read a flow sensor's (Biotech FCH-C-PA-N) output. The output needs to be processed by an Arduino or Raspberry. The problem I have: The flow sensor's output (a square wave with varying frequency) has a Voltage between 0mV-220mV where the ~220mV represents the HIGH state.
How do I amplify the 220mV to 3.3V (RaspberryPi) or 5V (Arduino)?
I thought about using a darlington-circuit. But even with three Transistors (BC547) somehow I can't read a HIGH state at the microcontrollers input. The darlington circuit doesn't open.
edit: Problem solved
As stated by WhatRoughBeast this sensor works as intended without trying to amplify the signal (which was done wrong by me anyway due to a misunderstanding of the amplifying effect of transistors). It turned out that the used sensor was "FCH-C-PA" and not "FCH-C-PA-N" (I got the wrong manual by the seller). This one uses NPN open collector sinking, so WhatRoughBeast's answer is correct.
The reason why I couldn't get the correct signals at first (after plugging in the pull-up resistor) was a short-circuit at the electrical connection which was encased.