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Posted Jan 25, 2012 at 11:09 am
Figure 1
The output of the flow sensor of the SwissFlow SF800 is displayed digitally using this frequency meter.
A display in liter/minute can be obtained directly when the sensor puts out an open collector square wave signal like a fan RPM monitoring signal between 50Hz and 2000Hz which is divided by 10 and the decimal point is put at the right place. The circuit is based on a counter IC MM74C925 which is a 4-digit counter that has 7-segment decoder and multiplexer with count and latch registers.
The other segments of the circuit comprise a precise timebase for the measuring time during the positive cycle of the clock that is based on the HEF4521 crystal oscillator and 24-stage divider. The output signal of the oscillator is taken from the output of Q23. The periodical latch enable signal is created by the integrator IC3A while the periodical reset signal is produced by the integrator IC3C.
The power supply consists of 7805 regulator that delivers 5V from the 12V PSU line supplying the 74C925, the HEF4521, and the CD4093. A ULN2003 buffers the segment driver output of the 74C925. The two PCBs are soldered by solder pads.
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