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I have a relay (VRS4H-S-DC5V-C), for which the triggering voltage required is 5V Min. If I directly use the 5V voltage of Arduino I'm able to trigger it, but if I use any digital pin in OUTPUT mode in digitalWrite I'm not able to trigger the relay.

Earlier I thought that it must be that because it's a small board its digital pins would be giving HIGH state as 3.3.V, but the fact is when I checked both the voltage of 5V (which is obviously shown 5V on multi meter) and that digital pin one, both show 5V. So the question is why the hell is digital pin not able to trigger the relay, since trigger the relay by 5V is of no use for me since I can't control it, i.e., when it should trigger and when not?

  • From the datasheet, the coil resistance is 70 ohms at 5V, so it is trying to draw 5/70=0.071A, or 71mA, which is more current than a nano can source. – Dave X Apr 18 '16 at 17:24
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    Okay @Dave X... – Shreyan Mehta Apr 24 '16 at 4:52
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The digital output pin cannot output sufficient current to drive the relay directly. Iirc this question was already asked. You need to add between the Arduino and the relay a driving stage, for example with a FET. Or you can use a shield that already has this additional circuitry.

Just google for "Arduino relay driver circuit" and you will get various options.

This is pretty close to your setup.

  • BTW, I just found out that at least some companies produce what they call "digital transistors" which is a fancy way to say that the component already contains some resistor integrated. Example: the resistor on the base of a BJT. – Igor Stoppa Apr 12 '16 at 17:54
  • I used to use those for a while. You can get them with just a base resistor, and also with a base pull-down resistor integrated. Useful if space is a premium and/or you have thousands of them on a board. Other than that, for the cost of a couple of 0603 or 0402 resistors, they're pretty pointless. – Majenko Apr 12 '16 at 18:09
  • Well firstly thank you both for such a quick response....the answer was quite okay...but the comments ...i found that quite overwhelming ...as in I dont belong to Electrical Domain by core I m a mobile Developer...so could you just explain it in rather simple words...i mean the comments part...i got the answer quite well... – Shreyan Mehta Apr 14 '16 at 5:28
  • When you follow the link I provided, you can see that it instructs you to build a circuit. Such circuit contains a transistor and several resistors. The comments were referring to special components called "digital transistors" which put, inside the same package, both the transistor and one or more resistors, thus reducing the number of individual packages that you have to interact with. It's just an optimization that you can safely ignore. – Igor Stoppa Apr 14 '16 at 6:10
  • well okay got that...i found something after googling the keys u gave me....as in using 2 resistors , a transistor and a diode...i didnt remember exact values of them... but using them i can increase the current rate and therefore do my needful thing... :) – Shreyan Mehta Apr 14 '16 at 7:20

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