Experience with photo-reflector anyone?

Jeremy Leach

Senior Member
This is PICAXE-related as it feeds my PICAXE (!), so here goes ...

I'm having some difficulty getting my photo-reflector to work on my encoded disc and I'm just wondering if anyone else has had any experience. I'd appreciate any advice.

The one I'm using is an SY-CR102 from Maplin <A href='http://www.maplin.co.uk/module.aspx?TabID=1&amp;ModuleNo=12709&amp;doy=search' Target=_Blank>External Web Link</a>

Unfortunately it doesn't have a datasheet, the diagram is difficult to understand and I can't find one on the net, except for one in chinese, which I can't open.

I've gone by the example this guy used <A href='http://www.jeffree.co.uk/Pages/speedmeasurement.html' Target=_Blank>External Web Link</a> and have this circuit:

<code><pre><font size=2 face='Courier'>

+5V o------------------o----------------o
| |
.-. .-.
| | | |
220R | | | | 10K
'-' '-'
| |
| |
| o-------------o Output
| |
| |
.----------------------------------.
SY-CR102 | | | |
Opto-reflector | IR | |/ |
| LED V -&gt; -| Phototransistor |
| - |&gt; |
| | | |
| | | |
| | | |
'----------------------------------'
| |
GND o-----o------------o----------------o
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
</font></pre></code>

I've made a disc that is coloured in black marker pen with aluminium foil stuck over in the desired encoded pattern. The opto sensor is 1mm from the surface of the disc as per the maplin details.

The trouble is, I'm getting a voltage of 0.16Volts for the foil and only 0.19Volts for the black areas. I'm not sure why but there seems to be a few possibilities:

1. Photo transistor series resistor too high?? Although I'd still think there would be a fair swing of voltage.

2. My 'black' marker pen marks, although they look black may be as relflective as the foil ??

3. The other possibility is that this isn't the right circuit to use. The Maplin diagram hints at an emmiter-follower type arrangement:

<code><pre><font size=2 face='Courier'>


+5V o------o----------------o
|
|
|
|
|/
-| Photo transistor
|&gt;
|
|
o------------o Output
|
.-.
| |
| |
'-'
|
GND o--------o-----------------o
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
</font></pre></code>


and I've read a brief statement in another electronics book that this arrangement will give faster photo transistor response - although I'm not clear why.

Anyone got any tips?

PS: Oh and I made the fatal mistake of not breadboarding this first, thinking it's bound to work - so I'm just asking for tips before brandishing my soldering iron ;-)


Edited by - Jeremy Leach on 20/08/2006 17:26:44
 

Rickharris

Senior Member
amost certainly your &quot;black&quot; marker isn't at IR this is quite common - it may reflect or be transparent - same happens with black paper.

The indications are the your transistor is on all the time. Try shielding it to see of the voltage goes up then at least you know it works.
 

whizzer

Senior Member
Hi Jeremy,
My thoughts on this coincide with Rick&#8217;s contribution above, i.e. that the problem could be caused by the 2nd possibility that you listed in your post above. Maybe you need to use some ultra-flat black paint instead of the marker pen&#8230;
But here&#8217;s another thought: try reducing the overall illumination from the LED -to get a larger voltage swing on the output of the photo transistor, since it seems to be permanently in saturation at the moment. Perhaps a variable resistor in place of the 220R may help in determining an optimum illumination current for the LED.

Hope this helps
 

eclectic

Moderator
Jeremy.
Other non-electronic thoughts.

Agreeing with Rick and Whizzer,

Lots of &#8220;black&#8221; markers aren&#8217;t black. They&#8217;re pigment mixtures.
A quick solvent chromatogram of three different
&#8220;Permanent&#8221; markers showed a high proportion of red.
(Actually, I splashed a black spot with cellulose thinners)

As alternatives, perhaps try

a. Using a Graphics program to print your Gray scale, using arcs of Red (?reflect) and Cyan (?absorb)

b. Drilling an arc with holes, instead of using black ink.

e.

 

manuka

Senior Member
Tried the old digital camera &amp; &quot;is my ******* IR remote working&quot; trick? Most digital cameras are very sensitive in the near IR, allowing easy trials of masking paints etc. However, as mentioned by others,drilled holes in wood/metal will give best results.
 

Jeremy Leach

Senior Member
Thanks guys, that's really useful. I feel a bit of an idiot because I didn't try a reading away from the disc - 3.6 Volts ! So I'd say a slightly lower resistor, with holes would probably do it, although for strength and reliability I think I'll experiment with the colours.

Also, reducing the overall IR level is another good idea Whizzer as it'll reduce overall current consumption too.

Ha .... I should have thought of the chromatogram eclectic, very clever ! And interesting too. So Cyan should absorb ...hmmm

I tried the digital camera trick to make sure I could see some IR - but my camera just wouldn't see anything - despite checking the circuit many times. Could be because it's a very small device? Actually I say this is an IR device - but there's no mention of what wavelength it operates at.

Anyway, that's really appreciated.
 

Rickharris

Senior Member
some cameras (often web cams) have an IR filter in front of the lens you need to remove this to get good IR performance - They can generally still see some IR as if you point the TV control directly at them from hand to hand but sensitivity is reduced.

Edited by - rickharris on 21/08/2006 13:56:27
 

hippy

Technical Support
Staff member
A word of caution on using digital cameras etc to detect IR which I've also suggested doing in the past.

There are some ultra-bright IR LED's out there these days which can damage the camera's sensor ( leaving permanent dark areas and streaking across the image ), as I found out recently. Thankfully this wasn't an expensive or needed camera, but an annoying lesson to learn the hard way.

I'd recommend using an old PC webcam or similar which you don't mind getting damaged for things like this. Putting developed film negative in front of the lens works as a good IR attenuater.
 

Dippy

Moderator
Jeez you must have super sensitive camera Hippy. Daylight photos will have a hundred times more infrared in them than an IR LED.

Neither my Fuji nor Kodak are very sensitive to IR. In fact my Fuji is nealry non-responsive, even to laser IR.

However, my Sony Camcorder with 'Nightvision' button is supersensitive and absolutely ideal for doing IR tests.
Even better is my el-cheapo CCD mono camera from Maplin, I would guess that most mono cameras are. The catalogue has suitable speccs for you to read.

The dead giveaway (Sherlock) is if the cheapo mono camera has IR illuminators.

By the way Jeremy, Halfords matt black spray paint is a good IR absorber. So, is matt black anodised aluminium and so are the 1998 black jeans from Marks &amp; Spencers even though they don't fit me any more.
 

Jeremy Leach

Senior Member
Thanks Dippy ...I took a trip down Halfords, picked up a small pot of 'plasti-kote Flat Black B2' paint and it works a treat : Output now varies very consistently between 0.15 Volts (foil) and 4 Volts (paint) give or take a bit.

So the moral of this story is : don't use a marker pen !
 

hippy

Technical Support
Staff member
The now smeared camera was a cheapo 'Far East sweat shop' B&amp;W 2.4GHz CCTV monitoring thing with a ring of IR LED's I picked up for a few pennies at a garage sale. Yes, it is hyper-sensitive ! Just pointing a TV remote at it washes out the whole image.
 
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