The project: An Arduino DIY Relay-controlled aquarium with a water pump, a heater, and a light bulb. All 3 have the exact same wires according to the EU standards: 1 "hot" brown wire and a blue neutral one.

EU document

If I solder them all in parallel, and add proper isolation, all while taking care of my safety, will this work? Of course, with a property thick cable for the current draw.

Here's a schematic with the 4 relays module I got: Schematic

The Relay

Back of the Relay

  • \$\begingroup\$ Many devices have one mains cable and internally power multiple mains subsystems. Your device does not sound any different. However, you don't say what you consider as proper isolation or insulation, and how you are going to take care of your safety, but there seems to be no fuses anywhere in the circuit. And soldering wires may not be the best option, as crimping, screw block terminals and other more proper methods exist. \$\endgroup\$
    – Justme
    Jan 25, 2023 at 20:34
  • 1
    \$\begingroup\$ I'm not sure I understood you correctly. You should never solder mains cables. Instead use screw terminals or something appropriate. \$\endgroup\$
    – Velvet
    Jan 25, 2023 at 20:36
  • \$\begingroup\$ By insulation I mean heatshrinks and 3d printing a case for the junction. However I'm NOT fixed on this solution, but open to new ideas and I just want to know what's the best way to save AC cables since having to connect all 3 of the components + the Arduino 12V power supply just sounds like a waste of 4 power sockets to me... \$\endgroup\$
    – Mito
    Jan 25, 2023 at 20:40
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    \$\begingroup\$ Tropical fish might not like it if the arduino bombs and the water heats up to 110 degC or more. Ditto if it drops to 50 degC \$\endgroup\$
    – Andy aka
    Jan 25, 2023 at 21:23
  • \$\begingroup\$ Why 110 Deg? The heater I'm using has a range from 17 to 35 degrees C, and I plan on heating it to 25. That should be fine for my little buddies as far as I know \$\endgroup\$
    – Mito
    Jan 25, 2023 at 21:34

3 Answers 3


Additional to other ansers:

In principle your diagram is correct.
Others have commented on pratical aspects.

An extremely important point that someone mentioned in passing - If connecting stranded wires to a screw down terminal block system the wires MUST NOT be fully tinned/soldered - the strands must be free to move relative to each other. It IS acceptable to tin the very ends to keep the wire tidy - but the strands where the screw clamps MUST be unsoldered copper.

The reason is that if you screw down onto a soldered bundle of wire strands the solder will creep with time and the joint will loosen. This is a genuine real-world problem and causes genuine disasters. I discovered this for myself about 60 years ago (! :-) ) but all reputable regulatory authorities also ban this practice.

Very bad things can happen.
Best case is a bad connection.
Worse is fire or a wire with mains on coming loose and shorting to something unintended.


Yes. That's basically what happens inside the walls of your house, or inside of a power strip.

  • 4
    \$\begingroup\$ As to safety -- this is up to you, but you want to make sure that the wires can't be reached from outside the device, they can't short out, and all of the above even when it's been knocked around. If you're going to 3D print this, be mindful of the fact that it'll melt when it gets hot -- personally, I'd put it in a good old fashioned metal or thermoset plastic (bakelite) box, and reserve the 3D printed stuff for less safety-critical items. \$\endgroup\$
    – TimWescott
    Jan 25, 2023 at 20:48
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    \$\begingroup\$ good point. Also Mito should be careful about connections so they don't accidentally get disconnected or short out. You can look up how they properly install wiring inside house walls, for some ideas, but beware because some of those types of connectors are for solid wire and don't work for stranded. \$\endgroup\$ Jan 25, 2023 at 22:07
  • \$\begingroup\$ Ok, glad to hear it'd work! But what's a safer way to do it? I heard of electrical panels/distribution panel that looks something like this: electricexpert.ro/accesorii-electrice/… or screw/crimp terminals. I'm new to the terminals side so any useful learning resources would be appreciated. Also, would the one from the link above work for my application? @TimWescott \$\endgroup\$
    – Mito
    Jan 26, 2023 at 13:27
  • \$\begingroup\$ Since the 1920's, the ARRL Handbook has included information about building safe equipment. I believe the RSGB has equivalent info. What they consider "safe" certainly has evolved (in my 1933 Handbook it's "oh, by the way, don't touch that 2000V line, you'll die" -- from the 1950's or 1960's it's fairly modern). So -- get a Handbook, if you can. It gets published every year, and you don't need a new one; if you live someplace economically connected to the US you should be able to get a used one fairly cheap. Somewhere in the back will be one or more chapters on safe building. \$\endgroup\$
    – TimWescott
    Jan 26, 2023 at 15:50
  • \$\begingroup\$ The applicable Code here is the UL standards. However they're not free. NEC would also play a role; it's free-to-see. \$\endgroup\$ Jan 30, 2023 at 18:05

Messing around with mains like this is madness.

You could do the whole show with 24 volts AC or DC, supplied by a quality, certified transformer or power supply. Or 12 volts, though current is starting to get large at that point.

  • You can get 24 volt lights.
  • I'm sure you can get 24 volt pumps, AC or DC (and not hard to make DC from the AC).
  • You can get 24 volt heaters, or simply get 120V heaters at 25 times the power rating you want. For a resistive heater, power is proportional to voltage squared.
  • \$\begingroup\$ In wouldn’t call it madness, but a trade off. Lower currents at higher voltages - this matters in practice. Overall, I’d think lower current mains option may end up being safer long-term since the fire risk is decreased, as long as there is a fuse on the mains entry into the box. \$\endgroup\$ Jan 30, 2023 at 13:10
  • \$\begingroup\$ @Kuba you're overlooking arcing. With higher voltages, series arcing at a poor connection is more likely to start, more likely to sustain, and be more desctructive especially if a switching power supply is involved (which will only increase current as the arcing causes more voltage drop). Also this cheap Chinese crud Isn't quality - no BSI Kitemark here - and looking right at the PCB you can see it's not well designed for mains voltage. So that invites parallel arcing, again more likely to strike and MUCH more destructive at higher voltages. There's shock risk too - RCD is not a panacea. \$\endgroup\$ Jan 30, 2023 at 18:18

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