Suggest comb like thing to pass through an optical switch?

MrCostington

Hi, I'm looking to measure about 150mm of linear movement using an optical switch. If you don't know them it's basically a small device with an infra red LED on one side and a receiver on the other side, you pass something through it that blocks the light and you get electrical signals.

This would be a typical usage to measure rotary movement:

https://www.robotshop.com/en/cytron-simple-rotary-encoder-kit.html

However I want to measure linear movement so need something a bit like a hair comb (or a garden rake) - long (150mm), thin (< 3mm) with teeth or fingers about 0.5 to 1mm apart that will pass by the IR beam and block/unlock it.

I don't have any experience of these, but I guess metal/plastic/even card may do so long as it's opaque to IR light.

This is just going to be used a few times, so looking for something I might perhaps re-purpose. I did think of old camera film, to use the sprocket holes at the edges, but IIRC they are about 3 or 4mm apart, I need better resolution than that.

Any ideas? This is the switch I ordered:

https://ie.rs-online.com/web/p/slotted-optical-switches/6666473/

PS the length of the "fingers"/teeth should be roughly 10mm, and something with holes will do too, like the film I mentioned.

Miscreant

You could try 3D print something perhaps? Would be quick and cheap if you know someone with such a device.

Either way, this is the Motors Projects & Builds forum so your query may be better elsewhere on the site. Here could be a good place to start: Technology & Internet — boards.ie - Now Ye're Talkin'

ozmo

https://www.boards.ie/discussion/2058253041/suggest-comb-like-thing-to-pass-through-an-optical-switch

Interesting project... above is about right - but the teeth could be finer and you can measure the level of light rather than just on/off if you need finer resolution...

Maybe the sprockets on a bit of old camera film negative or Cine Film might do? 3d printed would be best though.

You may want to check this out - its how to determine the direction of movement from such a setup...

https://www.youtube.com/watch?v=b8B8PCy-sA0

Also could consider magnets (reed switch sensor or hall effect sensor) - its easier as no light leakage to deal with.

Also - could a simple slide resistor be used? would make it even easier again (https://www.amazon.com/Slide-Potentiometer-Single-Linear-Taper/dp/B00L9YDRU6)

Victor

Rare these days, but you used to be able to get aluminium combs. A quick search suggests they are available for €2+.

MrCostington

@Miscreant unfortunately I have no access to 3D printing

@ozmo I'm already familiar with the type of encoder covered in that video, thanks. And also with hall effect sensors - the light switch is the way I want to go. If ambient light is an issue I'll just cover it with something.

@Victor aluminium comb sounds like a potential solution. Where did you find them online, I just looked myself and can't find any, local anyway

Victor

@MrCostington I just searched Google and there were a bunch of ads at the top. https://www.parfumdreams.ie/Hercules-Saegemann/Hair-care/Travel-and-Pocket-Combs/Travel-Comb-Model-4204/index_70687.aspx Amazon claims to have them. Farm suppliers seem to have coarser ones for animals, but for €50.

ticktock

If you get an old inkjet printer and strip it down . You will find what you need a slotted opto with clear graduated plastic ribbon passing through it . The graduations on the plastic ribbon are counted using the slotted opto so the printer knows the print head position. Hopefully this will help with your project.

MrCostington

@Victor thanks for the link, I actually saw that before I posted - I think that is just silver colored plastic, plus it looks like a German site (see footer) so would take a while to get here. Anyway, the switches arrived just after lunch so was able to try several materials - pretty much most things work including paper folded into 4, and white translucent plastic sheet folded. so I figured a plastic comb will do and picked up a pack of 12 (yes, 12!) in Mr Price for €1.50 on the way home.

The best one is 86mm long with 56 teeth all spaced evenly, this gives a resolution of about 0.75mm - I'll pick up another pack to get another similar comb to give the required travel.

@ticktock that sounds good, if my combs don't work out I'll pounce on someone on their way to the recycling center.

ticktock

looks promising👍️