[help] i need to create a circuit for a optic speed sensing

[STINGERX]

First forgive me for my bad English :|
Hi
I'm a student, and as a homework assignment i required to design a simple way to sense the speed of the rotation of an DC motor, and to display the result in a voltage scope display, my plan is to sense the speed by using photodiode that use a laser beam to know when exactly the disc (load of the motor) reach specific point. The laser hit a mirror that located in the disc and reflected back to the photo diode, and then the photodiode send pulses to an integrator that summarizes the pulses.
i am suppose to see something like this:


Because this system can confuse and think that the motor rotate in infinite speed (when the motor stop at the point when the mirror and the laser at the same point), I decided to use the device One-shot or known as 74123

here is the datasheet:
View attachment Texas Instruments 74123 datasheet.pdf

to make the output from the photodiode to the integrator to a pulses at the same size (same time).

I tried to simulate this circuit in pspice but i had some errors, I don't understand what is wrong with this.

The pspice simulation and more details about the error
**broken link removed**


Thank you.

 

[jiripolivka]

To measure motor speed in rpm you need an optical sensor with a LED and a photodiode, and frequency counter. On motor shaft you mount a disk with a hole so the sensor will respond by one pulse per revolution. The counter should be reset after one minute, the reading will show "revolutions per minute", or rpm.
To measure the rpm faster, use more holes in the disk and reset the counter after the time that equals one minute divided by hole number.

You can convert the pulses by integration into a voltage as you mentioned. Use a simple RC integrator circuit. To calibrate it, use the above procedure. Instead of SPICE models, do an experiment. You can learn more by lab work.

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You can use the rotating mirror, laser and a photodiode, too. Such system is used for very fast rotation. For slower rotation you will need more mirrors on motor shaft, to get more output pulses per revolution.

 

[amitjagtap]

I think use of mirrors is not that much good idea...As other than mirror some part of disk can be equally reflective as mirror. OR if holes are there then be sure that the beam will not get reflected from any other surface behind the hole...In this case you have to have proper threhold on photodiode to detect correct reflected beam. Again mechanical arrengment if quite tuff.
You can think of using Hall effect sensor which are readily available in chip form from various manufacturers.
Best of Luck..

 

[STINGERX]

thank you both,
i can't make holes in the motor disc, i use a DC motor kit, (**broken link removed**). all my measurements done in external way.
i asked the teacher about using frequency counter but he still want's me to go on with my original plan (to use 74123 and integrator), i followed the instructions in the datasheet and i don't understand what is wrong.
all the devices connected as i understood but i guess i missed something.
can you please show me where is the problem? and how can i fix it?

Thank you.