Scantlings

Why is a single pole, double throw switch, which has 2 operating positions, called a three-way switch? Or am I misinformed?

I thought a three way had a center off…

A Single Pole Double Throw switch is one that has one pole on one side and two on the other (logically). Using it you can:
- Connect A to B, or
- Connect A to C, but
- Can NOT connect A to B and C simultaneously, and
- Can NOT connect A to nothing (Know as "off")

Usually, this means that there is no "off" setting in the middle and not "both" setting. Sometimes the phrase is misused for a switch that has three settings:
- Connect A to B,
- Off
- Connect A to C

It's all about one of those tiny household fix-it chores that has so many very minor issues arise that it's not so tiny anymore.

The light switch at the stop of the stairs turns on the family room light, as does the light switch at the bottom of the stairs. The upstairs switch has gotten a bit wonky and needs to be replaced. Remembering from my youth (say 5th grade when I played with batteries. wires, and knife switches, I knew I needed a single pole, double throw switch. I confirmed it searching the very helpful internet.

I went down to the hardware store. The Leviton switches were in cages at stoop down height and below with distinguishing signs in mouse print. I found a "sales associate" to help me. Instead of a Leviton switch ($9-$19), he pawed through a tray of cheap, unbranded, China-made switches. When I told him I needed a single pole, double throw switch, he said he had never heard the term. When I described the situation at hand, he said I needed a 3-way switch, handed me a Chinese unit, and disappeared. I was pretty annoyed, but took the switch, paid $2.39, and left.

Back home, with circuit breaker turned off, I took off the faceplate, unscrewed the old switch, and pulled the unit out of the box. The old switch is from an earlier era, with terminals differently arranged. Nothing matched any of the diagrams I had found on the web. I put it all back together.

Today, I found confirmation that an SPDT and a 3-way are the same, and a diagram that I think matches my situation exactly. Yes, my house was built in 1958 and wired with BX or whatever it was called back then. I haven't gone back to it. My wife is in a state of total anxiety about this and other things.

I've always thought of SPDT as switches that are sometimes used in "three way" circuits.

So my question is: Why are "three way" circuits called that?

My best guess is it comes from the electricians trying to simplify things for themselves. A 3-way switch has 3 connections (neutral or hot, and two tracers), whereas a 4-way switch has 4 (4 tracers). (excluding ground).

If you tried defining the switch by the # of control locations you'd end up with 2-location for 3-way but that falls apart for 4-way since you can stick as many 4-way switches as you want between 2 3-ways as you want, although keeping track of the wiring and number of required wires gets messy fast. Back in my engineering school days, one of my professors liked handing out hypothetical corridor lighting arrangements with lights shown, and then having us locate a switch by each door at the ends and then draw out the circuiting with "tick" marks needed for the hot, neutral, tracers and grounds that would be required. It sometimes got really messy.

Thankfully now in most of what I do we skip all that, drop in a relay pack and use low-voltage switches for the really messy situations like that.

Switch installed per diagram. All functionality normal.

My father had a house with low voltage wiring and relays. It is a better system though my father had a relay fail and have to be replaced. The house's first owner was an EE.

SemiSalt wrote:Switch installed per diagram. All functionality normal.

My father had a house with low voltage wiring and relays. It is a better system though my father had a relay fail and have to be replaced. The house's first owner was an EE.

Semi, we have that same system. The previous owner and builder of the house started EG&G, an engineering firm started by a bunch of MIT guys, Edgerton, Germeshausen and Grier. Mr Germeshausen had a lot of interesting ideas. As far as I know the relays are original, but we replaced the switches, which are hard to find these days. House was built in 1960. Prof Edgerton invented the high speed photography to take these pictures, which are well known among older nerdy types like myself.

https://edgerton-digital-collections.org/docs-life/wartime-strobe

Yeah, the older relay based lighting control systems, especially on the residential side were a bit more one off custom type things up until the mid 2000's when the energy codes started coming into play (at least from an enforcement angle) and the various manufacturers started making more controls for the commercial side. Residential stuff is still lagging behind a bit due to less code pressure, but there are more options from manufacturers that have been around awhile. Every once and awhile I consider trying to move my house over to a full house lighting control system as I could probably get a decent discount through my work, but it would still be expensive and other than my own personal interest, probably wouldn't add much to resale.

Looking at whole-house lighting from a different perspective....

I think what will drive this is a combination of lower cost and consumer ease of use. It'll cost less, once folks can agree on a set of interface standards. Wires are smaller, switches are less expensive, etc... It will be MUCH easier to use for consumers. Indeed, it already is. ignoring what sort of wires are in the walls for now, as the installed base isn't going to change. None of my kids use "light switches" anymore. They have all replaced all that with much more expensive network-attached switches. They do things like: "Hey, Siri, turn on the lights in the kitchen." or "Hey, Siri, all lights on!" They do this for precisely the same reason all our sprinklers, outdoor lighting, door locks, etc... are on the network. They'll pay a LOT for greater ease of use. (Similar example: There isn't a "key" to a Tesla. You either have a fob or you have a phone. If you choose to use the phone the car wakes up when you walk up to it. You also get notifications if you've left a window open or the trunk lid slightly ajar.) While there are certainly still problems with some of the interoperability between phone, switches, outlets, etc... The fact that a significant percentage of younger folks are self-automating their homes is an indication of just how valuable that ease of use is to the consumer.

Keith - I had that picture of the bullet going through the apple as my screen saver for years. I called it: "The way we do product development." LOL!

BeauV wrote:Looking at whole-house lighting from a different perspective....

I think what will drive this is a combination of lower cost and consumer ease of use. It'll cost less, once folks can agree on a set of interface standards. Wires are smaller, switches are less expensive, etc... It will be MUCH easier to use for consumers.

What you describe with phones etc is using WiFi instead of wires.

Beau, Ann came home the other day with a story of helping a woman who'd locked her phone, i.e., her key, inside her Tesla. Ann offered her flip phone to call her husband, who of course didn't answer the call from a strange number.

kdh wrote:Beau, Ann came home the other day with a story of helping a woman who'd locked her phone, i.e., her key, inside her Tesla. Ann offered her flip phone to call her husband, who of course didn't answer the call from a strange number.

Our Prius won't lock with a fob in the car.

kdh wrote:Beau, Ann came home the other day with a story of helping a woman who'd locked her phone, i.e., her key, inside her Tesla. Ann offered her flip phone to call her husband, who of course didn't answer the call from a strange number.

I don't understand how one could do that. With our Tesla, it simply won't lock with the key/phone inside it. That is really weird.

All these teething problems aside, technology is rapidly moving towards very small devices serving as "keys". Just as we have small fobs that serve as "trackers", we'll eventually get to the point where we can us a watch or some piece of jewelry that identifies us to our various devices (cars and homes). Some have predicted that eventually, we'll simply implant an identity device that makes all our "stuff" work. One can only image the resistance to that sort of device from some folks.

Alexa....turn on the kitchen lights.

(although I have to spend a lot of time integrating all that too.) It is getting better though..now my Alexa app on my phone has started discovering new devices automagically after I install them via their native Chinese app. We have about 20 smart bulbs installed around, plus a few switches to power things like a bathroom light array, sprinkler and stuff like that. We had an older smart bulb start freaking out...could not display the desired color assigned to it at sunset, and it subsequently is also the porch light I use to cook on the grill with now that we are EST, so I removed the bad bulb from the list, changed the bulb out, gave it a temp name, which Alexa app promptly noticed, and then changed the name back after deleting the "unresponsive" bulb and re-added it to the proper group.
Laura likes to sit in the living room and say things like "Alexa, change the outside lights to Blue (red) (green) etc. and giggles when it actually works.
I just need the kitchen counter LEDs to light up as I tell Alexa to turn on the counter lights when I am trying to find the coffee in the morning!

The biggest issue with technology like this is the actual fixtures are still generally powered by conventional switches..just like Semi's dilemma. When friends come by to check on the cat, they forget and start throwing switches, which messes everything up .

Oh, three and four way 110v AC switches always baffle me.

For the record, my wife got us that Alexa light switch stuff for most of the lights in our house. She loves it and uses it constantly.

I walk over and use my finger.

BeauV wrote:

kdh wrote:Beau, Ann came home the other day with a story of helping a woman who'd locked her phone, i.e., her key, inside her Tesla. Ann offered her flip phone to call her husband, who of course didn't answer the call from a strange number.

I don't understand how one could do that. With our Tesla, it simply won't lock with the key/phone inside it. That is really weird.

All these teething problems aside, technology is rapidly moving towards very small devices serving as "keys". Just as we have small fobs that serve as "trackers", we'll eventually get to the point where we can us a watch or some piece of jewelry that identifies us to our various devices (cars and homes). Some have predicted that eventually, we'll simply implant an identity device that makes all our "stuff" work. One can only image the resistance to that sort of device from some folks.

I wonder if the phone's battery was dead. My pacemaker already talks to my phone, so theoretically it could talk to anything.

BeauV wrote:

kdh wrote:Beau, Ann came home the other day with a story of helping a woman who'd locked her phone, i.e., her key, inside her Tesla. Ann offered her flip phone to call her husband, who of course didn't answer the call from a strange number.

I don't understand how one could do that. With our Tesla, it simply won't lock with the key/phone inside it. That is really weird.

I know I'm very late to this thread, but I had to laugh when I read this. When I read KDH's comment, my first thought was "that shouldn't happen..." but my immediate second thought was "considering that roughly 25% of the time that I approach my car with my phone in pocket, it does not actually let me open the door. I have to admit that I actually *do* see how it can happen.

I have a sneaking suspicion that the up-market S and X may simply have a much better system for phone-keys & fob-keys. Then maybe Tesla dumbed it down to protect margins on the 3 & Y. The unreliability of the locking system is probably my #3 pet peeve about my Tesla.

What I have found is that if I try different door handles, that seems to trigger something that results in it "waking up" and unlocking. I have in fact left my phone in my car, and had it lock, and did this to get in. Note that I keep one of the "key cards" in my wallet at all times, so I'm never actually screwed when this happens. I just don't like getting using the key card, it is finicky in its own way, and it appears that when I use the key card to unlock the car, then "walk away door locks" doesn't work (perhaps it's trying to protect me from leaving the key card in the car).