I am working on a project which checks the pressure sensor in washing machine that is ok or nok. First of all, let me explain how a pressure sensor works. It is a circuit which uses pressure as input and gives square wave with variable frequency according to the pressure as output.
If we are able to regulate the pressure in the input side and we know what the frequency is as output according to that pressure…
Is there anyone has an idea how I can process that signal? I mean how I can light on a LED in special values?
You could make a simple RC filter (resistor and capacitor) to convert the pulses to a smooth DC voltage which could be read by an analog to digital converter (ADC) on most microcontrollers like Arduinos. You could also use a small inductor in place of the RC filter to achieve the same smooth DC voltage to be read by the ADC. If the voltage is above 5v DC you would also need to add a simple voltage divider to get the max voltage down to 5V. If you are using a 3.3v microcontroller then you would need to ensure the voltage to the ADC is below 3.3v not 5v. If you are using an ESP8266 you need the voltage on the ADC to be below 1v.
You can find RC filter calculators online if you want to calculate the exact values of the resistor and capacitor required based on the output frequency of the sensor or you could simply start with something like a 100nf to 1uf capacitor and a couple of ohm resistor and then see if you get readable value at all the desired pressures.
EDIT: For the above you could eliminate the microcontroller and simply read the voltage output of the RC filter / Inductor with a volt meter. Either a dedicated voltage meter or a multimeter.
If you want to read the frequency of the pulses rather than check if it produces a range over its output you would need a microcontroller that is capable of reading its inputs at a faster rate than the sensor outputs its pulses. That would likely be something like a Teensy or an ESP8266 or Arduino DUE etc.
Here’s an example of an RC filter I found on circuitlab.
In your case your sensor is the source of the square wave labelled “V1” and you would read the voltage via a multimeter, volt meter or microcontroller at “Out”.
I do not believe an RC filter will give you what you need. A phase-locked loop is a better solution. See the circuit: CD4046. What is the frequency output of your transducer?