Infra Red Tacho

I seem to encounter problems when they try to scan the print it yourself type tickets which are so prevalent now.

Having read this thread helps me understand possibly why this happens, we only have an inkjet printer so cannot test any theories.

I wonder if it is perhaps the clear plastic coating reflecting the light, how does it go uncoated? Ian S C

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... as described and illustrated here

MichaelG.

Given that "dark" is usually taken to mean outside the visible spectrum, I wonder if they define "dark" in this instance.

I seem to recall from way back that a piece of unexposed, but developed, slide film makes a tolerable IR filter.

Looking at the other end of the spectrum, I have found inkjet prints satisfactory for photo-etching with a UV box.

Neil

Edited By Neil Wyatt on 05/07/2014 16:29:22

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Neil,

That must qualify you for the "off-topic" post of the week award

MichaelG.

Hi Michael,
I don't see why UV is off topic. We had already started bringing shorter wavelengths in to the topic with visible light. We seem to think something is black just because we see it as black in our very limited part of the electromagnetic spectrum. Maybe black should be defined as something that absorbs all radiation from long wave radio waves to gamma rays. It is even possible that one of the colour inks may absorb infra red.

Les.

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Sorry, Les, it was just a joke

... hence the stupid smiley thing.

I had already made my serious contribution.

MichaelG.

Edited By Michael Gilligan on 05/07/2014 19:01:25

Edited By Michael Gilligan on 05/07/2014 19:01:52

Nearly all inkjet blacks are dyes, they charge a lot more for pigments! . In the original article MEW115p53 the author said that in his experience that Aluminium, steel and cast iron don't seem to be particuarly reflective so he used Humbrol white paint and black paint 1.e. the inverse problem!. Most reflective sensors have a sharp peak in their output when the reflector distance is changed so a simple control is to vary the target/ sensor distance.

Because reflective sensors are exposed to ambient light they often have visible light filtering built in, they often work around 900nM where many dye blacks are light greys and the optical filter is "black" plastic. There are a lot better sensors than the now obsolete SYCR102 which has a very slow detector.

Worth trying changing the sensor distance in any reflective sensor application.

Billy.

Now it is dark, just tried a quick experiment with an IR illuminated security camera. The IR is provided by LEDs that are just visible so probably no more than 850nm.

Printed 'Laser' on one sheet of A4 with the laser printer and one with 'Inkjet' printed on the inkjet.

When both sheets of paper are viewed side by side in front of the camera the word 'Laser' can be seen clearly but the inkjet printed paper looks like a plain sheet of paper.

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Les,

Microsoft recommends an ink called Epolight 8771

I have no idea as to availabilty or price.

MichaelG.

Hi John,
I had not thought of using one of these cameras with short wave infra red illumination (Just about visible to the human eye.) for testing. It confirms the test I did last night using the Maplin device with longer wavelength (Non visible.) infra red and proves that my original feeling that the inkjet print could be made to work was wrong. It would be interesting to see a plot of the reflective coefficient of the inkjet print against wavelength covering the visible and infra red spectrum.

Les.

Instead of paper, can you mount a gear anywhere on the shaft? Place the IR emitter one side, the photocell the other (photo-transistor may be better) and use the teeth to break the light beam between the two. You'll get more pulses than you do currently, but the PIC shouldn't be too difficult to reprogramme for that.

An elegant alternative is a Hall Effect sensor looking at the teeth of a gear. No worries about light reflection/absorption, etc.

Iain

Does the Hall sensor not need a magnet to trip it, thats how They work on the IC ignitions I use on my engines

You could use a gear and a sensor from a car ABS system.

Roy.

Jason,

Not necessarily. If the gear is ferrous, the change in magnetic field can be enough to activate them... might need some conditioning before measurement, amps etc., or careful thought about the size of teeth on the gear - the bigger the teeth, the more change in signal. The sensors used on ABS are very often Hall Effect devices.

Iain

I used a hall effect sensor when I built mine. I just stuck a tiny neodymium magnet on the lathe spindle. Worked a treat, and not expensive.

Luke