Control one light from two circuits

mbailey5 · Nov 29, 2021

I have four exterior 110v light fixtures in the front of my house in series on a single switch. On a different circuit, I have six 110v back yard light fixtures in parallel on a single switch. I want to add another light fixture on the side of the house that will turn on if either or both existing switches are on. I tried wiring the side yard light independently with a motion sensor, but tree branches in the breeze and neighboring lights caused frequent false activations. Turning down the sensitivity with Arizona temperatures near 100 caused the light not to activate. Can I wire connections to the side light from both the front and back yard circuits in parallel with the existing lights with relays from the first circuit that interrupt the line and neutral connections of the second circuit when the first switch is on? Is there an alternate or better way to allow either or both switches to turn on the side light?

mbailey5 · Dec 7, 2021

I received no responses so I came up with a proposed circuit. The following details my issue, attaches the proposed circuit, and asks for input. Thanks for any input you can give me:

Goal: Control one outdoor 110vac light fixture from switches in two different circuits so the light fixture is on if either or both switches are on. This is not a 3-way switch situation because two different circuits are involved.

Scenario (see proposed circuit diagram below and attached):

I have four outdoor 110vac light fixtures (Light 1) in my front yard controlled by Switch 1 on Circuit 1.
I have six outdoor 110vac light fixtures (Light 2) in my back yard controlled by Switch 2 on Circuit 2.
I added an outdoor 110vac light fixture (Light 3) in my side yard that I want to turn on if either or both Smart Switch 1 and/or Smart Switch 2 are turned on.
Light 3 was originally controlled by a motion sensor but tree branches in the breeze and my neighbor’s lights constantly triggered it and reducing the motion detector sensitivity caused it to turn on late or not at all.

Experience: I am not an electrician but I’m familiar and comfortable with most home electrical circuits. I took electrical classes in school and was an amateur radio enthusiast in the 60’s and 70’s, building my own stations (vacuum tubes, transistors, solid state). However, I have never worked with relays.

Circuit diagram explanation and operation (see below and attached):

Circuits 1 and 2 are separate 110vac circuits.
Smart Switch 1 is at the front of the house and controls the front yard lights.
Light 1 is actually four lights but appears as a single component for simplicity.
Smart Switch 2 is at the back of the house and controls the back yard lights.
Light 2 is actually six lights but appears as a single component for simplicity.
Relays 1 and 2 are normally closed, allowing current flow when not energized.
Relays 3 and 4 are normally open, blocking current flow when not energized.
In the proposed circuit, assume that Smart Switches 1 and 2 are initially off:
- a. When Smart Switch 1 is turned on:
  - i. Current flows through Relay 1 and energizes Relays 2 and 3.
  - ii. Relay 2 opens, isolates Light 3 from Circuit 2, and prevents Relay 1 from being energized.
  - iii. Relay 3 closes and powers Light 3.
  - iv. Relay 4 is open and prevents current flow from Circuit 1 past Light 3 that would energize Relay 1.
- b. Turn on Smart Switch 2 at this point. Only Light 2 is powered by Circuit 2 because Relay 2 is open.
- c. Turn off Smart Switch 1 at this point (while Smart Switch 2 is still turned on):
  - i. Relays 2 and 3 are de-energized.
  - ii. Relay 2 closes, allowing current from Circuit 2 to energize Relays 1 and 4.
    - 1. Relay 1 opens, isolates Light 3 from Circuit 1, and prevents Relay 2 from being energized.
    - 2. Relay 3 opens and prevents current flow from Circuit 2 past Light 3 that would energize Relay 2.
    - 3. Light 3 is no longer powered by Circuit 1.
    - 4. Relay 4 closes and powers Light 3.
- d. The circuitry after Light 1 and Light 2 is identical so the same logic applies if Smart Switch 2 is turned on first.

Concerns and questions:

Is there an alternate method or circuit for powering Light 3 from either or both circuits?
Is my logic in the “Circuit diagram explanation and operation” section correct?
If both Smart Switch 1 and Smart Switch 2 are turned on at exactly the same time:
- a. Current initially flows through both Relay 1 and Relay 2, energizing both Relay 3 and Relay 4.
- b. This would send power to Light 3 from both Circuit 1 and Circuit 2.
- c. At the same time, both Relay 1 and Relay 2 are energized.
  - i. This opens both relays which immediately de-energizes and closes both relays (current flow through each relay energizes and opens the other relay).
  - ii. If perfect synchrony occurs, Relays 1 and 2 would enter an infinite loop of energizing and de-energizing each other as well as the later relays in the circuit (Relays 3 and 4).
- d. Could the issues in 3b and 3c be addressed by using fast relays for Relays 1 and 3 and using delayed relays for Relays 2 and 4?
I’m not an electrician and have not worked with relays previously. What make/models would you recommend for Relays 1, 2, 3, 4?

Proposed circuit diagram:
Same as attached file

brianmoooore · Dec 7, 2021

First though is that relays need mechanical interlock to prevent any possibility of cross connecting supplies.
Would be a lot simpler if light 3 was actually two lights, mounted beside each other, and one fed from each circuit.

Avo Mk8 · Dec 7, 2021

I did see your original post, but I'm afraid I wasn't able to respond.

With regard to your proposal above, I do agree with brianmoooore's concern that there's the possibility of the two sources of supply getting connected together. Eg if any make contacts close before break contacts open. Presumably these two 'hots' are from the same phase - there's not 220V between them?!!

Not knowing the lamp characteristics/currents you need to switch, the physical distances involved, or the practicality of getting a permanent live/independent feed to Light 1, I realise the suggestion below may not be practical or attractive.

How about just a wireless receiver switch/module in each circuit, and then Kinetic type (no batteries or wired connections) transmitter switches, each paired with the combination(s) of lights you need, and put them wherever you need them?
The Quinetic products in the UK that I'm thinking of may not be available to you, but there seem to be equivalent products around.

mbailey5 · Dec 7, 2021

Brian Moore and Avo Mk8 - Thank you both for your input. Circuit crossover based on relay timing was what I was worried about but I was unfamiliar with mechanical interlocks. I'm looking into that now. I had suggested adding an additional light fixture so each one is connected to a different circuit but my wife was unhappy with the esthetics. I'm not sure how to integrate a wireless switch into the existing smart home system (Alexa) which currently uses smart dimmer switches.

Avo Mk8 · Dec 8, 2021

mbailey5 said:
I'm not sure how to integrate a wireless switch into the existing smart home system (Alexa) which currently uses smart dimmer switches.

In principle, if you used another smart switch/dimmer module (the same type as the ones you already have) to control your new floodlight, you will be able to combine the operation of the lights as you want.

Your existing smart switches either have an app, in which case you can group lights there, or your switches are directly controlled by Alexa voice command, in which case you use Alexa groups.

You just put a smart switch on the new floodlight, and set the light combinations you want as groups.

I didn't understand what was already in place, so I'm sorry to have complicated the concept by suggesting additional hardware. You already have 2/3rds of what is needed!
I really feel this would be easier than building the hardware version!

What Are Smart Home Groups? - Amazon Customer Service

Smart home groups let you use Alexa to control multiple devices at once.

www.amazon.com

Megawatt · Dec 8, 2021

mbailey5 said:
I received no responses so I came up with a proposed circuit. The following details my issue, attaches the proposed circuit, and asks for input. Thanks for any input you can give me:

Goal: Control one outdoor 110vac light fixture from switches in two different circuits so the light fixture is on if either or both switches are on. This is not a 3-way switch situation because two different circuits are involved.

Scenario (see proposed circuit diagram below and attached):

I have four outdoor 110vac light fixtures (Light 1) in my front yard controlled by Switch 1 on Circuit 1.

I have six outdoor 110vac light fixtures (Light 2) in my back yard controlled by Switch 2 on Circuit 2.

I added an outdoor 110vac light fixture (Light 3) in my side yard that I want to turn on if either or both Smart Switch 1 and/or Smart Switch 2 are turned on.

Light 3 was originally controlled by a motion sensor but tree branches in the breeze and my neighbor’s lights constantly triggered it and reducing the motion detector sensitivity caused it to turn on late or not at all.

Experience: I am not an electrician but I’m familiar and comfortable with most home electrical circuits. I took electrical classes in school and was an amateur radio enthusiast in the 60’s and 70’s, building my own stations (vacuum tubes, transistors, solid state). However, I have never worked with relays.

Circuit diagram explanation and operation (see below and attached):

Circuits 1 and 2 are separate 110vac circuits.

Smart Switch 1 is at the front of the house and controls the front yard lights.

Light 1 is actually four lights but appears as a single component for simplicity.

Smart Switch 2 is at the back of the house and controls the back yard lights.

Light 2 is actually six lights but appears as a single component for simplicity.

Relays 1 and 2 are normally closed, allowing current flow when not energized.

Relays 3 and 4 are normally open, blocking current flow when not energized.

In the proposed circuit, assume that Smart Switches 1 and 2 are initially off:

a. When Smart Switch 1 is turned on:

i. Current flows through Relay 1 and energizes Relays 2 and 3.

ii. Relay 2 opens, isolates Light 3 from Circuit 2, and prevents Relay 1 from being energized.

iii. Relay 3 closes and powers Light 3.

iv. Relay 4 is open and prevents current flow from Circuit 1 past Light 3 that would energize Relay 1.

b. Turn on Smart Switch 2 at this point. Only Light 2 is powered by Circuit 2 because Relay 2 is open.

c. Turn off Smart Switch 1 at this point (while Smart Switch 2 is still turned on):

i. Relays 2 and 3 are de-energized.

ii. Relay 2 closes, allowing current from Circuit 2 to energize Relays 1 and 4.

1. Relay 1 opens, isolates Light 3 from Circuit 1, and prevents Relay 2 from being energized.

2. Relay 3 opens and prevents current flow from Circuit 2 past Light 3 that would energize Relay 2.

3. Light 3 is no longer powered by Circuit 1.

4. Relay 4 closes and powers Light 3.

d. The circuitry after Light 1 and Light 2 is identical so the same logic applies if Smart Switch 2 is turned on first.

Concerns and questions:

Is there an alternate method or circuit for powering Light 3 from either or both circuits?

Is my logic in the “Circuit diagram explanation and operation” section correct?

If both Smart Switch 1 and Smart Switch 2 are turned on at exactly the same time:

a. Current initially flows through both Relay 1 and Relay 2, energizing both Relay 3 and Relay 4.

b. This would send power to Light 3 from both Circuit 1 and Circuit 2.

c. At the same time, both Relay 1 and Relay 2 are energized.

i. This opens both relays which immediately de-energizes and closes both relays (current flow through each relay energizes and opens the other relay).

ii. If perfect synchrony occurs, Relays 1 and 2 would enter an infinite loop of energizing and de-energizing each other as well as the later relays in the circuit (Relays 3 and 4).

d. Could the issues in 3b and 3c be addressed by using fast relays for Relays 1 and 3 and using delayed relays for Relays 2 and 4?

I’m not an electrician and have not worked with relays previously. What make/models would you recommend for Relays 1, 2, 3, 4?

Proposed circuit diagram:
Same as attached file
View attachment 92706

Why don’t you just change out your switches and install 3 way switches and use the NO screw and see if that works. Sorry for the delay, we are having issues with this forum and I can’t reply to questions on the original post. I have to wait for someone else to reply like you did on your second post. We apologize for the inconvenience.

EyelashViper · May 13, 2022

I just ran across this thread as I'm attempting to do the same thing: control one light from two other light circuits. However, this is new wiring and, therefore, it seems simple to accomplish by adding a 3rd circuit (to power the new light).

I'm thinking I simply wire up a logical OR circuit using two AC relays, each with a N.O. output. Each relay's input is wired in parallel with an existing light - if the light is on, the relay will close its output. Then the outputs of the relays are wired in parallel and break/close the hot leg of the new (3rd) circuit which feeds the new light.

With this, the circuits are all isolated. The new circuit is switched only by the relays (either one will close the circuit and the state of the other is irrelevant).

If you agree this is no problem (the real problem the OP had was only have the two existing circuits), I'd appreciate you letting me know. If you see a problem with this, please share that as well. Thanks!

Avo Mk8 · May 13, 2022

Are your "two other light circuits" on different breakers at the board - ie completely independent circuits?
If so I believe your proposal is OK, as long as it is safely done with correctly rated parts.

If the "two other light circuits" are actually fed from the same breaker/source, I think you could implement the 'OR' function by replacing your two existing light switches with double pole versions, putting the second poles in parallel to the new light.

Double-pole switches are easily available here in the UK, but don't know the situation where you are!

EyelashViper · May 16, 2022

Avo Mk8 said:
Are your "two other light circuits" on different breakers at the board - ie completely independent circuits?
If so I believe your proposal is OK, as long as it is safely done with correctly rated parts.

If the "two other light circuits" are actually fed from the same breaker/source, I think you could implement the 'OR' function by replacing your two existing light switches with double pole versions, putting the second poles in parallel to the new light.

Double-pole switches are easily available here in the UK, but don't know the situation where you are!

The "two other circuits" are each on their own breaker.

BTW, as I was wiring this for testing, I realized that there is no need for a third circuit. It looks like the OP can easily accomplish what is desired with 2 N.O. SSRs. The third light can be powered from either of the existing circuits (an unswitched tap) as the third light is just a parallel circuit on whichever original circuit you choose. I'm not sure why the complexity of the OPs diagram, but here's what I'm doing and there is complete isolation as far as I can tell (of course using SSRs does mean a small leakage current on the open output, but with typical lights, I don't think this is a concern).

The below turns on light 3 when either or both of lights 1 and 2 are on.

mbailey5 · May 27, 2022

Sorry to be so late updating this but I solved my problem with a much simpler circuit that always powers the side lights from Circuit 1. Instead of trying to use multiple relays to switch between circuits, a single relay is used to bypass the Circuit 1 switch when the Circuit 2 switch is on. I specifically wanted to avoid a 3-way switch because the side lights are not visible from the front and back so you would have no way to know if you were turning them on or off or whether they had been left on. This circuit simplified things so that the side lights are on if either the front or back light switches are on and the side lights are off when both front and back light switches are off, which is what I wanted. It works like a charm.

Megawatt · May 27, 2022

mbailey5 said:
Sorry to be so late updating this but I solved my problem with a much simpler circuit that always powers the side lights from Circuit 1. Instead of trying to use multiple relays to switch between circuits, a single relay is used to bypass the Circuit 1 switch when the Circuit 2 switch is on. I specifically wanted to avoid a 3-way switch because the side lights are not visible from the front and back so you would have no way to know if you were turning them on or off or whether they had been left on. This circuit simplified things so that the side lights are on if either the front or back light switches are on and the side lights are off when both front and back light switches are off, which is what I wanted. It works like a charm.
View attachment 97977

We’ll my friend it seems like you have it under control. Nice work

Control one light from two circuits

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