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.... hitting the fan
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StillTrying:
Putting the 2k2 resistor in parallel to the photodiode - what does this do to make the diode faster?

A PD can only produce a small-ish current, and can't sink any, with only the scope's 1M to discharge the (PD + scope's) 20-100pF the fall time will be very slow, so it's a pull-down resistor.

Now, when I put the 10k resistor to Vcc, I see the pulses as promised! Why when I just connected the 'scope to this wire did I see nothing? Is it because very little current was flowing?

Open collector of course, that's a pull-up resistor. :)

On 3 wire fans the 3rd wire is a speed output.
PerranOak:
Ah, thank you. I didn't realise that the photodiode generated current - I though of them as being, essentially, a type of variable resistor and so never bothered to find out about them.  :-[

How did you know it is an open collector!? I wish I had your knowledge ... hence all the questions!
StillTrying:
Open collector and open drain is VERY common.  https://en.wikipedia.org/wiki/Open_collector

I don't know whether it's the fan speeds, or the viewing of the blades through a photo diode that you're interest in.

Why don't you spend a year (on and off) reading 50 websites, 50 pdfs, and flashing LEDs and bulbs at photodiodes just to see what the light shapes look like, I did as in the link above. :-DD
janoc:

--- Quote from: Brumby on August 01, 2019, 11:41:52 pm ---
--- Quote from: PerranOak on August 01, 2019, 05:13:00 pm ---Any advice welcome.

--- End quote ---

I'm going to guess that you have your light source and sensor on opposite sides of the fan and placed at right angles to the plane of the fan blades.  If the fan you have is anything like the ones I've seen, there isn't much of an optical gap between each blade, so my suggestion is simple - turn the body of the fan so that the angle of the blades allows more light through, increasing the "on-time between blade rotations".

--- End quote ---

An even better way in this case could be sticking a piece of white or reflective tape to a blade and sensing reflected light. I.e. put the LED and the phototransistor/diode on the same side, just don't forget to put something between them so that the LED doesn't shine directly on the sensor. That's how most of the commercially sold tachometers work.
Jwillis:
A typical 4 wire brush less fan motor wiring has a Gnd , Pos, PWM signal in and a signal out or speed sense. The PWM controls the duty cycle to make the motor run faster or slower. The Signal out or speed sense wire sends a signal back to the PWM to help regulate the speed  and/or computer to display the speed. You can use the signal out to a simple 555 tachometer circuit to get a speed of the motor.You can also use that same type of circuit with a photo transistor to detect RPM of a fan.The internet has lots of 555 tachometer circuits. Although most are for connecting to a distributor ,that's not a problem since the point contacts only have a voltage of 6-12 volts. Simply connect a photo transistor collector to power source and the emitter to signal input .A photo transistor is much more sensitive than a photo diode and really would only need ambient light to function.
Just as a side note , you will have to calculate the relation between the frequency of light pulses and number of blades to get the RPM of the fan motor.

Household LED lamps work with a PWM.Frequencies can be quite high. So can receive weird readings on a light sensitive tachometer.I seem to get readings around 7200 Hz on a Digital Tachometer for RC aircraft engines. Incandescent bulbs of course "flicker" between 100 and 120Hz.Florescent lights can be as high as 60kHz .
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