how fast pulses can a phototransistor detect??

[settra]

hello forum. i was thinking of using a ir receiver, and an emmiter, in order to count the speed of a wheel, which has holes in it..
the thing is, that te time between each on/off would be less than 2ms....
is a phototransistor ok with that?? if not, what other way, to do it opticall??

(EDIT) Last part deleted, cause i solved it

 

[ericgibbs]

hi settra,
A photo diode or photo transistor will be fast enough for 2mSec.
E

 

[settra]

qiool, thanks! what would be the lowest it could work with?? 1ms? us??

 

[ericgibbs]

qiool, thanks! what would be the lowest it could work with?? 1ms? us??

You can get photo-trans to 1uSecs or photo diodes to nSecs.

Do you have circuit diagram you could post?

 

[settra]

naic. the circuit is the classic circuit:

https://imgur.com/TfoJa4o
the led in the circuit is not IR led. is just a led, that is turned on, when signal is detected

it might need some changes. in general, i will use it, to outpout a 5v pulse, every time light is detected (diferent circuit)

 

[Reloadron]

Your circuit looks like it will turn the LED On/Off depending on light striking the photo diode. The part labeled as photo transistor is drawn as a photo diode. I believe what you want is a common simple photo transistor circuit with an output of pulses to be counted to determine RPM or speed of a motor shaft.

There are no component values assigned because we don't yet know what parts will be used.

Ron

 

[settra]

cool. one last think.
the outpout, will be connected to an AVR interupt pin.
do you think there will be any sort of debouncing needed?? or will the signal be "Clean enough"

 

[Reloadron]

cool. one last think.
the outpout, will be connected to an AVR interupt pin.
do you think there will be any sort of debouncing needed?? or will the signal be "Clean enough"

i was thinking of using a ir receiver, and an emmiter, in order to count the speed of a wheel, which has holes in it..

If the holes or slots are clean there should not be a need for a switch debounce. You will need to know the number of holes per one full revolution and let the AVR (or whatever) do the math. As to pulse pickup there are reflective sensors like this one you may want to consider using. The unit I linked to is merely an example and units like this typically run about $8.00 (USD).

Ron

 

[settra]

Reloadron. thanks for the links!! if i will be able to find a refleftive sensor like that, i agree it will be the best. but i dont think because, the wheel i will be measuring, dosen't have "Teeth" but rather holes.
in any case, if the led is 5mm , then a hole of 5mm will be good enough i think...
the wheel will be rotating with 4000rpm (max) , and will have 35holes, which means, 0.4ms , between every hole... it is of the out most important, that no hole , will be lost... :/

 

[Reloadron]

I understand holes and no teeth. With a hole there is no reflection and no hole is reflection. Anyway any good optical sensor should work fine. 4,000 * 35 = 140,000 pulses per min / 60 = 2,333.333 pulses per second. That gives you 428.6 uSec. That really isn't all that fast for an optical sensor, good ones run much faster than that. I assume this plate is already pre drilled with the holes equally spaced so things are dynamically balanced? Anyway, optical sensing should work just fine.

Ron

 

[settra]

yiap. coool thanks allot man!

 

[settra]

so. i got ahead and bouthg a sensor like this one :
http://ecx.images-amazon.com/images/I/41QwOx4vZZL._SL500_AA300_.jpg
its not just a pair of IR emmitter and receiver, but also has circuity.

the "wheel" looks like that :
http://img717.imageshack.us/img717/1193/newbits01.jpg
but with smaller gaps...

everything works fine with less than 500rpm... but with higher Rpm's, it looks like as if, the sensor cant see the gaps any more.... it seems to be the sensors fault, since i cant find anything but , with the coding...
the only think i could find locally , is this :
http://www.sparkfun.com/datasheets/Components/GP1A57HRJ00F.pdf

but it dosent say maximum sampling rate...
but since it's just a pair of IR emiter and phototransistor, it should work... what do you guys think??

 

[Reloadron]

Sorry so late getting back but have been on vacation less a computer or even my phone.

I count 35 teeth in the wheel. I don't know how narrow the beam is. It should work.

Ron

 

[Dean Huster]

Why not use a simple optical interruptor since you're looking at rim teeth. Alignment is a snap. Only problem is if the wheel is too thick.