3-digit frequencymeter for SwissFlow flowsensor


This schematic 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 I 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.

The other sections of the schematic include a precise 0.5Hz timebase for the measuring time (during the positive cycle of the clock = 1 second = 1measure/2 seconds), based on a Philips HEF4521 crystal oscillator & 24-stage divider. This oscillator oscillates at 4.194308MHz and the output signal is taken from output Q23 which represents a dividing factor of 2^23 (=8.388.608) and serves as gate signal for IC3B (CD4093 NAND Schmitt trigger). Integrator IC3A creates the periodical latch enable signal and integrator IC3C produces the periodical reset signal, also from the 0.5Hz timebase signal.

The 7805 regulator delivers a stable 5V power supply from the 12V PSU line for the 74C925, the HEF4521 and the CD4093.

The segment driver output of the 74C925 is buffered by an ULN2003. With the shown values of R6-R12 (150 ohm) the peak output current per segment through the displays is about 40mA to compensate for the duty cycle of 25% and the multiplexing frequency of 1000Hz.

The 7-segment displays are (ofcourse) blue displays, the 0.56" common anode LT-5501 from Liteon. Blue leds have a forward voltage of about 3.8-4.0V, and taken into account the 0.9V saturation voltage of the ULN2003 output and the 0.7V saturation voltage of a BC327, +5V power supply was insufficient, and the displays are powered from the the +12V line. Since the digit driver outputs of the 74C925 only outputs 3 to 4 volts, it's not sufficient to saturate driver transistors T2, T4 and T6 (BC327 PNP's) and I added T1, T3, T5 (BC547 NPN's) to bypass this issue.

My Eheim 1250 pump delivers a flow of 1200 liter/minute at 0" head. Taken into account the pressure drop from radiator, water blocks, tubing and flow sensor it's quite possible that the final water flow will be under 10 liter/minute (600 liter/minute). In that case the first digit will always indicate "0". Therefore I will show both the schematics and PCB layouts for XX.X liter/minute display and for X.XX liter/minute display.


Flowmeter schematic for XX.X display
Flowmeter schematic for X.XX display


Flowmeter PCB layout for XX.X display
Flowmeter Display PCB layout for XX.X display

Download PCB layout in Circad'98 format

Download PCB layout in Circad'98 format


Flowmeter - Solder side
Flowmeter - Component side


Flowmeter Display - Solder side
Flowmeter Display - Component side


Mounted PCB's
Front view - In action

The two PCB's are soldered together by means of the solder pads at the edge of the main PCB, and the pads in the middle of the display PCB.

Fed with a square wave of 1280Hz
Idem - square wave 2630Hz

Mounted in a Lian Li bezel with blue plexi windows and together with my CPU speed indicator...






Mounted in the front panel of my PC-70...


General views of the front panel...