Pull Pin Switch - Thoughts?

geo_leeman

I needed a switch like this for a recent project, so I designed it up (literally still hot off the mill)! A spring loaded pin gets retained in the body to turn the microswitch on/off and turn your project on/off. A few applications immediately come to mind: high altitude balloon activation, safety power down, quick deploy instruments, detachment activation. I couldn't find anything like this out there other than people using 1/4" stereo plugs or 3D printed parts.

So what do you think? Would this be useful for anyone other than my narrow niche of instrumentation? Demo video:

Hal Albach

That is a nice piece of work! Well done!
Similar in function to what all jet ski watercraft have for an engine kill lanyard which pulls out a plastic "C" shaped clip which shuts down the engine if the rider is ejected from the craft, provided the operator clipped the other end of the lanyard to their life vest or wrist strap.
https://www.walmart.com/ip/Sea-Dog-Kill-Switch-with-Lanyard-Universal/44554681

WBA Consulting

That's pretty slick, nice and simple and could be used for normally open or closed setups.

I made something similar with one of these:
https://www.digikey.com/product-detail/en/magnasphere-corp/MG-B2-6.5-L/735-1005-ND/1680359

Putting metal near the top activates it.

geo_leeman

@"Hal Albach" - Thanks! It is indeed similar in function, just a bit more industrial in flavor!

@"WBA Consulting" Indeed! It's a pretty versatile little product I think. Neat reed switch as well - hadn't seen that form factor before.

Forgot to mention it's a washdown safe micro-switch so it should be resistant to some ingress, especially when mounted in any setup.

kwinn

Definitely has several uses in addition to those already suggested. E-stop, home sensor, and lockout switch came to mind immediately.

Publison

I second Hal's recommendation. I have used these on all my hovercrafts. They where all in the salt and fresh water envoronments and where exposed to the elements for long periods of time, (outdoor, UV, etc.).

msrobots

I really like @geo_leeman's version. It is solid and bulletproof, besides it looks like a hand grenade pin, so it really implies importance.

just my 2 cents,

Mike

geo_leeman

Thanks Mike - planning on getting them up for sale on my website this week. I've thought of a few accessories/variations. Maybe they are useful? Maybe not?

* Currently it mounts with 4-40 through hole fasteners. Those could be threaded as to eliminate the need for a washer/nut set. That does eliminate stacking them with a long 4-40 as an option though. Maybe that's not important?

* Laser cut acrylic cover that allows you to fill the cavity and completely cover the recessed terminals with silicone for really tough environments.

* Panel mount with threaded holes on either side of the pull pin.

It's possible to put all of that into one version (add panel mount holes and make all mounting holes threaded), but I don't want to kill any useful features trying to be overly general.

tonyp12

It looks nice.
But the problem with over-engineering is the final cost.


Industry is going solid-state, no moving parts, so try to sense magnetism or induction instead.

kwinn

tonyp12 wrote: »

It looks nice.
But the problem with over-engineering is the final cost.

Industry is going solid-state, no moving parts, so try to sense magnetism or induction instead.

While I agree completely with both statements I have to disagree with applying them to this particular device. It's simplicity and rugged construction makes it a good choice for applications where safety and reliability are paramount and cost is a secondary consideration. The cost would also be very reasonable if it were mass produced.

geo_leeman

I also agree over-engineering is expensive. In this case they're going to "retail" (largely to myself for a specific project) for $35... considering the instrument ($~5k) requires a hard power shutoff, it's a deal.

Mickster

Just throwing in my $0.02 because this stuff interests me (been through an awful lot of napkins, over the years).

Maybe consider machining the housing from ABS, Delrin or nylon to avoid the possibility of the pin binding due to corrosion?

Maybe use a magnet for the pin and encapsulate a reed switch in the housing to avoid the possibility of contamination getting to the mechanics of the microswitch?

Edit: DOH! Only just saw WBA's reed switch.

kwinn

Mickster wrote: »

Just throwing in my $0.02 because this stuff interests me (been through an awful lot of napkins, over the years).

Maybe consider machining the housing from ABS, Delrin or nylon to avoid the possibility of the pin binding due to corrosion?

Maybe use a magnet for the pin and encapsulate a reed switch in the housing to avoid the possibility of contamination getting to the mechanics of the microswitch?

Edit: DOH! Only just saw WBA's reed switch.

Used a lot of lunchtime napkins as well. Never hurts to re-hash a topic. Reed switches are great, particularly in harsh environments, however they do have their own set of shortcomings. They generally have low current and voltage ratings which limits their use as e-switches, and can also fail in the closed position, which is even worse for that application.

geo_leeman

While I do agree moving parts generally are best avoided, there's also something to be said for KISS here. If the pin is in/out your device will be on/off with no passive current draw in the off mode. When I make a run of these for my instruments I'll be anodizing the cases, just too expensive to do in low quantity. I had considered plastic, but was worried about wear from the pin. Might be worth giving UHMW a shot and seeing what happens. The microswitch is sealed (washdown safe) so intrusion shouldn't be an issue, but debris getting in the pin pocket could be an issue. I'm hoping that the pin pushing it out the back side is enough to keep the channel clear.

Mickster

..... and can also fail in the closed position, which is even worse for that application.

Ah, I considered it a bit too nitpicky to mention, earlier but dual-redundant contacts have become popular for safety circuits. I am with you regarding dry contacts when it comes to safety.

geo_leeman

Right - I assume that drove this design choice for model rockets: https://www.labratrocketry.com/product-page/double-pull-pin-switch-kit

Thanks for all of the feedback. I may look into a mag version at some point, but this is what I needed for my particular job. Just hoping someone else will need it too!

Mickster

geo_leeman wrote: »

Right - I assume that drove this design choice for model rockets: https://www.labratrocketry.com/product-page/double-pull-pin-switch-kit

Thanks for all of the feedback. I may look into a mag version at some point, but this is what I needed for my particular job. Just hoping someone else will need it too!

Looks like it. I have been through my share of frivolous lawsuits so I take safety pretty seriously. Even my Start-Cycle triggers consist of both N/O & N/C contacts. I have to see both inputs change state and overkill it with software debounce.

T Chap

One issue for some applications is you have a narrow angle that it must pull out at else it binds. If you put a loop on the back side of the pin that can hinge like on a chain or small nylon rope then the effective pull angle becomes 360 as the module will move in an direction you are pulling from. Otherwise if you are on a boat for example and you fall in a direction that does not allow the pin to pull out then you don’t kill the engine and you run over swimmers. Delrin is a good choice but not easy to finish although 400 and a buffing wheel with blue rouge does a decent job.

Mickster

T Chap wrote: »

Delrin is a good choice but not easy to finish although 400 and a buffing wheel with blue rouge does a decent job.

Yeah it can be a bit slippery

I don't believe you can even paint the stuff, right?

geo_leeman

Lawsuits are something to be avoided for sure! In this case, this is really an on/off on an instrument more for rapid deployment vs as a safety mechanism.

I don't think delrin can be painted, correct.

One question I always face on things like this - do you thread the part or put clearance holes and let the user deal with it? I tend to lean that way, but can see arguments for both.

Mickster

geo_leeman wrote: »

I may look into a mag version at some point

I have a thing for magnets lately, they make great fasteners.

When I retrofit a piece of equipment, I always dreaded installing my components in an existing electrical panel because of the drilling/threading and having to mask off the other components for fear of stray metal chips, etc. So I decided to try putting neodymium feet on my components and it works beautifully. Heck of a time-saver which is always a concern when you take a machine off-line.

The product range is amazing

geo_leeman

I generally like magnet mounts as well. I recently had a project for a magnetically sensitive instrument (in fact it was enclosed in a MuMetal shell) so everything had to be Aluminum or Brass and the motors feet from the sensing elements. A nightmare of design! Would have to get some pretty strong magnets to mount this and have the pull work correctly.