I am a Computer Science student so this is not quite my area of knowledge but I have tried to find as much information as I could.

I am creating a project using an Arduino and an electromagnetic door lock. I know that the Arduino can output either 3.3V 0.8A DC or 5V 0.8A DC which means the coil of the relay must be able to activate with one of these. The electromagnetic door lock runs on 12v 110mA DC so I know that the output side has to support that from the common.

Also for what I am doing I know that I need an SPDT relay so that I can use a No and NC line.

What numbers do I need to need to look at on the relay to know if it can run using what I have.

This is one that i have found and I don't know if it would work: https://uk.banggood.com/3Pcs-5V-Relay-5-12V-TTL-Signal-1-Channel-Module-High-Level-Expansion-Board-p-1178211.html?rmmds=search&cur_warehouse=CN

Any help would be much appreciated,


  • \$\begingroup\$ Tip: look at the part number on the relay and find the real datasheet for it \$\endgroup\$
    – pjc50
    Jun 30, 2020 at 15:09
  • \$\begingroup\$ What to check for when buying an electronic component or module \$\endgroup\$
    – Andy aka
    Jun 30, 2020 at 15:11
  • \$\begingroup\$ Typically you use a transistor to boost the signal from your Arduino, to power the relay coil. I think you will be able to control the door lock with the transistor, without needing a relay in between. You can still use a relay anyway, if you aren't comfortable with mixing 12V and 5V in the same circuit (could possibly fry the Arduino if done wrong), or if you want your circuit board to make a click noise. \$\endgroup\$ Jun 30, 2020 at 15:50
  • \$\begingroup\$ Thanks,I have considered using something like a transistor but I decided to use a relay as it would reduce the risk since I have no physical experience with any of this yet. The relay that I have does use one to boost the voltage to get the coil to function an change the switch. I trust somebody else much more than my self to pick the correct transistor. \$\endgroup\$
    – Marcel
    Jun 30, 2020 at 21:25

2 Answers 2


Basic relay selection guide:

  • are you switching AC or DC? The requirements are very different. In this case it's low current DC.

  • is the switched current and voltage less than the contact rating? It looks like it

  • what is the "pull in" and "hold" of the coil? Usually the first is much larger, and your driving circuit has to be able to supply that much current.

  • Don't forget the flyback diode (may be included in this module, not clear)


Thank You,

I have found the information on a data sheet and documentation on from other people that have used the same relay.

Not the data sheet but it helped explain that it would work correctly for me: http://www.techydiy.org/keyes-sr1y-relay-module/


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