The instrument circuit is shown in Fig.2. The
sensor assembly uses an infrared LED – photodiode pair
to sense the passage of drops through the drip chamber.
Use of infrared sensor reduces the effect of ambient
light and also the effect of variation in transmittivity of
different fluids. Normally the light from diode D1 after
passing through the chamber is incident on the reverse
biased photodiode D2 resulting in voltage Va. When a
drop passes through the chamber, the passage of light is
interrupted, resulting in a voltage drop. Voltage Vb,
obtained by low-pass filtering Va using R3-C3, serves as
reference input to the microcontroller’s internal analog
comparator. Dipping of Va below Vb results in a pulse at
the comparator output.