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hello, i intend to pulse the transmitter (ultra bright led) at a certain frequency using the 555 IC and use a phototransistor as a reciever , im using  that for a line follower , i wanna do that to remove the ambient light ,i know that i should use bandpass circuit at the reciever but i dont know how the reciever circuit will look like ? and can the phototransistor works in that method ?


so if you don't mind switching to infared rather than visible light there are several components on the market that are sensitive to IR light modulated at around 38 kHz.
(i've used them at around 40kHz without realising at the time and they worked fine.)
these are used as infared receivers for TV remotes and similar devices.
there's a list of such devices here: http://www.lirc.org/receivers.html
you supply them with 5V and the 3rd pin goes high when you shine 38kHz infared light at them.
a lot of robot builders use them for object detection (reflected IR to detect object) but i see no reason why they wouldn't work for you.


thanks for your reply ,
ya i know about the TSOP , but im using just infrared transmitter and reciever (a photo transistor) for line follower robot , not for object detection so the TSOP wont work ...

but i thought what you were saying was you were trying to build is a circuit that will only be triggered by a modulated IR signal?
that is all these TSOP type sensors do. they are infrared receivers with the filter you described built in.

this method would work as long as you can adjust your transmit power so you get a substantially different number of positive readings depending on whether you are over the line or not.

the disadvantage with using one of these sensors is you can't really adjust how sensitive they are. if they see modulated IR they switch on.


I agree with dunk – infrared should work fine for detecting a white line. We use a number of IR LIDAR units on our robot, and the only color they seem to have a problem with is a particular shade of blue. Using an off-the-shelf transmit/receive IR diode and detector pair, like those listed by dunk, or the ones made by Omron, will likely save you time and money over doing your own modulation of a red LED – and will save you grief over interference from ambient visible light sources. Even if you mask your detection of background light by rejecting the “wrong” frequencies, your source may still be masked by ambient light. This can still happen with IR, but seems less of a problem. If you are building your modulator circuit strictly to learn from your efforts – go for it. If you are mainly trying to solve the detection problem, look at the off-the-shelf options first. Have a look at Digi-Key’s section on optics - http://pdfcatalog.digikey.com/T063/SectO.pdf


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