> On Thursday, October 11, 2012 12:30:26 PM UTC+1, Simon Howes wrote:

>     I made a speedo for my cnc mill *really* cheaply one day in a pinch.
>     Basically we'd hacked the pulleys to make it much faster but we
>     didnt have any idea of what speed it was now running (because of
>     belt thickness etc) as the pulley was quite small the variances were
>     large and I wanted to know how fast it was spinning.

>     Solution? Get a thin metal rod, drill a hole in it.
>     Get an ir led and ir led sensor - it my case I only had ir leds -
>     but thats okay, they work as sensors too!

>     Get a square of any material that will block the light. Drill two
>     holes through it at 90 degrees - one the thickness of the rod the
>     other the thickness of the leds.

>     Clamp the rod in the chuck of the spindle.
>     And you now have an interruptor  sensor. Measure the waveform on an
>     oscilloscope and you can calculate the speed.

>     It was a nice 3600rpm in the case of my mill :)

>     On Oct 11, 2012 8:11 AM, "Adrian Godwin" <artg...@gmail.com
>     <javascript:>> wrote:

>         A laser tachometer is about £10 on ebay (eg
>         http://www.ebay.co.uk/itm/270913948041
>         <http://www.ebay.co.uk/itm/270913948041>).

>         These come with a few reflective squares of scotchlite-style
>         retroreflective material, and work reasonably well as long as the
>         speed isn't too low.

>         -adrian

>         On Thu, Oct 11, 2012 at 4:18 AM, 930913
>         <proxify....@googlemail.com <javascript:>> wrote:
>          > A small laser pointer and an LDR?
>          > If it's possible to attach a small magnet, a bike speedometer
>         may suffice.
>          > Is this for a one off calibration, or for constant reading
>         during normal
>          > use?

>          > On Thursday, October 11, 2012 1:59:58 AM UTC+1, Will Beaufoy
>         wrote:

>          >> From talking to some people yesterday, it seems we will be
>         painting a
>          >> white dot at the end of one of our centrifuge arms, and
>         using the reflected
>          >> light to judge its speed. We initially thought of using a
>         strobe, but then
>          >> it was suggested to just fix a reflective sensor where the
>         light from the
>          >> dot would be reflected to, and count it this way.

>          >> Does anyone have a spare sensor of this kind, or a strobe
>         light if they
>          >> think the strobe method is better?