# Home automation: Adding relays in series vs parallel



## jiyeon (Feb 23, 2017)

Recently I bought an Arduino Uno to discover electronics. I am a computer science student, hence the programming part is not a problem, however, I lack some knowledge about electronics.

Now I want to do some simple home automation, mainly to control the lights in my house. I was thinking about adding an Arduino controlled relay (in the central electrical box of my house) for each light I want to control.

However, for each light in my house I have a switch. Suppose, I'd add Arduino controlled relays in series with the switch (as in the figure below):








The problem is that with the relays being in series, I can only switch on and off the light if the switch is closed. Hence, like above, the switch is open and my Arduino controlled relay will have no effect.
Now, suppose I'd add the Arduino controlled relays in parallel:








In parallel, we observe the inverse problem. If the switch is open, we are able to control the light using the Arduino controlled relay. However, if the switch is closed (like above), the current will flow through it anyway, i.e. the light will glow without the Arduino controlled relay having any effect.

How does one tackle this problem in home automation systems, without the need to rewire my entire house?
PS : Excuse me if the diagrams are wrong in some way. I absolutely have no electronics background.


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## Bob Sanders (Nov 10, 2013)

Use a double throw relay.


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## LawnGuyLandSparky (Nov 18, 2007)

A double throw relay won't all you to turn off locally.


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## LawnGuyLandSparky (Nov 18, 2007)

jiyeon said:


> Recently I bought an Arduino Uno to discover electronics. I am a computer science student, hence the programming part is not a problem, however, I lack some knowledge about electronics.
> 
> Now I want to do some simple home automation, mainly to control the lights in my house. I was thinking about adding an Arduino controlled relay (in the central electrical box of my house) for each light I want to control.


That won't work. Entire circuits are fed from your circuit breaker panel, not individual lights.


> However, for each light in my house I have a switch. Suppose, I'd add Arduino controlled relays in series with the switch (as in the figure below):
> 
> 
> 
> ...


You mean open.


> Hence, like above, the switch is open and my Arduino controlled relay will have no effect.


 You mean closed.


> Now, suppose I'd add the Arduino controlled relays in parallel:
> 
> 
> 
> ...


The method you're trying to implement requires rewiring your entire house. It also needs a means to integrate local control. 
I strongly suggest you learn a lot more about home automation systems before you start installing what will amount to be a useless "improvement."



> PS : Excuse me if the diagrams are wrong in some way. I absolutely have no electronics background.


Relay controlled automated lighting is good for places like airports or train stations. But in a home, you need local switches, dimmers, speed control (ceiling fans) and quite frankly, hooking all that up to a computer is IMHO a colossal waste of money. So you can whip out your iPhone and turn your living room lights on and off. Big f'n deal, I'm not impressed and the feature, though novel, is of practical use to no one.

This is a good example of "just because you can, doesn't mean you should."


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## Bob Sanders (Nov 10, 2013)

LawnGuyLandSparky said:


> A double throw relay won't all you to turn off locally.


If the idea is to turn off a manual switch remotely then yes, I agree. You can't do it.

The only thing I could suggest aside from rewiring the house is a timer switch or a motion switch. This would ensure the light goes out after a period of time (or when no motion is detected) and therefore can't be left on. At least the light won't glow all night long if someone forgets to turn off the manual switch. If you need it on after the switch times out then of course you can trigger it remotely.

The relay would be hooked up as normal in parallel.


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## user_12345a (Nov 23, 2014)

you could do a double throw light switch with a single pole double throw relay.

They do the same with 3-way switch setups, but with two double throw switches instead of the relay.


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## LawnGuyLandSparky (Nov 18, 2007)

user_12345a said:


> you could do a double throw light switch with a single pole double throw relay.
> 
> They do the same with 3-way switch setups, but with two double throw switches instead of the relay.


Which means that an "on" command via the home automation would translate to turning the device off if it is already on. And vice versa. 

This is progress? This is energy savings? This is convenience? I'm not seeing it.


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## user_12345a (Nov 23, 2014)

there would have to be a way to monitor current to see if it's on or not. i think there r current sensing relays - would initiate another input.

To be honest i don't think any home automation stuff beyond basic timers and programmable t-stats has anything to do with progress or saving energy or makes life easier.

more like toys for rich people.

i'm all for experimenting just to learn though.


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