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Author Topic: How to build a safe knife switch?  (Read 9183 times)
Der Tinkermann
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« on: July 07, 2009, 07:18:37 pm »

I'm looking into building a knife switch,but am not quite sure how to go about this(as in wiring it).Any ideas?
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sidecar_jon
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« Reply #1 on: July 07, 2009, 09:31:35 pm »

How about a standard switch and extend the switch bit, so the knife bit isnt live just use to throw the real switch. A standard light switch extended by say 6 inches would give a good long "throw" without any danger.
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aquafortis
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« Reply #2 on: July 07, 2009, 10:28:52 pm »

Use a low voltage on the knife switch, say 12v, and use it to trigger a relay or contactor Wink You can have it as exposed as you like and not present any real risk. Contactors engage with a nice "clunk". If you can't get one with a 12v coil, trigger it from a small relay. That said, even a small sugar-cube relay will switch 5-10 amps.

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Gunny001
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« Reply #3 on: July 08, 2009, 01:03:53 am »

If it were I designing such a thing...? i would simply buy a standard rotary switch and use it for one of the 'hinges' on the purely decorative knife switch...

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Angus A Fitziron
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« Reply #4 on: July 08, 2009, 11:25:57 am »

I'm looking into building a knife switch,but am not quite sure how to go about this(as in wiring it).Any ideas?
Rather than make assumptions ~ what is it you want to switch with this? What sort of knife switch do you have? Are you switching one wire? What voltages are involved, what current and ac or dc?
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Der Tinkermann
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« Reply #5 on: July 08, 2009, 07:32:14 pm »

Ah yes,I probably should have put that in my original post.......
The switch would be build from scratch,involving very low voltages(most likely battery powered,9 V ,or a 12 V adapter),using one wire.And used in an automaton and/or lamp.
This is what I'm after:
Spoiler (click to show/hide)
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jringling
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« Reply #6 on: July 08, 2009, 08:55:32 pm »

Both of the switches in your picture look safe to me. Are you just wanting to build it yourself for personal satisfaction, or are there size/material limitations...

In other words, why not just buy one?
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Der Tinkermann
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« Reply #7 on: July 08, 2009, 09:00:53 pm »

Because it's much more fun to build one  yourself....
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Gunny001
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« Reply #8 on: July 08, 2009, 09:51:53 pm »

Because it's much more fun to build one  yourself....

The 'original' was, quite literally, a knife - not double-poled, but single: a knife with a blunted tip, drilled to act as a hinge, with 'clips' to receive the portion of the conductive blade near the haft/guard/pommel (depending on type of knife).

That, I am thinking, would be.... 'cool'.

(*Grin*)
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Angus A Fitziron
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« Reply #9 on: July 08, 2009, 10:37:08 pm »

..and the twin bladed one is just two knives joined across the top (or formed out of a U shaped piece of material). The two blades just double the contact area. From memory (a long while ago) the power or common wire was taken to the centre pair of posts (knife pivots) and the the output from one pair of outer contacts. Each pair of contacts are paralleled. This also allowed you to switch to two different circuits. These kind of switches were used for low current devices, like voltmeters, ie connect a VM to the pivots and two measured circuits. The contacts that the knife blade slid into are quite critical as they need to have a good area of contact and needs to be sprung to provide a good grip on the blade. Have a good look at some originals.
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Gunny001
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« Reply #10 on: July 08, 2009, 11:01:38 pm »

..The two blades just double the contact area...

Correct - when dealing with voltage, you are dealing with resistance - and resistance, to simplify, can be considered as a funtion of available mass. (Thus, 12-gauge wire handles more current than a 14-gauge wire, with the same 'resistance factor'; or, alternately: 12g offers more 'leeway' than 14g... assuming 14g is spec'd to handle said (assumed) load in the first place... *Grin*)

Alas, however, I fear we are wandering a touch O/T - judging from the OP, we are well and truly below a point where any such consideration would possibly become 'non-trivial'.

No, my friends; in the here and now, we are discussing... Æsthetics!

(*Grin*)
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OldProfessorBear
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« Reply #11 on: July 09, 2009, 02:08:39 am »

Spoiler (click to show/hide)
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Angus A Fitziron
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« Reply #12 on: July 09, 2009, 11:59:11 am »

Bravissimo OldProfessorBear!  Grin
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jringling
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« Reply #13 on: July 09, 2009, 12:53:00 pm »

Building one yourself is a good enough reason...

Instead of a blade contacting a clip, maybe the contacts could be small sections of tubing and the blade a rod that slides into the tubes closing the circuit...

Just a thought...
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Angus A Fitziron
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« Reply #14 on: July 09, 2009, 02:58:23 pm »

There is a variation (sorry, no pics) that provided a "quick break" disconnection. The blades were as shown, but the section of the blade that made the contact, was hinged and held close to the remainder of the blade by a spring. When the lever was pulled to disconnect the power, the sprunk part of the blade remained in contact, held by the contact side pressure, until it reached the point where the tension in the spring overcame the resistance in the contacts and the 'knifelets' shot out of the contacts with a most satisfying noise (and smoke and sparks if you were really lucky careless.)
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Professor J. Cogsworthy
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« Reply #15 on: February 27, 2012, 04:51:35 pm »

Just found a book I need to add to my Library.

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Cubinoid
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« Reply #16 on: March 20, 2012, 02:23:55 am »

If it were I designing such a thing...? i would simply buy a standard rotary switch and use it for one of the 'hinges' on the purely decorative knife switch...



Elegant solution. I like that, Gunny!
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bicyclebuilder
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« Reply #17 on: March 20, 2012, 10:52:05 am »

How about a standard switch and extend the switch bit, so the knife bit isnt live just use to throw the real switch. A standard light switch extended by say 6 inches would give a good long "throw" without any danger.

I like this. Simple, safe and splendid.
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Professor J. Cogsworthy
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« Reply #18 on: March 20, 2012, 11:45:08 am »

I've decided to go with using the knife switch to control a low voltage relay
that closes the higher voltage circuit.

Use an old wall transformer to step down from ( in the US ) 120VAC to 12VDC and that goes to the switch.
« Last Edit: March 21, 2012, 12:01:27 pm by Professor J. Cogsworthy » Logged
Cubinoid
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« Reply #19 on: March 21, 2012, 12:01:08 am »

There used to be a door operated lightswitch that would work well...When the door closes, it turns off the switch and when it is open, the light goes on.

Similar to the one these guys sell:

http://www.toolstation.com/shop/p52171

Not bad for £3.50p

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techsean
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« Reply #20 on: February 10, 2014, 06:06:59 pm »

This is a low voltage one but I thought it worth posting up

http://www.instructables.com/id/Knife-Switch/

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RJBowman
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« Reply #21 on: February 23, 2014, 10:32:29 pm »

Part of the appeal of the knife switches in the horror movies is the shower of sparks given off when the knives touch the contacts. Is there a way that this can be achieved without putting the user at risk of electrocution?
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Keith_Beef
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« Reply #22 on: February 24, 2014, 12:23:54 am »

Part of the appeal of the knife switches in the horror movies is the shower of sparks given off when the knives touch the contacts. Is there a way that this can be achieved without putting the user at risk of electrocution?

This is what I thought, too. While it is a good, safe idea to have a low voltage circuit energise a relay to switch the high voltage, you're not going to get the dramatic spark.

But as I understand it, the spark is caused by high voltage, while the danger comes from high current.

So to combine dramatic sparks from high voltage with safety from low current, my idea would be a circuit to transform 240V (or 110V for you colonials) on a 5A circuit to supply the blade switch with something stupidly dramatic like 1000V at 100mA that then energises the relay.

MODERATOR NOTE: 1000V @ 100 mA = 100 W, which is quite enough to make you permanently dead.  DO NOT ATTEMPT THIS!
« Last Edit: December 26, 2015, 02:26:50 am by von Corax » Logged

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« Reply #23 on: February 24, 2014, 05:40:30 am »

Part of the appeal of the knife switches in the horror movies is the shower of sparks given off when the knives touch the contacts. Is there a way that this can be achieved without putting the user at risk of electrocution?

This is what I thought, too. While it is a good, safe idea to have a low voltage circuit energise a relay to switch the high voltage, you're not going to get the dramatic spark.

But as I understand it, the spark is caused by high voltage, while the danger comes from high current.

So to combine dramatic sparks from high voltage with safety from low current, my idea would be a circuit to transform 240V (or 110V for you colonials) on a 5A circuit to supply the blade switch with something stupidly dramatic like 1000V at 100mA that then energises the relay.

I understand that when they film Dramatic Sword Fights for the cinema, they electrify the swords somehow to make them draw sparks when they clash. They generally have very expensive actors beating on each other with those selfsame swords, so they can't be that dangerous; perhaps something like that would serve your purpose if you could figure out the specs?
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RJBowman
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« Reply #24 on: February 24, 2014, 04:33:12 pm »

I just remembered that one of my childhood books on electricity described how to make sparks with a battery, some wires, and a nail file; I tried it back ten, and it worked. Wire file to one battery terminal; brush wire lead from other terminal across the rough side of the file. Small sparks fly from where the wire contacts the file.

This method could possibly be employed to produce a spark effect for mock-up props.
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