Need some help selecting a photomicrosensor switch…

[mbu]

Hi,

I’m putting together a tachometer and have all the parts except a photomicrosensor. The problem is that I have never used a photo sensor and don’t know how to go about selecting one. Could someone give me some help?

I’m thinking of using a photomicrosensor that will pick up from either a slotted disk or one that will pick up from a reflective tape on a shaft. Either way, the sensor will be permanently fastened and placed in a location where it is dark and in a fairly clean environment. The rpm range is from 20 rpm to 2500 rpm.

So here are some questions:

1.) If I use a slotted disk with perhaps 10 slots will the sensor have trouble counting the slots at high rpm?
2.) If I use a sensor and picks up on a reflective mark on the shaft (one mark) will that give me trouble getting accurate rpm readings at the low end of my scale?

So… based on your recommendation could you give me a sensor model number so I can at least get an idea of what is needed?

Thanks for your help!

Mike

 

[dch222]

Using any photodiode or phototransistor you should be able to detect pulses easily at the speeds you are talking about.

To avoid the uncertanties due to a noisy rising edge at the beginning of each detection you will need some sort of de-bounce circuit (eg monostable) to avoid counting each pulse more than once.

The low RPM should be no problem although if you use capacitive coupling anywhere, the size of the capacitor will need to increase to cover the low RPM end.

 

[mbu]

dch222,

Thanks for the response! Would something like this work or is it the wrong thing entirely?

https://www.mouser.com/search/refine.aspx?Ntt=opb732wz