Electronic – arduino – Photodiode and Arduino to measure the speed of a projectile

If this is the type of fan you are using: http://www.formfactors.org/developer/specs/REV1_2_Public.pdf

-then the output sensor claims it is an open collector or open drain. (Meaning it only pulls down.) It also says "Motherboard will have a pull up to 12V, maximum 13.2V". Written this way I would take this as a recommendation. If the output is truly just an open collector/drain then you should be able to add your own pull-up resistor as needed. For the Arduino input you would have the resistor pulled up to +5 only. (You could add the 5k resistor as shown, or if you know how to enable the internal pull-up on the Arduino port pin use that, in which case you would not need the 5k resistor shown.) With a resistor in place the fan sensor pulls down (to 0v) then releases the pin, so it then climbs back up to what ever voltage the pull-up is connected too.

The hook up should look like this:

^{simulate this circuit – Schematic created using CircuitLab}

But just as a precaution, before connecting the Arduino Speed input pin to Sense, you should measure the voltage on the Sense fan pin while the fan is slowly rotating (with 12v on the fan power input).

The spec lists the Fan's Control input to be a 5v PWM signal. So coming from the Arduino no extra driver circuit is needed.

Also the spec says the pulses from the Sense pin is 2 per revolution, so measure the pulses per second then divide by 2.

First, the Lighthouse puts out a lot more optical power. Instead of a puny little 5 mW laser, they apparently use a string of lasers, and you can easily get 10 times more power from that sort of setup.

Second, you seem to have missed the basic idea about emitters/receivers entirely. If your laser produces a visible line (and it seems to do so), why on earth would you try to detect it with an IR receiver? The BPW34 data sheet www.vishay.com/docs/81521/bpw34.pdf gives it a peak sensitivity at 950 nm, but it also works at visible wavelengths, but at lower sensitivity. You're using a visible laser, so I'd expect lower sensitivity from your setup than is possible with an IR emitter of the same power. And going to a BPW FS will only make matters worse, since the FS incorporates a visible-rejection filter, so it won't see your visible laser at all. This also explains why you could not detect other lasers with an IR receiver, although there are other issues such as modulation which you need to learn about.

What you do need is a narrow bandpass filter at your laser wavelength. You can find them on eBay. If you get something, keep in mind that, for it to be really effective, you'll have to make a light-tight housing for your diode so that only light which has gone through the filter hits the diode. You really need to do something like this anyways. Try this experiment. With the laser off, put a box over your PD and take a reading. Call this your zero point. Now take the box off and take another reading, and call this your ambient level. Finally, turn on your laser and take a third reading - call it your active level. If the ambient is significantly different from zero (and it will be) this represents a laser light level which is unmeasurable, since the laser is being masked by the ambient.

An amplifier is probably a good idea, but this is clearly going to be a learning experience for you.