# Measuring revolutions

#### Rickharris

##### Senior Member
I need to measure RPM of a motor shaft - about 2000 RPM - I intend to use a photo transistor LED etc - and will get the bits tomorrow at school - But a thought struck me I am concerned that as the speed increases the pulse width will decrease - am I correct in thinking that I could coloured the shaft half black and half white i.e. a 50% duty cycle i would still get the same count as if i put a narrow stripe on the shaft.

It sounds logical but I have been struck by doubt. Can always try it tomorrow of course.

#### papaof2

##### Senior Member
I have an old (i.e., analog meter, discrete components) commercial reflective RPM meter that uses a focused light source (flashlight bulb with built in lens) to provide a relatively high light intensity in a small area.
I find the aluminum tape used on heating/air-conditioning ducts works well at 3000RPM on a dark surface (shaft of small gasoline engine) measuring from 25-50cm away. I use a piece of tape that covers 10-20% of the circumference of the shaft.

John

#### hippy

##### Senior Member
@ Rickharris : I think you are worrying unduly. There will be the same number of pulses, one per revolution, regardless. The issue you have is whether, as the pulse decreases, it becomes too small to register as a pulse.

At 2000RPM that's 33.3RPS, one revolution per 30mS, a 15mS pulse with 50:50 marking. Call that 18mS, 18000uS for 180 degrees, and a 1 degree wide mark should still give 100uS, which a PICAXE should be abe to detect. Even with poor rise times on your sensor.

Unless you have a very narrow mark I reckon you'll be okay. All theoretical, but I'd stand by it in the absence of contrary evidence.

#### Jeremy Leach

##### Senior Member
I think you can either measure the pulse width using Pulsin, or count the revs. Counting will give an accurate average but will lose the responsiveness of pulsin. It will be easier to do the maths to convert a count value to RPM than the maths to convert a Pulsin value to RPM. So all depends on what's more important in your app.

#### Michael 2727

##### Senior Member
From memory Pulsin starts to bottom out at around
12,000 Hz, so 2,000 RPM should be a piece of cake.

#### papaof2

##### Senior Member
You may want to build the unit for the learning involved, but be aware that a photo tachometer can be had for less than \$30US, including shipping.

Search Ebay for
photo tachometer

John