Electronics > Beginners

.... hitting the fan

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PerranOak:
I have a fan from an old PC which is in really good nick and would like to do a few things with it for education purposes.

Firstly, I'd like to measure the speed of rotation. I tried a bright light and a photodiode but the 'scope signal was mostly mains interference. I could see the change in voltage when I passed my hand in front of it so the circuit "works" but I guess the light intensity is too low to register over the noise given the short on-time between blade rotations.

Is there a better way? Is a photodiode (jelly bean) going to be fast enough? Should I use a laser diode for higher intensity?

Any advice welcome.

bdunham7:

--- Quote from: PerranOak on August 01, 2019, 05:13:00 pm ---I have a fan from an old PC which is in really good nick and would like to do a few things with it for education purposes.

Firstly, I'd like to measure the speed of rotation. I tried a bright light and a photodiode but the 'scope signal was mostly mains interference. I could see the change in voltage when I passed my hand in front of it so the circuit "works" but I guess the light intensity is too low to register over the noise given the short on-time between blade rotations.

Is there a better way? Is a photodiode (jelly bean) going to be fast enough? Should I use a laser diode for higher intensity?

Any advice welcome.

--- End quote ---

Why mains interference?  Does your light (or the room lights) flicker?  You might try a bit of aluminum foil on one blade.  Or, how about a small microphone placed right near the ends of the blades to pick up the pressure pulses?

To answer your other question, I've never heard of a photodiode so slow it couldn't pick up fan blades.  An old CdS resistance cell, maybe.

soldar:

--- Quote from: PerranOak on August 01, 2019, 05:13:00 pm ---I tried a bright light and a photodiode but the 'scope signal was mostly mains interference.
--- End quote ---

What "bright light" did you use?

Can you show us the schematic of the detector?

magic:

--- Quote from: PerranOak on August 01, 2019, 05:13:00 pm ---I have a fan from an old PC

--- End quote ---
It doesn't have a three or four pin connector by any chance? ;)

One thing about those fans is that they contain a magnet in the rotor. I'm not sure how much of a coil it would take to induce measurable voltage from its rotation, but that should be possible in theory.

Brumby:

--- 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".

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