In article ,
wrote:

It is not a mechanical switch. The pulse generator consists of an IR
LED that shines on IR sensor with a plastic disk in between that has a
series of black stripes painted around the edge of it. When the wind
cups spin, the disk spins so the black stripes interrupt the IR path
between transmitter and sensor generating pulses. The pulses then to go
to a base of a transistor that I assume is a switching transistor that
is either fully on or fully off. It is an NPN transistor in common
emitter configuration and the output of that is fed directly into the
CPU for counting the pulses in a given time frame for wind speed
determination.

Sounds to me as if there may not be enough (any?) hysteresis in the
IR-sensor / transistor-switch system. If the disk comes to rest in
just the right position (with one of the stripes partially but not
completely blocking the IR path) the amount of IR energy reaching the
sensor, and thus the amount of current going into the switching
transistor base might be right around the transistor's switching
threshold. Varying amounts of random electrical noise in the
photosensor or transistor could then cause the output to switch state
unpredictably.

If I were going to try to fix this, I'd do one of two things:

- AC-couple the output of the sensor to the transistor's base, adding
a biasing network to hold the base a comfortable distance on one side
or the other of the switching threshold. That way, only fairly
substantial (and relatively fast) sensor transitions would dump enough
current into the base to cause the transistor to switch state.

- Use a Schmitt trigger, or some other sort of transition-detection
circuit with explicit hysteresis (e.g. a comparator with a bit of
positive feedback added).

--
Dave Platt AE6EO
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