how to use a digital tachometer with an arduino

Hey guys,
I am wanting to somehow use a digital speed tachometer With inputnto an arduino.
The values dont have to be exact, I just am looking to make an led dim or grow brighter with the speed of a fan.
Does anyone know of any smart ways to do this with a cheap rpm laser tachometer you can buy
On ebay?

If you're using an existing tacho sensor then you need to know how to get an output from that sensor. Devices sold as self-contained tacho sensors+displays may not provide an accessible output. Depending what sort of fan you're referring to, there may be ways for you to sense its speed without using a commercial tacho sensor.

It would probably be easier to do without the laser tach. Just use an LED and phototransistor.

  1. super-bright crystal clear LED fed with 5V through 180 ohm resistor
  2. Phototransistor fed with a 100K ohm resistor from 5V to collector, emitter to ground
  3. Take output of collector - resistor junction to Arduino input.
  4. Place both LED and phototransistor next to each other pointing in the same direction with a bit of back tape between them.
  5. Point them at the fan blade, from short distance (2cm-3cm)
  6. Read the pulses coming off the phototransistor

Something like that.

I agree, doing it all with an Arduino would probably be easiest. Although I haven't built it yet my next project will be based on this: Arduino Tachometer - Hardware | PyroElectro - News, Projects & Tutorials

I'm actually not using it with a fan, I just said that to make explinations as simple as possible haha, but it didnt go that way.
What I want to do is have it control underglow led strips under my longboard.
I have made underglow under my longboard before with 12v led strips,
but i thought it would be cool to add a little extra.

What I want to do is have the tachometer pointed at the ground and vaguely detect the speed I am going and change the colour of the lights accordingly.

Why not use a hall effect and small magnet in the wheel and have it read the real speed ? Then you could even record the speed if you wanted .