3 Digit Frequency Meter for SwissFlow Flowsensor
This electronic circuit is ment to digitally display the output of my 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. Resistor R1 (2k2) is there for accomodating the open collector output of the SF800 to a 5V TTL signal, available at connector J1.
The schematic designed is based on a old well known Fairchild Semiconductor counter IC, the MM74C925. It's basically a 4-digit counter with seven segment-decoder & multiplexer and counter & latch registers.
Source: http://www.turbokeu.com/myprojects/flowmeter.htm