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I'm clueless when it comes to the information on datasheets for switches. When I see a datasheet for LEDs or servos, I'm used to seeing a fairly straightforward call out for the required volts and amps.

A project I'm working on now requires turning on an LED using a float switch like this one: Water Level Switch

The max switching voltage and current values, and minimum voltage info is really throwing me. The minimum voltage, for instance, is 250v DC! Does a $5 switch really require 250 volts? I haven't been able to find many schematics where they are just using a switch like this without a pump or other hardware, but I seem to remember seeing someone run a switch like this with 5v pin. And I'm totally unsure what the current would be/if I need a resistor, etc.

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    I think that "minimum voltage" must be the "minimum damage voltage", i.e. the switch is guaranteed against damage until 250 Vdc. – jfpoilpret Oct 29 '14 at 19:50
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    It's just a reed-switch, so it should work just fine with any voltage (below 100v). I think the 250V is the "minimal breakdown voltage". I see that term in some reed switch datasheets. – Gerben Oct 29 '14 at 21:10
  • Ah, interesting. I haven't seen that called out in any Arduino materials I've read so far, but I'm pretty new to this. If one/both of you post as an answer, I'll accept. – Gregir Oct 29 '14 at 21:43
  • Thinking about this again, datasheets for these kind of switches don't seem to indicate what the minimum voltage and current are. How might I go about figuring that out? – Gregir Dec 13 '14 at 6:27
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A reed switch like this will not require any minimum voltage. It should work quite happily down to millivolt levels. If you want to switch on an LED from a 5V supply that is switched by the reed switch you will need a resistor that limits the current to no more than 20mA. The forward voltage drop over a red LED is around 2V so your resistor will drop 3V which means it needs to be at least about 150 ohms.

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It says "maximum breakdown voltage: 250 V DC". That's maximum, not minimum. Above 250 V DC the arc which is created by an opening contact isn't guaranteed to extinguish. It's defined for DC voltage because then the arc remains; for AC the arc will extinguish automatically when the current crosses zero.

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  • Looks like they changed the data in the listing or the datasheet since I originally posted this, and you're right, now it says maximum breakdown. Either way, I hadn't see anything about arcs and your info is useful and I appreciate the comment. – Gregir Feb 8 '15 at 5:21

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