Discussion in "Project Doubts" started by    Arpit    Nov 15, 2006.
Wed Nov 15 2006, 05:22 AM

Hello every one!

this is my ifrst post on this forum and i hope i'll sort out my problem here.

The problem is rather a simple one - just to interface a photo-transistor to the AT89C51 using a comparator (op-amp) or a monostable multivibrator.

But, I dont know How i design the circuit exactly.
Some boddy having usefull suggestions for me, please be good to me.

[email protected]

Wed Nov 15 2006, 02:30 PM
well try to make a test circuit.. with
phototrasistor and a IR LED and test it if u are getting the right thing. i am sure u do.

Circuit is like this
          /          /
  220R \          \ 10K or something
          /          /
          \          \_____________ To Controller Pin
         _|_       _|_
         |L|       |P|
         |E|  → |T|
         |D|     |R|
         ¯|¯       ¯|¯

i hope this will work good luck!
Wed Nov 15 2006, 04:32 PM
thanks Rickey! for replying.

just have a look to the circuit i composed for it, if somebody have any suggestions or corrections, please let me know.

[email protected]

Wed Nov 15 2006, 07:41 PM
you know what you are doing in that circuit.. you are amplifying that volatage.... its no more TTL logic level.. moreover the gain of second opamp is 10 which is too high and such level of voltage is not able to handle by our controller.

I gave you a simple circuit in my previous post...
if your photodetector is detecting something i.e. if IR light is coming on the detector then u will get a logic 0 as trasistor will turn on and the transistor will be a closed switch.

if no light is falling on Photo detector.. the trasistor base will not provide sufficient current and transistor will act as open switch and u get Logic HIGH or 5V at controller pin.

i think my design is really simple.. and surely gonna work.. please give it a try and.. with opamp i ment to use as comparator and just keep the o/p between the Logic High and Logic low level.
Thu Nov 16 2006, 09:40 AM
the above circuit is fine and the first stage is a buffer and the voltage comming to it is very less so if we amplify it 10 times there will not be any problem to the microcontroller.
Thu Nov 16 2006, 11:31 AM
try it... practcally and tell me.. se the voltage is pulled up to 9V
Fri Nov 17 2006, 06:39 AM
can u gimme any circuit simulator like orcade or simula.
it would be better to work on simulators before i d the circuit a try.
Sat Nov 18 2006, 12:00 PM
how can i provide u that.. i mean.. its not difficult to make a simple circuit like that.

just an IR LED and a Photo transistor..
make something on Test Board and test it and u will get better results than a simulator

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