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 Post subject: proximity switches
PostPosted: Dec Thu 05, 2013 4:45 pm 
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Joined: Jan Thu 03, 2013 5:17 pm
Posts: 131
Location: Corvallis Oregon
Does anyone have any experience with these imported proximity switches? They cost as little as $3 and run on 6 to 36 volts dc. They have some type of oscillator in them and set up a field where a metal object in proximity of the control head will result in an output control voltage. They have 3 wires
I'm just wondering how reliable they are and how long they last.
I'm building a piece of control eqpt. that has to work right, first time, every time. I need a margin of failure that's next to zero.
Thanks


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 Post subject: Re: proximity switches
PostPosted: Dec Thu 05, 2013 5:30 pm 
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Joined: May Tue 30, 2006 4:46 pm
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Location: Santa Rosa, CA
It would help if you could post a link to the kind of switch you are considering.

If it operates on metal parts, it may be Hall Effect, not and oscillator.

Rich


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 Post subject: Re: proximity switches
PostPosted: Dec Thu 05, 2013 7:14 pm 
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Joined: Jan Thu 03, 2013 5:17 pm
Posts: 131
Location: Corvallis Oregon
I'm not sure how to post a link but it is Ebay item number 121216192950 . If you go to Ebay and put this number in the search box You'll find it.
Thanks


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 Post subject: Re: proximity switches
PostPosted: Dec Thu 05, 2013 7:39 pm 
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Location: Littleton, MA
thompsonwayne1 wrote:
I'm building a piece of control eqpt. that has to work right, first time, every time. I need a margin of failure that's next to zero.

In that case, I wouldn't buy some possibly counterfeit part on eBay for $3, and would instead spend the money for something from a known reliable manufacturer purchased through a known reliable supply chain. Allied Electronics has them, at only about 10 times the eBay cost you quoted.

https://www.alliedelec.com/search/searc ... 4294882497

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 Post subject: Re: proximity switches
PostPosted: Dec Thu 05, 2013 7:47 pm 
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Location: Los Angeles
Here's how:
http://www.ebay.com/itm/121216192950

You stick all the characters from (and including) http thru to that final "/", then tack on your eBay number.

You can paste that into a Notepad ".txt" file and save it in a folder, on your computer.

Here's a bunch they sell at Mouser:

http://www.mouser.com/Sensors/Proximity ... c0&FS=True

These are commonly used industrial sensors. An industrial control vendor, who sells things like PLCs, PIDs, and DIN rail power supplies, is probably more appropriate. When I was part owner of a laser cutting business, on very rare occasion one of the 5'x10' material beds wouldn't move on our laser. It usually was because a metal protrusion, on the bed, wasn't properly aligned with just a sensor. Someone would have to go into the danger zone, and fake it, by sticking a piece of metal in front of the sensor. The ones used in industrial control, usually have a simple "dry contact" output. That's basically a pair of relay contacts, that are electrically isolated from the rest of the world. Usually normally open. They should also operate on 24VDC, since that's commonly available in that setting.

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 Post subject: Re: proximity switches
PostPosted: Dec Fri 06, 2013 1:15 am 
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Location: NJ, 07645
Automation Direct is where I purchase all my industrial control items.


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 Post subject: Re: proximity switches
PostPosted: Dec Fri 06, 2013 1:43 am 
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Location: S. Dartmouth MA 02748-1225 USA
thompsonwayne1 wrote:
I'm building a piece of control eqpt. that has to work right, first time, every time. I need a margin of failure that's next to zero.
Thanks
Specifying a device for a critical application that does not have traceable manufacturing standards or part sources as well being a known manufacturer to back it up, doesn't really fit into what your requirements are... Despite the "cost" if the product injures someone because of PPE, the "LLC" will only cover the business so far then its a lawyer feeding frenzy...
YMMV

Chas


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 Post subject: Re: proximity switches
PostPosted: Dec Fri 06, 2013 2:33 am 
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Joined: Jan Thu 03, 2013 5:17 pm
Posts: 131
Location: Corvallis Oregon
Thanks everybody,
I looked at the ones from the different suppliers mentioned and couldn't find out anything about where they were manufactured.
Sometimes I wonder if the one you buy direct from the overseas manufacturer isn't the same one that gets imported and then passes through a few middlemen on its way to an American parts house.
Then the consumer pays top dollar and assumes that because it costs so much it must be so much better.
If this device I'm building doesn't work no one is going to get hurt or die. Some plants won't get watered correctly, that's all. Still in the developement stage here.
I've got one of these $3 proximity sensors and have been putting it through its paces and it just keeps working perfect.
I'm going to try to find out where the Panasonic switches are made. I bet it's not Japan anymore.
Years ago I needed an 8 volt car battery and the auto parts place said they could have it in a day or 2.
I wanted it right away so I talked them into letting me pay them for it and going to the battery warehouse and picking it up myself.
At the battery warehouse they got out an 8 volt battery and looked at my paperwork which I think was from Pep Boys so they went over to this wall of containers of stick on battery labels and got a Pep Boys sticker and slapped it on the battery. They had a sticker for every type of parts and supply house you could imagine. If you bought a Silver edition battery you got a Silver edition battery sticker. If you spent more and got a Gold edition battery you got the Gold edition battery sticker. The kid that worked there told me they were all exactally the same 8 volt batteries
Trust, but verify


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 Post subject: Re: proximity switches
PostPosted: Dec Fri 06, 2013 4:02 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 30698
Location: Pocasset, Cape Cod, MA
You could build in a little redundancy by using two.


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 Post subject: Re: proximity switches
PostPosted: Dec Fri 06, 2013 3:00 pm 
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Joined: Jan Thu 03, 2013 5:17 pm
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Location: Corvallis Oregon
I've thought about using 2 for fail safe operation. I'm building a different model with reed switches and a magnet. I use 2 reed switches in parallel for redundancy. If I used 2 proximity switches in parallel I think I would have to isolate their outputs from each other.


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 Post subject: Re: proximity switches
PostPosted: Dec Fri 06, 2013 10:06 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 30698
Location: Pocasset, Cape Cod, MA
A couple of diodes?


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 Post subject: Re: proximity switches
PostPosted: Dec Sat 07, 2013 8:08 pm 
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Location: Sullivan, MO
thompsonwayne1 wrote:
I've thought about using 2 for fail safe operation. ... If I used 2 proximity switches in parallel I think I would have to isolate their outputs from each other.


If you are looking for redundancy then wire them in series. Connect the commons together, connect power to one and connect it's output to the power lead of the other. Then use the signal from the second one to trigger your control. That way both prox switches have to be tripped to get an output.

At work we use a lot of Balluff brand prox switches. Some are made in the USA (others are made in Germany) and their "Global Prox" series are usually $20-$30. They also have a lifetime warranty. Those are about the lowest cost proxes that I trust to use on our machines. Other proxes I have bought in the past with what I would consider to be the same quality are in the $50-$80 range.

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If it ain't broke, take it apart and see how it works.


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 Post subject: Re: proximity switches
PostPosted: Dec Sun 08, 2013 4:59 pm 
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Joined: Jan Thu 03, 2013 5:17 pm
Posts: 131
Location: Corvallis Oregon
I don't see how wiring them in series is going to give me any redundancy, or fail safe operation. If they both have to operate to make the device function you have made it more complicated to get less reliability.
If you wire them in parallel either one operating will make the device function. If one fails you've still got the other, that's redundancy.


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 Post subject: Re: proximity switches
PostPosted: Dec Mon 09, 2013 1:17 am 
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Location: Sullivan, MO
Fail-safe means that if a component fails the equipment it controls will be put into a safe state. Requiring 2 sensors to be made to allow operation of the equipment ensures fail-safe operation (although best practice is by using 2 channel operation). It is the basis of safe machine control. It seems that you do not want fail-safe operation. You just want to have a backup sensor in case one of them shoots craps.

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If it ain't broke, take it apart and see how it works.


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 Post subject: Re: proximity switches
PostPosted: Dec Mon 09, 2013 2:26 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Pocasset, Cape Cod, MA
It's just a matter of which mode is "safe," water or no water.


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 Post subject: Re: proximity switches
PostPosted: Dec Mon 09, 2013 7:32 am 
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Joined: Jul Sat 23, 2011 9:33 pm
Posts: 835
Location: Long Beach Ms. USA 39560
If you or your client want safety and reliability and you will sell the resulting item then you really need industrial-rated components. Allen Bradley, Square - D, Westinghouse, TI, and a few other companies with industrial catalogs can provide you with reliability information and exact installation and operation specifications for their switches, PLCs, and relays. (I left the Industrial controls field back in 1993, so there always are changes).
Please read the following.
Many years ago a magazine "Datamation" had a story about a client that ordered automation for machines to do the same job. He had two bids, so two machines were automated, one by each company. At startup both worked a while and died.
Company A was an industrial equipment manufacturer, their hardware was superb, but their programmers were unable to handle all the possible combination of logic conditions that could happen.
Company B was a computer company, their switches were modified doorbell buttons with sloppy actuators, loosely secured wiring, and programs that were beautiful examples of logic and covered all the possible exceptions.
The unhappy client gave the companies xx days to make it work or remove all the automation and put the machines back into manual production.
The resolution in the story was to combine the subject-matter-knowledge of both companies and make the machines work.
-=-=-=-
A real story I know of involved a boiler house where several boilers were fed with a high-pressure gas line. These boilers were each larger than a shotgun house (think Two-story double-wide trailers).
The automation company decided to remove all the hard-wired safeties and interlocks and trust their redundant computers to run the boiler house.
"Something" happened and all the gas flow valves were driven wide open, including the valves on the boilers not lit at the time. In each boiler a six-inch gas line was dumping full force into the firebox.
Since one boiler was lit, when the gas filled any unlit boiler and started up the stack, it would meet the flame of the lit boiler. The repairman working on something else in the building knew he could not run fast enough to escape the blast, so he stayed and shut down the manual valves.
Why do I tell you this?
You seem to have a project that needs reliability. You do not seem to know that reliability requirements are always tied to lawyers and possibly undertakers. Remember the CORVAIR?

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Unhappy tubes blush while unhappy power FETs scatter plastic


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 Post subject: Re: proximity switches
PostPosted: Dec Mon 09, 2013 4:26 pm 
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Joined: Jan Thu 03, 2013 5:17 pm
Posts: 131
Location: Corvallis Oregon
Thanks for the correction on what fail safe means.
This thing I'm building will control greenhouse water valves. If it stops working plants won't get watered in a timely matter. If someone left town for days and relied on it while they were gone and it failed plants could go without water long enough to die. Usually greenhouse growers don't take off for days at a time though and leave their plants totally under automatic control.
One thing I was concerned about was electrical grounding of the device, but I looked up the National Electrical Code and found out AC systems operating at less than 50 volts don't need to be grounded. There are a few exceptions but I'm still good, and it runs off the 24 volt grounded Rainbird transformer.


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 Post subject: Re: proximity switches
PostPosted: Dec Mon 09, 2013 6:09 pm 
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Joined: Nov Sat 27, 2010 6:15 pm
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Fail safe would be two or three devices independently supplied from redundant and completely independant power supplies physically seperated to withstand all known and anticipated perils and supported by another multi device switching infrastructure to provide failover service.

Now if it is temperature limits in a greenhouse or freezer or something like that, get the burgler alarm company to install their own stuff, and tie that in with proper insurance against whatever loss might be sustained.

Trying to trace origins of components or pieces of components is getting really hard, because stuff is mined, manufactured, refined, prepared, assembled, tested, certified, redistributed and rebranded so many times that in most instances the last vendor in the chain takes the hit when something goes wrong, and if assets are lacking, the food chain is slowly and expensively unravelled until the injured party gives up, dies, or is made whole, and you do know how that is likely to end up.

But, hey, I'm an optimist.


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 Post subject: Re: proximity switches
PostPosted: Mar Tue 13, 2018 3:57 pm 
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Joined: Mar Sun 11, 2018 8:44 am
Posts: 1
thompsonwayne1 wrote:
Thanks for the correction on what fail safe means.
This thing I'm building will control greenhouse water valves. If it stops working plants won't get watered in a timely matter. If someone left town for days and relied on it while they were gone and it failed plants could go without water long enough to die. Usually greenhouse growers don't take off for days at a time though and leave their plants totally under automatic control.
One thing I was concerned about was electrical grounding of the device, but I looked up the National Electrical Code and found out AC systems operating at less than 50 volts don't need to be grounded. There are a few exceptions but I'm still good, and it runs off the 24 volt grounded Rainbird transformer.

I'm not so sure that's the way these are being used.


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