wanted to calculate the speed of small dc machine using laser beam
i have figured out that i have to use decade counter along with the seven segment decoder along with seven segment display but could not figured out what to use for detecting the laser beam as the shaft of dc machine would cut laser beam
i have an idea to use either photo diode or photo transistor
but does not know what to use and how to use it

 

First, how will you calculate speed with a decade counter?

 

there would be a stick or some other kind of hinderence on the shaft of dc motor
the number of times the shaft cuts the laser beam the number of times the laser beam would not fall on the photo diode or photo transistor
thus it would produce negative clock pulse required for the decade counter

 

Look at something like the Fairchild H21A1 optointerrupter - www.isl.ee.boun.edu.tr/projects/motordriver/downloads/datasheets/h21a1.pdf.

 

there would be a stick or some other kind of hinderence on the shaft of dc motor
the number of times the shaft cuts the laser beam the number of times the laser beam would not fall on the photo diode or photo transistor
thus it would produce negative clock pulse required for the decade counter

Yes, but by counting the times the laser beam will be cut is not enough. You have to make some other calculations too.

 

Beenthere,
The link you posted got garbled due to the proximity of the period at the end of the line.
http://www.isl.ee.boun.edu.tr/projects/motordriver/downloads/datasheets/h21a1.pdf

 

Yes, but by counting the times the laser beam will be cut is not enough. You have to make some other calculations too.

what do you mean by some other calculations????
i am planning to calculate first the "rps" of the machine using stop watch from which the "rpm" would be calculated of the dc machine
if you have any alternative idea then give it to me
otherwise my only issue is about detecting the laser light