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Author Topic: Modulating  (Read 4137 times)

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Offline andreahmedTopic starter

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Modulating
« on: December 11, 2006, 06:36:29 PM »
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 ?

thanks

Offline dunk

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Re: Modulating
« Reply #1 on: December 11, 2006, 07:12:48 PM »
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.

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Offline andreahmedTopic starter

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Re: Modulating
« Reply #2 on: December 11, 2006, 07:17:05 PM »
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 ...

Offline dunk

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Re: Modulating
« Reply #3 on: December 11, 2006, 07:54:56 PM »
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.

dunk.

Offline Militoy

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Re: Modulating
« Reply #4 on: December 11, 2006, 08:57:20 PM »
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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Re: Modulating
« Reply #5 on: December 11, 2006, 10:27:19 PM »
Infrared light will work for line detection, Ive done it on several robots already. Its really common, actually. But I dont like to use it because its harder to debug than visible light. I found a green led with photoresistor is much easier, cause I can see what the robot sees.

As for modulating IR to eliminate noisee . . . I dont see how that will help. Modulation allows you to send a data signal without degradation from ambient sources, but the intensity (the thing you will be measureing, no?) will degrade no matter what. For color detection, you must measure reflection intensity . . . someone correct me if Im wrong . . .

I think the best way to go about it is to have a very high signal to noise ratio (a very bright LED with not so bright background).

Offline Cognaut

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Re: Modulating
« Reply #6 on: December 12, 2006, 12:47:35 AM »
I could totally be wrong about this, but the way I interpreted the concept was that the "off time" would be used to sample the ambient light in order to create a base line for the "on time."  That should allow the isolation of the reflection of the modulated source from the rest - I think - like a continuous autocalibration.
« Last Edit: December 12, 2006, 01:09:51 AM by Cognaut »

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Re: Modulating
« Reply #7 on: December 12, 2006, 05:55:40 AM »
Quote
the "off time" would be used to sample the ambient light in order to create a base line
hmmm ok ok that will work, my mistake  :P

Offline andreahmedTopic starter

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Re: Modulating
« Reply #8 on: December 12, 2006, 08:10:23 AM »
 thanks for reply , but i dont want to use something like TSOP , its very sensative to IR , i just wanna use a simple emitter ,detector infrared circuit, would the phototransistor respond  if i modulated the emitter at a certain frequency ?

Offline dunk

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Re: Modulating
« Reply #9 on: December 12, 2006, 08:14:00 AM »
yea, that's the advantage of using modulated signals.
you can look for the rising edge rather than a specific level. (think of looking for a change in the signal rather than for a set value.)
therefor far less susceptible to background interference.
this is true for radio signals as well as light, sound, etc.

i'm not convinced it will be a good thing for line detection though as with a modulated signal you see the difference between "on" and "off".
this difference will be similar wether the sensor is pointing at a light colour or a dark colour.

if i was designing this application i would have several sensors in a line. make sure the line of sensors is longer than the width of the line so there is always at least one sensor over a dark patch and one sensor over the lighter line. (providing your bot is not really lost...)
the sensors that are over the line would return a different value to the ones that weren't.
this way you could again look for a difference in values rather than a set "light" or "dark" value.

dunk.

Offline dunk

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Re: Modulating
« Reply #10 on: December 12, 2006, 08:20:36 AM »
Quote
would the phototransistor respond  if i modulated the emitter at a certain frequency ?
sure. the phototransistor would just average out the signal.
modulating the signal doesn't really help you if you do things this way though.
if your receiver isn't looking for a change between the high and low signal values at that frequency there would be no point transmitting them.

i'm not sure i'm explaining this very well.
let me think about it for a while and i'll try and think of a better way of explaining after work...

dunk.

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Re: Modulating
« Reply #11 on: December 12, 2006, 09:04:57 AM »
Quote
you can look for the rising edge rather than a specific level

this method works really well when transmitting data, such as from a tv remote - but it wont work for line following. for line following you must analyze amplitude of a signal (amount of reflectance from the line).  :P

pseudocode:
turn LED on
wait 20us then measure transistor with ADC (A)
turn LED off
wait 20us then measure transistor with ADC (B)
A - B = signal without ambient light (assuming transistor changes linearly)

Quote
would the phototransistor respond if i modulated the emitter at a certain frequency
check the datasheet for the signal response rate. the above 20us wait is based on a guessed response time delay.

Offline andreahmedTopic starter

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Re: Modulating
« Reply #12 on: December 12, 2006, 09:57:30 AM »
hmm , after getting the value without the ambient , can i use it for the thersholding method ?

Offline Militoy

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Re: Modulating
« Reply #13 on: December 12, 2006, 10:49:34 AM »
Quote
I found a green led with photoresistor is much easier, cause I can see what the robot sees

Was there any particular reason you chose green, or was it just the visibility in setup? It seems to me that the optimum situation would be one where the delta between light reflected off the white line and light reflected off the background was maximized. Maybe a high-intensity white or UV LED might give even better results - as long as the detector sees a big difference. Modulated or not, and even without any light source beyond ambient light, the detector should be selected to best react to the color of light reflected off the line. Theoretically, white should reflect most colors pretty well. I wonder if, on the average, one color or one type of detector may "outshine" all others?

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Re: Modulating
« Reply #14 on: December 12, 2006, 12:17:03 PM »
Quote
Was there any particular reason you chose green, or was it just the visibility in setup?

I did a lot of research into this - most photoresistors have its highest sensitivity with the color green (check the spec sheets). So a green LED made sense . . . The problem with photoresistors is that they have a low frequency response rate, so any modulation must be done at really low frequency.

Quote
after getting the value without the ambient , can i use it for the thersholding method ?

you would want your threshold to have ambient light subtracted from it already. it really depends on your algorithm, and the sensor, but thats how i would do it . . . you could always test it different ways to see which works best.

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http://www.societyofrobots.com/robotforum/index.php?topic=354.new;boardseen#new
« Last Edit: December 12, 2006, 12:19:55 PM by Admin »

 


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