Arduino control mains device

I am considering a new project to turn on an electric water heater on my way home from a trip. I have just found that the heater can waste 1kWh/day just to maintain the temperature, which can seriously affect the main project.

The inention is to use a surplus Uno and mobile phone, and get a new USB host shield and a mains relay - all generally in th manner of Oleg Mazurov in Circuits@Home.

I have an old 5.3v phone charger for auxiliary power, or simply tap in to the charger that I would have to have for the phone.

I'm looking at two different relays, both for DIN rail installation :


I understand this requires a momentary pulse to make and another to break. It does not specify the coil current to do this.


I understand this requires a continuous 5v 100mA to operate and a make-and-hold 5v 0.35mA from Arduino.

This is an all-new arena for me, and I would be glad of any comments, particularly on the relays.

What kind of electric water heater is involved?
If its a hot water system, then the heating elements in these heaters are usually 3600 watts, so you will need
a relay with a 15A rating.

I should have mentioned that... It's standard Rheem, but I replaced the element with a 1500 watt. This enables me to run it on an ordinary domestic 10A circuit.

the first one might be a viable option, but the contacts on the second one are only rated at .5 amps. If it were me i would derate it by 50%. So if i am confident that the load only draws ~6amps I would recommend a relay capable of 9amps (10 amps is the common commercially available rating).


I didn't see that. It's a nice neat installation, but clearly for a different purpose!