8
\$\begingroup\$

I would like to use a pull switch as a trigger for an alarm that I'm building. When the switch is triggered it will turn on a siren and potentially a strobe light. I've found plenty of options out there, but the prices are a bit staggering.

Take this one for example, it's $238. I was thinking $20 would be expensive! My alarm isn't going to be used in an industrial environment and the items I'm securing aren't worth enough to justify spending that kind of money for a switch.

I think a simple pull type light switch would be sufficient, but I need the cable to break away from the switch when activated and not damage it in the process.

I guess I have two questions:

  1. Why is this switch so expensive?
  2. What other options are there for a N.O. pull cable switch that stays closed after being activated? I'm open to ideas of building my own switch.
\$\endgroup\$
8
\$\begingroup\$

Why is this switch so expensive?

  • The switch you reference is made to be used as an emergency stop via a length of pull cord (think of a stop-request cord on a transit bus ). It is made to latch open once pulled (i.e. it's a mechanical one-shot). It is more than a simple pull switch.
  • It is rated IP67, which equates to dustproof and immersion proof. It is made for demanding industrial applications including washdown and nontrivial shock and vibration.
  • It is a safety device made by a name brand company.

What other options are there for a NO pull cable switch? I'm open to ideas of building my own switch.

Many switches can be used as a pull switch depending on the specifics of your application. Something as simple as this can have a string tied to it such that the switch actuates when the string is pulled.

enter image description here

If you need the cable to break away from the switch, there are plenty of mechanical ways to accomplish this. A breakaway lanyard is made to open at a certain tension. There are also more repeatable "tension fuse" type devices based on magnets for various applications; take a look through a big industrial catalog for ideas.

\$\endgroup\$
  • \$\begingroup\$ Thank you for your detailed response. I hadn't thought of using a breakaway lanyard. I was guessing that the switch I had found was way more sophisticated than I thought, but didn't understand the details. I guess I'm just surprised that there aren't any cheaper similar versions. I need something that functions mechanically the same, but don't need the robustness of it (i.e. no need to worry about vibrations, dust, or immersion). \$\endgroup\$ – Matt Ruwe Apr 1 '13 at 2:58
  • \$\begingroup\$ @Matt - the reason you are having trouble finding anything cheap is that the number of people who need something like this, but who DON'T need the industrial version is extremely small - there's just not much market for such a thing, so there's no cheap solutions. \$\endgroup\$ – Michael Kohne Apr 1 '13 at 11:29
  • \$\begingroup\$ I accepted this answer because it's the only one that answers the question of why the switch is so expensive and offers an alternative. However, I'll probably solve my problem with the audio plug idea in the end since the switch referenced here won't latch until reset (which is a requirement of my application). \$\endgroup\$ – Matt Ruwe Apr 3 '13 at 11:23
11
\$\begingroup\$

One simple way to make your own N.O. pull switch would be to use something like a 3.5mm audio or DC plug and a socket with the extra contact to detect if the plug is inserted.

enter image description here

So the lower two pins would be normally open with the plug in place, and closed once the cable had been pulled out.

\$\endgroup\$
  • \$\begingroup\$ Thank you, your idea looks like the sort of idea I need. Do you think this would work? digikey.com/product-detail/en/TR2A/SC1514-ND/1289104 \$\endgroup\$ – Matt Ruwe Apr 1 '13 at 2:50
  • \$\begingroup\$ @Matt, yes that has the three pins as per above. Most connectors do so if you're near a Radio Shack type retail components store you could also pick up and try a few different connector sizes to get a feel for the amount of pull required to remove the connector. \$\endgroup\$ – PeterJ Apr 1 '13 at 2:56
  • \$\begingroup\$ I've seen something like this used as a kill switch for things like scooters, with a cord around the rider's wrist so that the motor will be shut off if the rider falls off. \$\endgroup\$ – Jeanne Pindar Apr 1 '13 at 12:32
  • \$\begingroup\$ Many sockets are designed to hold plugs in place with a noticeable amount of force. It may be worthwhile to experment with various other objects to find some that could be removed more easily. Perhaps a nail could be good, if one files off the tip of the head so it wasn't sharp enough to cause injury. \$\endgroup\$ – supercat Apr 1 '13 at 15:22
  • \$\begingroup\$ This approach works well if the angle between the cord and the socket can be somewhat controlled. The pull force will change dramatically if the cord is pulled at an angle. \$\endgroup\$ – HikeOnPast Apr 1 '13 at 19:37
4
\$\begingroup\$

"What other options are there for a N.O. pull cable switch that stays closed after being activated? I'm open to ideas of building my own switch."

Here's my simple do-it-yourself suggestion:

Drawing of simple pull switch made with a ribbon and two spring contacts

(Sorry for the MS Paint drawing, I'm not on my own computer and don't have any fancy graphics tools installed.)

The thick blue line represents a cord or ribbon made of something non-conductive. The brown lines represent two bent metal clips under slight tension, such that they will close together if the ribbon holding them apart is pulled away, electrically connecting the red and black wires. The tension also serves to hold the ribbon in place until it is pulled.

For neatness and reliability, enclose the whole thing in a small box, with a hole in one end for the cord and at the other end for the wires.

Of course, there are plenty of variations you could make to this basic design. Heck, a plain old clothespin with some aluminum foil glued between the prongs for contacts would probably do the job in a pinch (pun not intended).

Edit: If you used a (more or less) rigid plastic stick tied to the line for the blue part in the diagram, you could even design the switch and its enclosure so that you just need to push the stick back in to reopen the switch.

\$\endgroup\$
  • \$\begingroup\$ I like the idea and was thinking of doing something similar. The problem that I ran into was that I can't think of a place to get the metal contacts. They would need to be some kind of spring material so they would return. Any ideas? I suppose maybe a hobby store would have something like that? \$\endgroup\$ – Matt Ruwe Apr 1 '13 at 10:58
  • \$\begingroup\$ A quick search turned up the following which I think would be exactly what I'm looking for: mcmaster.com/#standard-spring-steel-sheets/=m4p7ak \$\endgroup\$ – Matt Ruwe Apr 1 '13 at 11:25
  • \$\begingroup\$ You might be able to scavenge suitable pieces from old parts like battery holders. Or you could maybe make them out of the coil spring of an old wind-up toy (or buy a cheap one to sacrifice). Or, come to think of it, the clothespin design might actually be worth considering, maybe with bits of thin sheet metal instead of just foil for more wear-resistant contacts. Or maybe just conductive paint on a wooden clothespin? \$\endgroup\$ – Ilmari Karonen Apr 1 '13 at 12:21
  • 1
    \$\begingroup\$ I've personally used a clothespin before. Screw two brass screws into the ends and separate them with a strip of transparency film. \$\endgroup\$ – Bryan Boettcher Apr 1 '13 at 13:45

protected by W5VO Apr 2 '13 at 21:44

Thank you for your interest in this question. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).

Would you like to answer one of these unanswered questions instead?

Not the answer you're looking for? Browse other questions tagged or ask your own question.