I am wondering if this Arduino Relay Shield design would be good enough to handle 120v 1amp The transistor is a 2n4401. Also The Two capacitors on the other 3 circuits would be going to ground opps its to late for this enter image description here

  • You seem to be missing some earth connections: bottom of C3,C5,C7,
    – Jasen
    Aug 2, 2015 at 6:50
  • please explain the wiring on the output side, preferably draw in the supply and load.
    – Jasen
    Aug 2, 2015 at 6:51
  • The relays have 5 pins. Th logical arrangement is the 2 coil pins at ine end and the 3 contact pins together ar the other end. That way there is maximum coil to contacts separation. Some relays (and notably low cost Chinese ones used on premade relay shields) have the contact "common" pin at the coil end between the coil contacts. That means there is a short path from low voltage coil to high voltage contacts. This is gross stipidity but very common. Aug 2, 2015 at 12:38

2 Answers 2


As already mentioned, everything to the left of the relays is completely irrelevant when it comes to what the system can cope with. It's purely down to the relays what voltage and current they can deal with. As far as the circuit goes there's nothing wrong with it that I can see at first glance.

So to go from there to an actual physical shield there are a number of things you do need to consider:

  1. Ensure that the relays have a rating at least as high as the expected maximum voltage and current, ideally derate it by at least 50%. For CFLs (which present an inductive load as well as a resistive load) you want to dereate it even more.
  2. Ensure that the mains voltage track layout is kept well away from the low voltage digital signals and ground plane.
  3. Ensure that the mains voltage tracks are sized appropriately and kept as short as reasonably possible. Use a track width calculator to get the right width.
  4. Be sure to use appropriate connectors for the mains power. They need to be able to cope with as much current and voltage as the relays and at the same time be safe.
  5. Keep all mains voltages away from human contact. This means no exposed live metal on the upper side of your board where it can be inadvertently touched. Any components with exposed leads that have mains voltage in them should have their leads insulated.
  6. Be absolutely certain that, since this is a shield, no mains voltages even come close to the Arduino the shield is plugged into. This is especially critical around such areas as the USB socket where through-hole components (like screw terminal blocks) are likely to short circuit and cause very very nasty things to happen.

Remember: Mains voltages are dangerous, caution and safety should be your first concern over everything else!

One more tip: Do all your mains experimentation as safe as possible. Either ensure that you have an ELCB and an RCD on your ring main / power outlet (with the former you must ensure that you have your earth wires connected properly), or even better (and what I would certainly recommend above all else), use an Isolation Transformer to separate you from the mains circuit entirely. With an Isolation Transformer electrocution is only possible if you contact both terminals at once as opposed to just one terminal of the normal mains supply.

Edit: One thing that does puzzle me with your circuit is how you have wired up the common pin of the relays. That part of the circuit makes no sense to me at all. You should have 2 connections to each relay - in and out where in connects to the NO and out connects to the common pin¹. The only way your arrangement could work would be to designate one relay as a master power control and that one has the main power coming in which then feeds through to the common rail to supply the other three relays. If this is what you want, then you will have to ensure that the relay designated as master (and its connector) has at least three times the capacity of the other three relays and connectors (or all four relays have at least three times the capacity you would otherwise need).

¹ Note: doing it this way around ensures that when the relay is in the off position the NC pin isn't live. The only pin that would ever be live is the common pin. Safety first...!

  • Yes the relays are rated 250v 10amps, They wont be pushing anything to hard, Just A microwave fan, also only two relays can be on at once. As for the common pin it seems that i forgot to add that as the AC in for the relays. It wont be on a trace. I plan on using 22awg wire to make a physical connection
    – liljoey112
    Aug 2, 2015 at 12:09

The critical part is the relay.

I'd want relays with rated to switch the load and and handle the cold current that the lamp wants.

I also want to see a good creepage clearance distance between digital side and the relay outputs. Relays which put the common contact between the coil terminals are probably a bad choice here, relays that instead have the contact terminals at one end and the coil terminals at the other make the PCB layout easier.

  • The relays are rated 250v 10amps as for the coil clearance i dont quite understand what you mean.
    – liljoey112
    Aug 2, 2015 at 6:35
  • 250V 10A sounds suitable for lamps up to 100W or so, the other bit is about creepage distance (edit) you don't want to suddenly find that your arduino is live.
    – Jasen
    Aug 2, 2015 at 6:53

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