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I am working with relays and I noticed that connecting the trigger pin to ground (or a digital pin set to LOW) is not the same as having the trigger connected to anything. For the digital output pins, how is LOW different than no connection? Is there actually a current flowing into the LOW pin, or is there a tiny current flowing into the relay?

Is Ground and LOW the same voltage?

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    Connecting a relay directly to an output pin is not a good idea: you should use a transistor to provide enough current to the relay. You also need a diode to protect against reverse emf voltage. – jfpoilpret Mar 10 '15 at 5:28
  • I'm not sure exactly what you're asking but try giving this a read: arduino.cc/en/Tutorial/DigitalPins. What are you declaring the pin in question as? – MattEE Mar 12 '15 at 0:03
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(Edit for clarification: I've assumed in this answer that you're asking about the difference between connecting the relay to ground/LOW vs. connecting the relay to nothing at all. I've also assumed by the word "trigger" that your relay has some kind of digital actuation built-in, such as a transistor, or that it's simply a Solid State Relay.)

If something is unconnected, it is described as 'floating'. This means there could be random voltage spikes in it caused by nearby electromagnetic fields, making it appear to be switching on/off.

Connecting something to ground or LOW can stop this from happening by drawing away any induced currents.

  • What you describe is true for inputs, not outputs. The op is about outputs. – jfpoilpret Mar 10 '15 at 5:30
  • No. The electromagnetic environment does not create any currents on the floating pin. It creates voltage fluctuations. – Edgar Bonet Mar 10 '15 at 9:15
  • @EdgarBonet Oops, sorry. I wrote this post late at night. :) – Peter Bloomfield Mar 10 '15 at 9:16
  • @jfpoilpret - I think we've understood the question in different ways. I've edited my answer to clarify my assumptions. – Peter Bloomfield Mar 10 '15 at 9:21
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Ground and LOW are very, very close in voltage. They are the same when no current is being sunk into the output (LOW sinks current from higher voltages, HIGH sources current to lower voltages), but the voltage rises as the current does (the output has a resistance of about 25ohm).

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