For a small home project I would like to turn on a water heater using 2 NO reed switches. Once I put a small magnet ontop of the reed switch (turn the reed switch to Close), the water heater will be activated. Once one of the small magnets is separated from the reed switch, the water heater will be off. How can I accomplish that ? Reed switch always needs a current and I cant just connect it to 230V power right ? If so, How would a DC 3v powered reed switch turn on a AC 230v device ?

Also, I have no experience with arduino and such, so I would appreciate a simpler solution to my problem.

thank you in advance !

  • 1
    \$\begingroup\$ Why are you turning on water heating with a magnetic switch? Usually a thermostat is used. Also note that you will need a safety thermostat to switch off the heater if your control circuit fails. \$\endgroup\$ – Transistor Apr 4 '16 at 22:10

I highly recommend doing a lot more research before playing with 230V AC mains!

And don't hook your reed switches to a 230V water heater. Most reed switches are fairly delicate. I've never seen one that can reliably switch 230V, let alone the amperage required by a water heater.

To answer your question, you would hook up the reed switches to inputs of your Arduino (or whichever controller you will use). Then, have the Arduino output a signal to drive a relay to control the water heater.

You will likely need two relays. A low-power, low-voltage relay (or MOSFET) that can be driven by the Arduino, and another one that can power the heater. The small relay drives the large one.

Please note that the real circuit will need additional considerations, such as inductive kick diodes and perhaps an RC snubber network for the relay.

Here is an answer that talks about driving a larger load with a MOSFET. You could use the same ideas, but put your larger relay in place of the "motor"...

As far as a simpler solution, you could do it all with relays. Then you wouldn't need to learn how to program an Arduino.

Something like this:


However, this requires that your reed switch and relay are all spec'd for 230V. And as I said before, I don't think you will find a reed switch rated for 230V AC. A better alternative (safer, cheaper, and easier to find parts) would be to use something like:


Please keep in mind that these aren't complete circuits. Only guidelines to help you research.

  • \$\begingroup\$ Stick a fuse in there on the mains side. It gets folks used to thinking about them. ;^) \$\endgroup\$ – Transistor Apr 4 '16 at 22:36
  • \$\begingroup\$ Do not put 230v on a reed relay. No way does a reed relay that actuates on a few AT have enough gap to snuff a 230V arc, especially if that has an inductive component such as a relay coil. \$\endgroup\$ – Harper - Reinstate Monica Apr 4 '16 at 23:08
  • \$\begingroup\$ @Harper Agreed. I did mention that twice :) Yours is a good answer, by the way. \$\endgroup\$ – bitsmack Apr 4 '16 at 23:10
  • \$\begingroup\$ Thanks. I know how to do the mains stuff, but too long in the Code trenches has squelched my imagination of what else to do. \$\endgroup\$ – Harper - Reinstate Monica Apr 4 '16 at 23:15
  • \$\begingroup\$ @Harper Seems wise to me! Better to know the code and be safe, then to try to reinvent the wheel and burn down a building :) \$\endgroup\$ – bitsmack Apr 4 '16 at 23:19

Use a relay. Use the 24V low-voltage power already in your home to switch the relay.

By amazing luck, there are HVAC relays used in whole-home air conditioning units, which are typically rated to switch 240V at 30A or 40A and cost under $20. Oversize your relay to at least 125% of the water heater's rating. If your water heater is bigger than that, you'll be into industrial grade contactors, and figure a couple hundred bucks.

Since we're dealing with mains power, we are obliged to follow the National Electrical Code or whatever applies to your jurisdiction. That is not that big a deal, but you do have to learn it, and do it properly, and consider paying an electrician to bless it. The consequences for noncompliance are most grave - house fires, lawyers, bankruptcy, jail if a person dies. You need to get the relay from an HVAC specialist (or Amazon or McMaster-Carr) but for the enclosure and cabling, talk to your local "electrical supply", often locally owned or regional companies... don't waste your time at Home Depot.

Site that relay as close as possible to the existing wiring, preferably at one end. Bring the 24V to the 240V, not the other way round. When you cut the cable, it won't have enough slack to make the connections to the relay, and you must leave 6" of slack in the box. There's no way to do it without new cable on one segment, so let it be a short one. That cable costs about $1/foot.

Once your mains voltage stuff is absolutely tip-top, Code gives you more leeway on the low-voltage stuff (doorbells, thermostats and the like.) However it cannot be run in the same conduits, boxes or raceways as mains power (with the obvious exception of the relay; you have to put it in there.)

HVAC relays generally have a 24 volt AC coil, which counts as "low voltage". It's no coincidence your house probably has a 24 volt AC transformer already... furnaces supply it to thermostats to switch A/C, heat, fan etc. It's also sometimes used for doorbells.

Important question: Can the reed relay can comfortably carry enough current to work the HVAC contactor directly? If in doubt, get a much smaller "booster" relay in between - the reed switch switches the small relay's coil, and its contacts switch the HVAC relay's coil at 24V. This also allows you to use other voltages for the small relay's coil, in case you'd rather drive it with 12V, 5V or 3.3V.

Amazon links give me (or somebody) a small commission. It tends to cause me to recommend products which Amazon sells.

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  • \$\begingroup\$ Thank you for your detailed response ! I will definitely check the HVAC relay and read more about this subject. thanks \$\endgroup\$ – chegov Apr 6 '16 at 19:12

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