4 ms·

For some reason, last year I tried to rederive the circuit diagram for 3 way switches. I failed. It's actually a very clever idea, and higher numbers are fun to

by sn41 5y ago

For some reason, last year I tried to rederive the circuit diagram for 3 way switches. I failed. It's actually a very clever idea, and higher numbers are fun to think about independently.

bombcar 5y ago
There's two way, three way, and four way, and four way can support arbitrary numbers of "ways" - because a path is always found on one flip of the switch and not on the other.
- OJFord 5y ago
  No, GP is responding to the 'multiway switching' link in the top-level comment. This is about where, in the UK and I'm sure most developed jurisdictions, stairs for example must have a both on and off control at both ends - regardless of the state of the other switch. If they were simply in parallel as you describe, you could find yourself in the situation of going upstairs to turn it 'off up there', (because it'd need to be 'off everywhere' to be off) then realising it was already 'off downstairs' and falling back down. If they were simply in series, you could find yourself unable to turn the light on without first going downstairs at speed in the dark, because it's 'off there' no matter what you do upstairs.
  - sn41 5y ago
    yes... A way to look at 2 way switches is to look at the connections of usual circuit setups to Boolean logic. Taking two on-off switches, - parallel is or - series is and We can see that a 2 way switch is xor (or xnor depending on the setup). So now, for 3 way it feels like you have to beyond 2 state (on-off) switches... i don't have a proof of impossibility that 2 state switches cannot do 3 way switching, but in practice they indeed involve more states (multi-throw etc.)
    - OJFord 5y ago
      The closest you can get with '2 state' (by which you mean SPST - single pole single throw; SPDT is the standard way to achieve this but is usually still '2 state', it's just that 'off' (if you like) has a terminal too) switches is to add a 'don't care' state, and accept that first deliberate ('do care') switch wins. To do this you have a live rail and a switched live rail, and two SPST each end: one switch SL hot or not; the other to break its connection to the light. Open the latter for 'off', close both for 'on', close break & open SL for 'don't care'. (Then in the 'don't care' state, SL is hot iff switched on the other end, and if so is also closed through to the light.) Not legal, not (that I can imagine) desirable! Proving it is actually simple if you continue your Boolean line of thought: you can't build XOR without an inverter; an SPST switch is only a sort of 'half inverter', a mux between '0' and its input. Since it can never give you '1' from '0' input, you need a schematic for your XOR where each constituent truth table in isolation has '0' out for its '0;0' in line. Which can't be done, because clearly then you could never get '0;0=>1' overall.
  - twic 5y ago
    The key thing is that it's a double-pole, double-throw switch. You can put any number of them in series, and the circuit essentially counts the parity of the switches, so flipping any of them toggles the state of the lights.
    - OJFord 5y ago
      If it's only two switches, you only need SPDTs ('hot' & 'light' on each, throws connected, then toggling either switch takes either to the other).
  - bombcar 5y ago
    That's what a 3-way or 4-way (should be called X-way) switch setup is - three or four or more switches that no matter what the state of the light is, flipping ANY switch will change the state. https://www.electrical101.com/4way-switch-wiring-using-nm-cable.html https://www.electrical101.com/4way-switch-wiring-using-nm-ca...
    - OJFord 5y ago
      Sorry then, I misunderstood what you meant by: > a path is always found on one flip of the switch I thought you were taking the comment you replied to to be seeking a simple parallel arrangement of SPST switches, so that turning any one on would turn the light on. So I was explaining that No, point is you want 'off' on any switch (regardless of the state of the others) too. Seems you understood that though. :)
JKCalhoun 5y ago
They annoy me simply because I prefer the "up" position to mean "on" and "down" to indicate "off". Completely arbitrary on a two way (wait, they're called three way?) switches.
- mopsi 5y ago
  Get momentary toggle switches that stay in "off" position and toggle the light when pushed to "on" position.
- toast0 5y ago
  The switch way numbering is based on the number of terminals. A 2-way switch has two terminals and gives a closed (on) or open circuit. A 3-way switch has three terminals and gives you one terminal tied to either, but not both of the other two. A 4-way switch has four termimals and gives you two terminals which are each tied to the other two terminals and you can pick; these also give you a big headache when one switch of many fails and one circuit path is open, but many paths still work.
  - mauvehaus 5y ago
    Or in other words if you're used to seeing switches from the electronics side of the world: 2 way: SPST 3 way: SPDT 4 way: DPDT I once lived in a house with a 5(!) gang box with three of the switches wired to the upstairs hall light incorrectly and another switch upstairs. It was as big a mess as you'd imagine.
- D13Fd 5y ago
  There are electronic three-way switches that have either a single button or an up/down rocker that always reacts the same way (up for on, down for off). These are available in "smart"/connected form or regular electronics-only form. Example here: https://www.amazon.com/Lutron-Incandescent-Single-Pole-Multi-Location-MACL-153M-RHW-WH/dp/B008X3CH70/ref=sr_1_3?dchild=1&keywords=lutron+three+way+dimmer&qid=1628275248&sr=8-3 https://www.amazon.com/Lutron-Incandescent-Single-Pole-Multi... I agree with you and much prefer them to the traditional switches.
sebzim4500 5y ago
Isn't it just a few XOR gates? I'm probably missing something, but they aren't that hard to make out of AND and OR gates, or even out of transistors.
- jcranmer 5y ago
  These circuits don't run on digital logic, but are instead based on physically connecting and disconnecting an electrical connection between the live and neutral wires. This means you have to work out how to use the two switches to produce a connected circuit when only one but not both switch is in the up position. Digital logic works on lower voltage than mains voltage, and you'd need to maintain far more wires to hook up the connection (since the actual gates themselves need both power supply and ground drain wires which are separate for the inputs). Possibly also a relay to actually drive the higher voltage/amperage for the light bulb.

by sn41 5y ago

For some reason, last year I tried to rederive the circuit diagram for 3 way switches. I failed. It's actually a very clever idea, and higher numbers are fun to think about independently.

bombcar 5y ago

There's two way, three way, and four way, and four way can support arbitrary numbers of "ways" - because a path is always found on one flip of the switch and not on the other.

OJFord 5y ago
No, GP is responding to the 'multiway switching' link in the top-level comment. This is about where, in the UK and I'm sure most developed jurisdictions, stairs for example must have a both on and off control at both ends - regardless of the state of the other switch. If they were simply in parallel as you describe, you could find yourself in the situation of going upstairs to turn it 'off up there', (because it'd need to be 'off everywhere' to be off) then realising it was already 'off downstairs' and falling back down. If they were simply in series, you could find yourself unable to turn the light on without first going downstairs at speed in the dark, because it's 'off there' no matter what you do upstairs.
- sn41 5y ago
  yes... A way to look at 2 way switches is to look at the connections of usual circuit setups to Boolean logic. Taking two on-off switches, - parallel is or - series is and We can see that a 2 way switch is xor (or xnor depending on the setup). So now, for 3 way it feels like you have to beyond 2 state (on-off) switches... i don't have a proof of impossibility that 2 state switches cannot do 3 way switching, but in practice they indeed involve more states (multi-throw etc.)
  - OJFord 5y ago
    The closest you can get with '2 state' (by which you mean SPST - single pole single throw; SPDT is the standard way to achieve this but is usually still '2 state', it's just that 'off' (if you like) has a terminal too) switches is to add a 'don't care' state, and accept that first deliberate ('do care') switch wins. To do this you have a live rail and a switched live rail, and two SPST each end: one switch SL hot or not; the other to break its connection to the light. Open the latter for 'off', close both for 'on', close break & open SL for 'don't care'. (Then in the 'don't care' state, SL is hot iff switched on the other end, and if so is also closed through to the light.) Not legal, not (that I can imagine) desirable! Proving it is actually simple if you continue your Boolean line of thought: you can't build XOR without an inverter; an SPST switch is only a sort of 'half inverter', a mux between '0' and its input. Since it can never give you '1' from '0' input, you need a schematic for your XOR where each constituent truth table in isolation has '0' out for its '0;0' in line. Which can't be done, because clearly then you could never get '0;0=>1' overall.
- twic 5y ago
  The key thing is that it's a double-pole, double-throw switch. You can put any number of them in series, and the circuit essentially counts the parity of the switches, so flipping any of them toggles the state of the lights.
  - OJFord 5y ago
    If it's only two switches, you only need SPDTs ('hot' & 'light' on each, throws connected, then toggling either switch takes either to the other).
- bombcar 5y ago
  That's what a 3-way or 4-way (should be called X-way) switch setup is - three or four or more switches that no matter what the state of the light is, flipping ANY switch will change the state. https://www.electrical101.com/4way-switch-wiring-using-nm-cable.html https://www.electrical101.com/4way-switch-wiring-using-nm-ca...
  - OJFord 5y ago
    Sorry then, I misunderstood what you meant by: > a path is always found on one flip of the switch I thought you were taking the comment you replied to to be seeking a simple parallel arrangement of SPST switches, so that turning any one on would turn the light on. So I was explaining that No, point is you want 'off' on any switch (regardless of the state of the others) too. Seems you understood that though. :)

JKCalhoun 5y ago

They annoy me simply because I prefer the "up" position to mean "on" and "down" to indicate "off". Completely arbitrary on a two way (wait, they're called three way?) switches.

mopsi 5y ago
Get momentary toggle switches that stay in "off" position and toggle the light when pushed to "on" position.
toast0 5y ago
The switch way numbering is based on the number of terminals. A 2-way switch has two terminals and gives a closed (on) or open circuit. A 3-way switch has three terminals and gives you one terminal tied to either, but not both of the other two. A 4-way switch has four termimals and gives you two terminals which are each tied to the other two terminals and you can pick; these also give you a big headache when one switch of many fails and one circuit path is open, but many paths still work.
- mauvehaus 5y ago
  Or in other words if you're used to seeing switches from the electronics side of the world: 2 way: SPST 3 way: SPDT 4 way: DPDT I once lived in a house with a 5(!) gang box with three of the switches wired to the upstairs hall light incorrectly and another switch upstairs. It was as big a mess as you'd imagine.
D13Fd 5y ago
There are electronic three-way switches that have either a single button or an up/down rocker that always reacts the same way (up for on, down for off). These are available in "smart"/connected form or regular electronics-only form. Example here: https://www.amazon.com/Lutron-Incandescent-Single-Pole-Multi-Location-MACL-153M-RHW-WH/dp/B008X3CH70/ref=sr_1_3?dchild=1&keywords=lutron+three+way+dimmer&qid=1628275248&sr=8-3 https://www.amazon.com/Lutron-Incandescent-Single-Pole-Multi... I agree with you and much prefer them to the traditional switches.

sebzim4500 5y ago

Isn't it just a few XOR gates? I'm probably missing something, but they aren't that hard to make out of AND and OR gates, or even out of transistors.

jcranmer 5y ago
These circuits don't run on digital logic, but are instead based on physically connecting and disconnecting an electrical connection between the live and neutral wires. This means you have to work out how to use the two switches to produce a connected circuit when only one but not both switch is in the up position. Digital logic works on lower voltage than mains voltage, and you'd need to maintain far more wires to hook up the connection (since the actual gates themselves need both power supply and ground drain wires which are separate for the inputs). Possibly also a relay to actually drive the higher voltage/amperage for the light bulb.