I'm having issues controlling a single relay module with an arduino. It works fine when its up and running but the relay initially starts triggered until the arduino can send a "high" signal to turn it off again. I should have used a high trigger relay instead of a low trigger one.

My question is whether I can use a pull-up resistor on the input of the relay (being the output pin of the arduino)

both the arduino and relay are powered by a common 5v supply.

  • \$\begingroup\$ Switch from NC to NO. \$\endgroup\$ – Lundin Oct 16 '19 at 7:36
  • \$\begingroup\$ Does the relay have NC and NO terminals? \$\endgroup\$ – Ben Watson Oct 16 '19 at 7:41
  • \$\begingroup\$ I can switch from NC to NO, however I would like to avoid reprogramming the arduino if possible as its installed and hard to get to \$\endgroup\$ – Ben Davis Oct 16 '19 at 7:51
  • \$\begingroup\$ So is the relay itself actually active low? Or do you just have your load on the NC output of the relay which activates your load while the relay itself is not activated? \$\endgroup\$ – Ben Watson Oct 16 '19 at 7:59
  • \$\begingroup\$ yes, the relay is active low and the terminal used is the NO \$\endgroup\$ – Ben Davis Oct 16 '19 at 8:04

Yes this would work. You should size the pull up resistor roughly between 680 and 1000 ohms as the relay module description says that it requires 5mA to trigger the relay, but you dont want to load the Arduino more than necessary when it has to pull it down to activate the relay.

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  • \$\begingroup\$ this isn't a satisfactory answer. \$\endgroup\$ – Andy aka Oct 16 '19 at 7:45
  • \$\begingroup\$ @Ben Watson (banggood.com/…) \$\endgroup\$ – Ben Davis Oct 16 '19 at 7:48
  • \$\begingroup\$ @Andyaka I understand that there's really not much to my answer, but it does answer his yes or no question. I'm not sure how to best expound on my answer other than by asking some clarifying questions, which I did. I am happy to remove or edit it to meet any site specific requirements for answers. \$\endgroup\$ – Ben Watson Oct 16 '19 at 8:00
  • \$\begingroup\$ This should be a comment not an answer. \$\endgroup\$ – Lundin Oct 16 '19 at 8:08
  • 2
    \$\begingroup\$ You have radically changed the answer since I posted that comment. Your original answer said "From what you have described it sounds like that would work just fine. However, it would be nice to be able to look at the data sheet of the relay to ensure there are no conflicts. Can you provide that please?". When asking for clarification, we should do so in comments. Good answers shouldn't speculate or guess. \$\endgroup\$ – Lundin Oct 16 '19 at 8:41

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