This schematic is ment to digitally display the output of an electronic Swissflow SF800 flow sensor. This sensor puts out an open collector square wave signal (like a fan RPM monitoring signal) between 50Hz (=0.5 liter/minute) and 2000Hz (=20 liter/minute).
By dividing the measured frequency by 10 and putting the decimal point at the right place you get directly a display in liter/minute. I don’t use the first digit output (output D) from the 74C925 to obtain a display of 10 to 9990Hz or xx.x liter/minute.
3-Digit Frequency Meter: [Link]