magnetic reed switches - understanding ratings

My first Arduino project is a garage door open notification/control. I'm using a Uno and Ethernet shield; to determine whether a door is open/closed I've found some magnetic reed switches.

On the package of the switch it states 500mA @ 12VDC. The data sheet ( has the following:

  • switching current (max): 0.25A
  • switching voltage (max): 30VDC
  • insulation resistance: 100Mohms Min

My question is: can this be driven from one of the digital pins of the Uno, or do I need to use an opto-coupler to drive this from a 12V power supply? Does it really require 500mA?

many thanks, J.

I think you're confused as to what a "magnetic reed switch" is. It is a switch that is actuated by a magnet. It isn't driven by anything (it can't be switched by the Arduino - unless you wire up an electromagnet to switch it, then you've just made yourself a relay).

Sorry, I didn't mean "drive". I want to hook the reed switches to an digital input pin to sense whether the switch is closed or open.

The 500ma refers to the MAXIMUM current that the switch can carry. Since a reed switch is a sealed device it will happily switch extremely small currents (several microamps). It’s generally considered that many switches need a decent current flow so that the arc of breaking “wipes” the contacts clean, but this limitation doesn’t apply to reeds.

Thank you... that is what I was hoping to hear!

I will post further (hopefully) successes as I proceed with this project.