enter image description hereI'm thinking of building my own alarm clock because none of the one's I've found are meeting my needs, and because it's kind of fun. I am a bit out of my depth on at least on part though, and I want to make certain I don't do something unsafe. I'm not sure whether or not I can use a common neutral line for two circuits. There are two diagrams I'll try to attach to this post that should explain the two options I'm looking at. Finding double-pole switches is a little more difficult than single-pole, and a bit more wiring (not much of a problem for wiring though).

I'm kind of worried about a neutral line being shared because if one alarm device is activated, all the neutral returns will be 'returning' (for lack of certainty of a term to use). Since multiple devices could (and probably would in some circumstances) be plugged in to outlets that have 'returning' neutral lines and share a common but unheated hot line. Might the current be driven backwards through a device in this scenario (ie, Each Switch Controls Only Hot, Timer-less Feed Open, Timer 2 Open, Timer 1 Closed, Circuit 1 fed by Timer 1, Circuit 2 fed by timer 2).

Is there a great danger in having a store-bought wall timer plugged in but with the neutral and possible sometimes the hot wire both 'returning' and hot due to the devices they're connected to receiving power from the other timer or from the timerless-activation switch?

Also, is there something I missed with my design in terms of safety or functionality (or sanity)?

Is there such thing as an anti-backflow device for AC? Or for DC for that matter?

Explanation of the system:

  1. I want an alarm to go off with a bright light and a loud noise and maybe a strobe light and whatever else I might want to add later.

This is what Timer 1 is for. The night before I will set Timer 1 to the desired time, and will set Circuits 1 and 2 to be fed by Timer 1.

  1. I want to be able to turn off the bright light and loud noise while still leaving enough light to see.

The loud obnoxious items will be set to Circuit 1. The USB light will be set to Circuit 2. When the alarm goes off, I will make my way across by bedroom to quickly switch Circuit 1's feed from Timer 1 to Timer 2. Since Timer 2 is neither on nor is the switch for Timer 2 connected to bother Circuit 1 and Circuit 2 at the same time (... unlike a previous design iteration...), Circuit 1 will no longer be being fed power, and thus the obnoxious items will stop making me want to perform ungodly acts of destruction upon them.

  1. I then want to take medication set near the system the night before. I will then set Circuit 2 to be fed by Timer 2 instead of Timer 1, thus deactivating the USB light.

  2. I may want to add in a device outside of these circuits (like how the projector alarm clock and Wall-to-USB adaptor are wired in) that allows me to turn on a light for 30 seconds or a couple minutes until I'm back in safely bed (stubbed toes hurt). I might use the kind of timer you see in some bathrooms that activate a heat lamp or a fan by turning the knob to the desired amount of time, after which the lamp or fan turn off.

  3. I then want to go back to sleep until some time I'd set the night before (maybe half an hour later, maybe 5 hours, whatever).

This is what the second timer is for. Circuits 1 and 2 will have been set to be fed by Timer 2 when I got up for Timer 1, and so when timer 2 goes off, so does the alarm again.

  1. I want to be able to test the alarms before I go to bed without having to switch the timers themselves to "Always On" and then back to "Timer" mode.

This is what the Momentary-On switch before each timer is for (I press it and it closes the circuit as though the timer had gone off).

QUESTION: If I press the Momentary-On switch, creating a bypass with near-zero resistance, would this de-power my timer? I suspect yes. Also, if the timer were on and I press the Momentary-On switch, would this wind up with insufficient power through the timer, quite possibly buggering it up? I hope not but suspect yes...

  1. I want to be able to activate the items throughout the day without having to turn on the timers.

This is accomplished by a switch.

  1. I don't want to light anyone or anything on fire, nor do I want to electrocute them, unless they're reeeeeally really annoying me.

  2. I'm trying to make this portable (although I suppose that term means different things to different people). I'm not certain in what to place the switches I buy (probably not light switches, more like the kind of switches you see on, for example, a coffee maker), but I've found a wooden box from Dollarama that might make a good thing to use for a first try, since I have no idea what else to use. Any ideas? I seem to remember/remember reading of controls being placed in wood back in the day?

Location: Alberta, Canada

6 Answers 6


Just the hot.

Just switch the hot.

  • 1
    Always switch the hot. Never neutral or ground. You always want a safe path through the wiring to ground for a compromised circuit, never through you.
    – Suncat2000
    Apr 2, 2019 at 16:03

Orision, My take on this is that you understand what you want pretty well, and understand electricity only at a very surface level. That's not a great mix.

My suggestion would be for you to do some googling on home automation products and see if you can come up with a way to set up a system that accomplishes this with off the shelf home automation products and some clever computer based control.

If each of the items you are turning on and off were controlled by home-automation switches, and if you were to have a number of separate simple controller switches for each of the modes you describe above, you could accomplish your aims with no actual electrical knowledge. You would just have to become somewhat proficient at setting up and configuring whatever home automation system you decided to work with.

Probably a bit more expensive this way...but much safer.

  • Thank you for your reply, Jeff! What is it that makes you think I don't understand electricity beyond a surface level? I am not particularly interested in an extremely expensive alternate solution, but I thank your for your suggestion. I am interested in whether or not my design is safe if assembled in either one or both forms shown. If you know them to be unsafe, then I am very interested in the reason. Is there something wrong with my "Each Switch Controls Both Hot and Neutral" design?
    – Orision
    May 17, 2015 at 20:31

There's a lot going on here, and I think we could spend a lot of time drawing out the threads of this design and implementation; however, to answer your specific questions:

  1. No, current will not flow back through a device from the neutral in any but the most pathological circumstances, for two reasons. First, it is incumbent upon the builder to ground all of the neutrals at the same potential, so electrical current has no motivation to move back up the circuit. Second, the hot is switched, so there's no place for current to go, because, presumably, nothing else is electrically connected to the circuit.

(Skipping the questions about design functionality or sanity - come back to that at the end)

  1. Yes, there are anti-backflow devices for DC (diodes) and AC (a properly designed amplifier will accomplish this function); however, in this case they are completely unnecessary for the reason mentioned above. To use a water analogy: if all of your drains are emptying into a hole, dumping water out of one drain isn't going to cause water to flow back up another drain.

  2. Bypassing the timer to the control output will not de-power the timer. As drawn, the timer is wired in parallel (that's why it has it's own neutral). Therefore the voltage drop across the timer will be the same as the voltage drop across the bypass switch and whatever is in series with it (the alarm devices). It has to be wired in parallel this way, otherwise the timer wouldn't function when the alarms were turned off. This is assuming the device's power supply is robust enough to power both the timers and all of the devices at the same time. If it isn't, then the device won't function well anyway. So - The timer should function independently of the alarms.

  3. Enclosure - Safe enclosures for this type of device are rated and sold as electronics enclosures. There are many such boxes in all different sizes sold.

Functionality and sanity: There are a lot of design and construction details involved in such a device that aren't immediately obvious and may not be an issue for some time which could, nevertheless, kill you or burn down your house. Examples include: fusing (yes, absolutely it should be fused, with a fuse which will protect the wiring and switches in the device), insulation of wire connections, attachment (are you soldering these together?), wire size, power supply. Power supply? Are all of these devices 120V, or do you need a DC power supply for some of them? My advice would be to do some careful research on how these things are done before attempting to undertake such an adventure.

One thing you may consider is to use a plug-in DC power supply, the venerable black boxes you plug in to charge just about everything in your house. These can be obtained at many different voltages and current delivering abilities, and are way, way safer than trying to mess with a 120V, unregulated supply from the wall. Just keep in mind that a ground fault can set a fire at 12V just as fast as it can at 120V.


You only have to switch the Hot side.

You may also want to tone down this whole idea. The clock on any cheap Android tablet can be set for several alarms, with different loud (some obnoxious), sounds at any set time (with great time accuracy). And they can be set daily, for set days (Mon, Weds & Fri), weekly and to repeat. There are also "home automation" devices that an Android device (phone / tablet) can control. Set one alarm to flash lights and ring a bell, and another alarm to do whatever else you'd like - and control it easily from your phone.

Tone it down even more and use a medication / pill reminder app, like this or others and you may not wake the neighbors, but it'd sure be a lot easier.


The question is a bit long for me to follow fully but I will say that on a 120V circuit you are not aloud to switch the neutral. It is a code rule. You always switch the hot, never the neutral. I am not sure if your whole circuit is 120V but theres a part answer for you.

  • Thank you Janessa! The whole circuit is 120V. I'm glad to hear that you always switch the hot, never the neutral, because it means I'm not the only one who's wondering on this. I will look up why later when I have some free time again, and I suspect I will find something helpful! Thanks again!
    – Orision
    May 17, 2015 at 20:34
  • You are not allowed to switch the neutral by itself, but you can switch it at the same time as the hot. ( older computers had a switch that controlled both the hot and the neutral. ) May 19, 2015 at 1:00
  • Was that low voltage on the computers or 120V? May 19, 2015 at 1:02
  • @janessa on old computers (windows 95 days) it was the 120v that was switched off by the button on the front. They switched both hot and neutral. New computers just have a 5v signal switch that tells the power supply to turn off.
    – Grant
    May 27, 2015 at 15:29
  • @Grant thats interesting. Thanks for the info. May 27, 2015 at 17:08

You only switch the hot side but why not just buy a arduino with a relay sheild and have all that and wifi controlled lights?

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