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.
Best Answer
Your problem is that you have ignored the part of the data sheet which states that the sensor output is "open collector NPN". Disconnect the Darlington, and connect a 10k resistor from the sensor output to +5. The fact that you're seeing 220 mV is just an artifact of leakage.
With a 10k to +5, you'll see 0 - 5 volts pulses, and you can tie these directly to your Arduino.
And, just to make things clear, you must also get rid of your pulldown resistor.