.... hitting the fan

[Rick Law]

Thank you everyone for all the replies. So, in order:
...
Rick Law: wow mate! That’s fantastic! I’m gonna print it out and go through it once I’ve posted this reply!
...

Nice of you to say that.  Let's hope that indeed helps you narrow the problem.

As you work through that, the most important would be the Vdelta baseline which is the difference (in absolute value) between VmaxBlocking and VminBlocking.  That will tell you a lot.  Take note on the noise level there too.  Since you are moving the fan blade manually, any of significant noise there (compared to Vdelta) is going to cause problem.

I think there is a good possibility that your reflected light (plus ambient light) is overwhelming that light that hits directly your photo diode.  Looking at the size of your light source, it will leak a lot of light to eventually reflect on to your photo diode, so reflected light could be a significant problem-contributor.

It is not necessary good to have an overly strong light source.  Strong light source reduces ambient light problem but it also introduce more reflected light.  The idea is to maximize Vdelta. 

Beside moving the blade to get VmaxBlocking and VminBlocking, you should also note the reading when the fan is absent (VnoFan).  That will give you a sense of the best way to approach maximizing Vdelta.

Let us know how it goes.

Rick

[Circlotron]

Put a 10 ohm or therabouts resistor in series with the DC power lead and put your scope across the resistor and watch the rate the current gets chopped as the fan rotates. Could be 3, 4, or 6 times per revolution or similar.

I went and tried the above. At 12 volts supply there are two pulses per rev with my fan, approx 13mS apart. Scope says 77.878Hz which is  4673 pulses per minute. Actual speed measured with a Uni-t UT372 reflective tacho is 2344 rpm averaged over about 30 sec. 4673ppm/2344 rpm = 2 pulses per rev.

[ledtester]

ledtester: thank you but I don’t have a phototransistor and wanted to use the PD.

The methods described in this Analog Devices lab should also work with a photodiode:

https://wiki.analog.com/university/courses/electronics/electronics-lab-led-sensor

The Darlington-pair circuit in particular should be very effective at amplifying the current produced by the photodiode. Of course, you could also use an LED.

Also, Forrest Mims III also has written extensively on how to use LEDs/photodiodes as a light detector. Here's an article of his that appeared in Make magazine:

https://makezine.com/projects/make-36-boards/how-to-use-leds-to-detect-light/