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PICList Thread
'IR beam detection'
1999\09\21@084201 by White, Todd

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I have volunteered to be the "special effects" person for my neighborhoods
halloween party. What this means....automating a haunted house, really more like
a haunted room that's divided into narrow walkways. Because of young kids, the
effects will be fairly tame. A mask that lights up as someone walks by, a blast
of air at another spot, some sound effects. Its not practical to have someone
trigger these at the right moment, therefore, I will use a PIC to do the work.

Controlling the lights, air, and sound is no problem. However, Detecting people
as they walk by...I could use some help. I went to Radio-Shack and bought a IR
LED (model 276-143) and IR Phototransistor (276-145). The LED was driven by a 5
volt supply and 100 ohm resistor. The Phototransistor would conduct when the LED
was placed a few inches away, but when the LED was more that 4-5 inches away it
didn't.

I need to place the LED and Phototransistor about 3 feet apart to sense when
someone walks by. What am I missing? My TV remote works from 15 feet away. Do I
need a different LED/Detector?

Is there another inexpensive way to detect someone passing by?

Any other ideas for "tame" effects?

Thanks

Todd

1999\09\21@102258 by Dan Creagan

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The detector is tuned to 40 kHz.  You must modulate the LED at that rate and
you will get (usually) 12 -15 feet of response. However, focus the LED or
detector, add more LEDs, overdrive them temporarily,  etc and you can get
lots more.

Be sure you shield the detector from stray lights. They are overly sensitive
and things like bright flourescent bulbs will make them false trigger.

For ideas on using a PIC to modulate the IR LED, look at
http://204.233.101.40/robots/brb.html - which isn't exactly what you want,
but it should get you the basic information.

Dan

{Original Message removed}

1999\09\21@110204 by bill

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> The detector is tuned to 40 kHz.  You must modulate the LED at that rate and
> you will get (usually) 12 -15 feet of response. However, focus the LED or

I think he's using a plain phototransistor rather than a canned IR detector.  Ho
wever, the canned detector with IR LED modulated at 40KHz may
be a good idea.  Should be fairly reliable and will give good range.


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1999\09\21@111242 by Dan Creagan

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> I think he's using a plain phototransistor rather than a canned IR
detector.
> However, the canned detector with IR LED modulated at 40KHz may
> be a good idea.  Should be fairly reliable and will give good range.

<Groan.> Yes .. didn't read that very well.  Oh, well.  The detector should
make it better - cost is a bit more.

Thanks,

Dan

1999\09\21@121223 by Byron A Jeff

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>
> I have volunteered to be the "special effects" person for my neighborhoods
> halloween party. What this means....automating a haunted house, really more li
ke
{Quote hidden}

ED
> was placed a few inches away, but when the LED was more that 4-5 inches away i
t
> didn't.
>
> I need to place the LED and Phototransistor about 3 feet apart to sense when
> someone walks by. What am I missing? My TV remote works from 15 feet away. Do
I
> need a different LED/Detector?
You're missing the fact that most I/R is modulated and detection is done on
a particular frequency, typically 38 to 40 Khz.

You're missing that once the LED is pulsed that typically it can by run at a
higher current load due to duty cycle. This increases the range.

You're missing that often optics are entered into the equation, increasing
range.

Other than that you're fine. ;-)

What you're going to want to add to the equation is a IR receiver/demodulator
(Sharp GP1I52X, part number 276-137). It filters the IR at 40 Khz and gives
the demodulated signal.

Then it's pretty easy. Program the pic to generate a 40 Khz signal. Drive
an IR LED at max current for 40 Khz (or multiple IR LED's for that matter).
The detector should remain active untilthe beam is broken, then change states.

To simplify setup it's possible to set up a mirror on one side of the path
and have the PIC and the demodulator together on the same board. For separation
you may want to enclose the IR LED(s), detector, and mirror in black tubing
(like paper towel rolls painted black inside and out) and you can put an IR
filter over the end to block out ambient, but not IR light.

BAJ

1999\09\21@123451 by Matt Burch

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At 12:11 PM 9/21/99 -0400, Byron A Jeff wrote:

>and you can put an IR filter over the end to block out ambient, but not
>IR light.


If you'll permit me to mention my favorite shoestring-budget IR trick
here... :)  You can make a great IR filter for next to nothing by going to
your local photo place and having them develop a roll of *unexposed* Kodak
E6 slide film. The negatives that result from this will be completely
opaque to visible light but quite transparent in the IR range. We use
cut-up strips of slide negatives to make the IR windows on our telemetry
receivers look spiffy.

mcb
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1999\09\21@155142 by Art Allen, KY1K

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You don't need the Sharp detector or an expensive 40 khz oscillator!

LET'S NOT MAKE THIS ANY HARDER THAN IT HAS TO BE-

First, the transmitter....

These LED's are rated at 20 ma, 5v-3v/100ohms does indeed eqaul 20 ma, this
part is just right.

Now, for the receiver-

What value of resistor did you use in the collector of the phototransistor?
The value of the series resistor is what determines the sensitivity. Since
you do not need bandwidth, try starting with a 15k collector resistor.

Put your high impedance voltage detector at the jct of the collector
resistor and the collector-and you will see a BIG voltage there when the
beam is blocked! You might want to place a .1 uf cap between that jct and
ground, just to keep a small fast light spike from false triggering (a
flashlight sweeeping by the detector for instance).

Keep in mind it has to be dark for this to work, and you need to test it in
the dark-otherwise the ambient ligt will keep the thing triggered.

You can use a 3 or 4 inch tube around the detector to help keep out stray
light from the sides.

You can use the black film as suggested by another user too (I like to save
the black plastic that goes over the ir sensors from junk vcr's and use
that for filter material). You probably DON'T need a filter (the
phototransistor probably has a black plastic package, which is already
giving you a built in filter).

You should find this to give you 5 or 10 feet (or more) range.

If not, try going up to 50k in series with the transistor collector OR use
a small, cheap lens (anything you have laying around) to collect light and
concentrate it on the detector-just put the detector near the focal length
of the lens. A one inch lens provides about 200X gain due to its larger
light gathering surface. So, you don't need much/if any major expendature
for a lens::>

I guarantee it will work!

As a final comment, this ap doesn't need a pic and you can use a couple of
diodes and a cap to keep something turned on for a few seconds after the
beam is broken if you like. Sure, you can use a pic, but it's not really
necessary unless you are doing other functions with it, or unless your
timing needs is very very critical.

Write me off list IF I can assist further.

Art



t 12:11 PM 09/21/1999 -0400, you wrote:
>>
>> I have volunteered to be the "special effects" person for my neighborhoods
>> halloween party. What this means....automating a haunted house, really
more like
>> a haunted room that's divided into narrow walkways. Because of young
kids, the
>> effects will be fairly tame. A mask that lights up as someone walks by,
a blast
>> of air at another spot, some sound effects. Its not practical to have
someone
>> trigger these at the right moment, therefore, I will use a PIC to do the
work.
>>
>> Controlling the lights, air, and sound is no problem. However, Detecting
people
>> as they walk by...I could use some help. I went to Radio-Shack and
bought a IR
>> LED (model 276-143) and IR Phototransistor (276-145). The LED was driven
by a 5
>> volt supply and 100 ohm resistor. The Phototransistor would conduct when
the LED
>> was placed a few inches away, but when the LED was more that 4-5 inches
away it
>> didn't.
>>
>> I need to place the LED and Phototransistor about 3 feet apart to sense
when
>> someone walks by. What am I missing? My TV remote works from 15 feet
away. Do I
>> need a different LED/Detector?
> You're missing the fact that most I/R is modulated and detection is done on
>a particular frequency, typically 38 to 40 Khz.

1999\09\22@013333 by parkiss

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On 21 Sep 99 at 12:11, Byron A Jeff wrote:

> ...Then it's pretty easy. Program the pic to generate a 40 Khz
> signal. Drive an IR LED at max current for 40 Khz (or multiple IR
> LED's for that matter). The detector should remain active untilthe
> beam is broken, then change states.

Has this been the general experience?  My detector (Liteon
LTM-97AS-38) seems to activate only on the first few msec of a
constant 38 KHz source.  Some references suggest that most detectors
want 'short bursts' of IR at the designated frequency (i.e.,
modulation) and aren't designed for a continuous signal...

Steve

1999\09\22@021011 by Sean H. Breheny

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Yes,this was my experience the few times that I have tried to use similar
modules (the Sharp ones that Rat Shack sells or used to sell). If you look
at a block diagram of the inside of the receiver,I think there may be a
DC-blocking capacitor AFTER the main 38 or 40kHz detector. This is why you
only see a short pulse when you apply an external continuous carrier.

SO, I suppose the best answer,if you must use such a detector,is to
modulate the carrier on/off perhaps 100 times per sec,and use your PIC to
detect that. However, I feel that other solutions may be best for simple
beam-obstruction people detectors (using IR leds and
Phototransistors,perhaps combined with an LMC567 (IIRC) tone decoder,which
could easily provide a digital signal from a 40kHz carrier,using only four
external components (two resistors,two caps).

For more info on the type of IR "cans" that you are asking about, Wirz
Electronics has some info (I'm not sure how much) at

http://www.hobbyrobot.com/info

In addition, if you want a ready-made solution and you don't mind a small
expense (about $25), you could use an IR distance sensor. Yes,its
overkill,but it is ready to go,if you just hook it to a PIC,and you can
sense a person by just looking for the reading to pass a certain threshold.
For more info on these, see

http://www.hobbyrobot.com/info/gp2d02/index.html

(Which is also a gratuitous attempt to plug an app note I wrote <G>)

Good luck!

Sean


At 10:14 PM 9/21/99 -0800, you wrote:
>Has this been the general experience?  My detector (Liteon
>LTM-97AS-38) seems to activate only on the first few msec of a
>constant 38 KHz source.  Some references suggest that most detectors
>want 'short bursts' of IR at the designated frequency (i.e.,
>modulation) and aren't designed for a continuous signal...
>
>Steve
>
|
| Sean Breheny
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| Electrical Engineering Student
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1999\09\22@101409 by Wagner Lipnharski

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"Sean H. Breheny" wrote:
>
> Yes,this was my experience the few times that I have tried to use similar
> modules (the Sharp ones that Rat Shack sells or used to sell). If you look
> at a block diagram of the inside of the receiver,I think there may be a
> DC-blocking capacitor AFTER the main 38 or 40kHz detector. This is why you
> only see a short pulse when you apply an external continuous carrier.

Those 38-56kHz receivers have a pll-type-filter. They are designed to
have a high gain in those frequency bands, and they purely look for IR
delta intensity, so I guess they are not so good to receive a continuous
IR beam.

It is so easy to generate a 3X kHz oscillator (TTL, CMOS, Transistor,
etc) that I can't imagine why not use one to modulate the IR beam and
another to send a low frequency (100Hz as you said). A single chip 556
(double timer) can do both frequencies in a very small space.

This week I am playing with three diferent receivers from Digikey...and
an Atmel At89C2051 uC to decode it. Will post the results later on.

Wagner

1999\09\22@204026 by Russell McMahon

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Consider using surplus PIR detectors. These can be cheap in junk shops here
so should be there too (where-ever There is :-)).

_______________________________________
What can one man do?  Help the hungry for free at
http://www.thehungersite.com/

{Original Message removed}

1999\09\22@222626 by Mark Newland

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Just my 2 cents worth. Add to the options.

Use a LM567 (Tone Decoder) or equivalant.  Amplify your LED at whatever
frequency you want (PIC generated of course) and feed that freq to the LM567
also.  Bounce the signal off a reflected source and back to the
phototransistor.  The output of the phototransistor is fed into the input of
the LM567 and compared with the freq you generated.  The output is at logic
levels (fed back into the PIC of course).  Optics (mentioned earlier) help
ALOT also.

Worked for a major supplier of bowling ally supplies and this is how we did it
for foul line detectors, etc.  Can supply schematics in EagleCad if anyone is
interested.

Mental thought:  One of these days I want to program the pic to replace the
LM567.

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