Optical interupt switch implementation.

[owhite]

We all need limits...

This is a continuation of a thread that was started here.

As I am sure many of you know, limit switches detect the position of moving parts when they are near the extreme ends of their linear positions. This prevents collisions of things like ballnuts at the ends of the ball screws, and are also used as points of reference for when the software resets the table to go to the "home" position. Some systems use mechanical switches but in my case I chose to use optical interupter switches made by Fairchild. The advantages of optointerupters like the H21LOB are that they create TTL logic and are fairly accurate.

After some testing I came up with this circuit diagram ( jpg pdf ) for the interupters. In this circuit you'll note that in order to daisy chain the optointerupters together it was useful to solder a resistor directly to the interupter. The H21LOB wiring was bundled up with lots of heat shrink tubing (also shown here). The circuit diagram also shows the pinouts for each of DIN connectors which eventually will plug into the back of the electronics enclosure.

Although its very difficult to see in the picture, my cnc table came with a nice mounting bracket (note red arrow) which had some old school interupt switches. I pulled off the bracket, removed the switches, and strung in my interupter circuit into the mount (view here and here). Hot glue was used to make all the wiring that was crammed into the rail stay in place.

The pictures look kind of crappy but once the rails were bolted into place everything looked really good. There are two rails, one for each axis, and each rail has three switches: limit+, limit- and home. The table comes equiped with small pieces of sheet metal that project down onto the rails. When the table slides towards the ends of its total possible travel (around 18 inches) the sheet metal tabs travel between the opto-interupters. The state of the opto-interupters changes, the universal stepper controller picks up the change, and my software throws a complaint.

owen

[ESjaavik]

These interrupters are very accurate and dependable. But it may be worth mentioning that this is for a laser cutter. I don't think Lasers don't generate swarf or dust, so it will be fine. But for others reading this and thinking of using it for metal or wood cutters, please consider this: A small piece of debris landing in the reading fork, and it will signal "limit! end operation.".

[High Seas]

owhite -
Say Owen, thanks for that great summary! I've salvaged a bunch of similar opto-limit switches from printers and 5.25 floppy drives (thanks to Big AL for his connection notes too), so I may have a purpose for them now!
And Einar, sure understand your point regarding swarf and dust. I'll mount "high and dry" overhead - SYSTEM2 sorta sits "above" some of the mess.
Thanks guys! - Jim

[impact]

Sounds interesting. I think I am going to try something like this.

[AlmostHandy]

I'm not sure I understand what the OP means when he says "The advantages of optointerupters like the H21LOB are that they create TTL logic and are fairly accurate. "

I'm not familiar with the use of that type of opto interrupter. It looks like the phototransistor side is set up like a darlington pair. Is that for current gain?

How do the H21LOBs compare to a 4 wire slotted optical switch, like an H21A1 (commonly found inside old computer mice). I would really like to implement this type of limit switch in my design, but I'm having trouble wrapping my tiny brain around the wiring of the switches.

Can anyone help out on this?

I only ask about the H21A1's because that is what I have, but I'd really like to understand the differences between the two types, before I make a final decision. What is the benefit of using one type over another?

[AlmostHandy]

Also, my circuit will require the limit switches on all axises to be chained together in series, so that if one switch is closed, the whole machine stops.

This is what I had so far. Does this look like it would work?

[Al_The_Man]

I do not see this working at all, you have the LED's all in series across the 5v supply, with no resistance in series you will most likely blow one.
You need three wire between each switch, and you have the NPN transistors which have no final load resistor.
See post #7 http://www.cnczone.com/forums/showthread.php?t=77324
Al.

[AlmostHandy]

Thank you Al! That file is just what I was looking for! I understand how to connect them one at a time now, which is a step in the right direction.

My problem is that my circuit calls for limit switches to be connected in series, so that if one of them is broken, the whole loop is broken. They are all connected to the same pin on the parallel port (S6), and a 5v source with a 4.7K pull up resistor.

Is there a way to connect the opto-interrupters in series, so that they are all connected to the same loop? I can't figure out how to daisy chain them all together in series.

[Al_The_Man]

I will repost with the series circuit.
Al.

[Al_The_Man]

This is a series circuit, it has been tested, the resistor values may have to be adjusted if any opto's are added or subtracted.
Al.

[AlmostHandy]

Originally Posted by Al_The_Man This is a series circuit, it has been tested, the resistor values may have to be adjusted if any opto's are added or subtracted. Al.

Awesome! That makes sense! Even seems, dare I say, simple?


I found a box in my junk that has 6 of the same type of opto-switch. They are Sharp, model number GP1S58. I've looked at the datasheet, and it says that the typical rating is 1.2Vf @ 20mAIf on the LED side. Now, my math isn't that great, but I'm pretty sure that since I'm using a +5V supply, I'll only be able to chain together four of these in series, before the forward voltage becomes too low for the LEDs to make any light, closing the circuit.

Does that make sense? I'll be trying to put 6 of them in series.

What I've done is attempt to separate the power to the LED half of the switches, without breaking the chain in the output side. My attempt seems logical, but again, these little optical switches baffle me for some reason. Do you think this would work?

[Al_The_Man]

You could probably go with three LED's in each string with a 50 ~ 70ohm resistor.
Unless you want to keep them in pairs?
Al.