The project is using fan and optical RPM meter to define rotation speed. The speed has been converted using externaly calibrated TSI 4040. It's currently capable to measure 50 - 250 lpm. For highest values more sophisticated amplifier or more sensitive photoresistor/diode to be introduced.
Files for 3D Printing at CAD section.
Gerbers and DipTrace for PCB at PCB section
Source code for AttinyX4A is at Firmware section
Device overview:
PCB
BoM for 1.3v:
uC Attiny44A
U1, U4 AO3400
U2 HT7130-3
1400 NCP1400asn50t1g
OA AD8628
L1 18uH, CR32 D1 M3
C1, C6, C8 -
C2, C3, C4, C5 100n
C7 50u
J7, J8 18p /20p
R1 10k
R2, R5, R6 100k
R3, R7 1k
R4 330
R8, R11 51k
R9 100
R10 4,7k