Very special switch
By Madscientist267, Section Controls Posted on Thu Dec 18, 2008 at 02:41:30 AM MST
It's just a switch! How hard can this be?!!

Ok. According to the EPA, it appears that mercury is more deadly than the black plague, and is therefore extremely difficult (I avoid 'impossible' since seeing missile silos on ebay) to find them anywhere, even on the internet. Unfortunately, two of the classic SPDT mercury switches for thermostats would make excellent candidates for a toggling, snap-action limit switch design that I conjured up for the self powered tracker project. Anybody have any ideas? I know there are 'replacements', like ball switches, but they have a tendency to have accuracy issues (meaning they can be finicky). The best I have come up with myself is one of those sub-mini Radio Shack toggle switches with a fairly stiff spring attached to the lever to provide positive snap-action, but there may be force issues there. :( Also saw one teaser, a 'plunger' type microswitch. Looks like it toggles, but I cant seem to find it in anything but SPST!!!! The MS3 series, top left of the page --- http://www.switch-china.com/en/Product.asp?BigClassName=MICRO%20SWITCH In the end, the crucial specs are: DPDT Break before make VERY low force Snap action All contacts must switch at as close to ideal in timing as possible Must be mechanical, semiconductor losses (even FET) are too great to be of use Oh yeah, and a kitchen sink. Thanks ahead of time all... :) Steve

Very special switch | 6 comments (6 topical)

Re: Very special switch (3.00 / 0) (#1) by zap on Wed Dec 17, 2008 at 07:48:19 PM MST

Do they need to be DPDT?  I have some mercury thermostat switches plus the mercury part from 3 old "silent light" switches. zap

Re: Very special switch (3.00 / 0) (#2) by robl on Wed Dec 17, 2008 at 08:04:40 PM MST

You'll find more than you'll ever need here: http://search.digikey.com/scripts/DkSearch/dksus.dll?Cat=1114207&keywords=limit%20switches I use the Honeywell brand sealed dpdt models at 120VDC (with arc supression caps) to control my green house vents. Regards rob [ Parent ]

Re: Very special switch (3.00 / 0) (#3) by methanolcat on Wed Dec 17, 2008 at 08:40:39 PM MST

Ebay has quite a few mercury switches of many kinds, heres an interesting one I found, maybe find 2 of these and you'll be set. http://cgi.ebay.com/Bidirectional-Mercury-Tilt-Switch-NOS_W0QQitemZ180313497541QQcmdZViewItemQQptZBI _Connectors_Switches_Wire?hash=item180313497541&_trksid=p3286.c0.m14&_trkparms=72%3A1205|66% 3A2|65%3A12|39%3A1|240%3A1318|301%3A0|293%3A1|294%3A50    Matt

Re: Very special switch (3.00 / 0) (#4) by methanolcat on Wed Dec 17, 2008 at 08:41:51 PM MST

Hey look, he has 5 of them, good price too. Matt [ Parent ]

Re: Very special switch (3.00 / 0) (#5) by Madscientist267 on Wed Dec 17, 2008 at 09:45:09 PM MST

Thats exactly what I need. The only way it would be better is if they were dual tubes, already tied together by design as a DPDT! Missile silos and mercury switches... there truly isn't anything you can't buy on ebay is there?! I appreciate the feedback! Zap is gonna hook me up with a couple that he has around the house. Staring hard at the ebay deal tho... maybe for a future version... ;) Steve [ Parent ]

Gotum: Very special switch(es) (3.00 / 0) (#6) by Madscientist267 on Tue Dec 23, 2008 at 07:26:56 PM MST

Got the 3 switches from the ebay deal today (Thanks MethanolCat for the tip!). Let the building begin! Between waiting for shipping, and no usable sun for over a week, this whole thing has been on hold more or less. Theres a snafu; unlike their thermostat cousins, they are make before break. This is going to be a bit of a challenge to overcome, but I'm gonna see what I can do. Probably going to need a combination of mechanical vs electrical. Flop 'em quick to make switching as fast as possible, and a small amount of resistance inline to keep Isc down to just below magic smoke levels for the duration of the shorting that does occur. Delicate balance; too much ballast would defeat the entire purpose of using mechanical contacts... We will see what happens, wont we... On a less technical note, they're pretty - apparently never been used. No solder on the massive (14ga?) terminals and the blobs are flawless. Almost seems a shame to even use them, considering that they are so difficult to come by these days... :( Yes, I got lazy and swiped the pic from the auction page. Hope nobody minds... Steve

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