I could swear that it was working for a while. I got back to my desk, tried it again, and it's no longer working. Could I have fried the NO pins on both sides? This is a DPDT relay. Everything works normally on the NC pins. I have never applied more than 5V. I do hear the relay click when I apply 5V to the coil. But when I measure voltage on the NO pins, I get 0V. Has anyone else seen this? I have two of these relays and I can't seem to get voltage on the NO pins with either relay. I should clarify that I'm expecting the same 5V power source to power both the coil and the common pins. If the NC pins work then I don't see why the NO pins shouldn't. In both cases the 5V is shared between the coil and any load attached to the NC/NO pins. I did try driving the entire circuit off a 9V power supply, but that did not change the results (and that does contradict my earlier statement that I've never applied more than 5V to this relay). My circuit is based on Charles Platt's "Make: Electronics", p. 59.

Here's the link to the relay product page.


Schematic I am trying to implement1

  • 1
    \$\begingroup\$ Can you post the schematic for anyone that doesn't have the book? A few other things, I take it you're sure it's a 5V relay but it might also be worth checking the coil voltage when activated to make sure your supply is coping OK, sometimes relays click but don't fully actuate if the coil voltage is too low. \$\endgroup\$
    – PeterJ
    Nov 17, 2013 at 6:28
  • \$\begingroup\$ Peter, thanks for the comment. I am attaching a photo of the schematic from the book that I am following. The only difference is that my relay is a 5V relay (I am adding a link to the product page). Also, I am using a piezo buzzer instead of LEDs. Finally, I am not using a resistor. Regarding your point about the coil voltage possibly being too low, I did wonder about that and that is why I eventually tried a 9V power supply. But that didn't help. But just to be sure, I just measured the voltage across the coil when actuated, and I got 4.99V. I am using a USB port for power. \$\endgroup\$ Nov 17, 2013 at 15:01

1 Answer 1


The relay connections in the book are wrong. The Omron datasheet for that relay shows that the connections fro the contacts are Common, NC, NO, where the book appears to use NC, common, NO.

  • \$\begingroup\$ Correct, and thank you. Incidentally, I just realized the same thing after looking at the schematic printed on the top of the relay with a flashlight so I could read it more easily. \$\endgroup\$ Nov 17, 2013 at 18:31
  • \$\begingroup\$ @PuneetLamba Also, throw a diode across the relay coil pins, as a protection. \$\endgroup\$
    – Passerby
    Nov 18, 2013 at 2:00
  • \$\begingroup\$ @Passerby, thanks for the reminder. My diodes are still in transit from China. However, I wonder, does it matter which way the diode is facing when I connect it across the two coil pins? \$\endgroup\$ Nov 24, 2013 at 15:21
  • \$\begingroup\$ @PuneetLamba: the diode should be connected with the cathode (bar) towards the positive supply - so it doesn't conduct when the relay is powered. \$\endgroup\$ Nov 24, 2013 at 17:24

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