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I’m trying to build a simple circuit with a relay controlled by an Arduino that will turn on/off a little LED light string I have that was previously powered by two AA batteries (that would be 3V, according to the diagram on the battery case). I have the circuit wired and the Arduino programmed so that it triggers the relay on every .5 seconds and off every .5 seconds. I would expect the lights to blink with this configuration, but they do nothing.

I have tested quite a few different configurations, I have tested the lights (they work perfectly fine with power directly from my breadboard power supply), and I have tested other known-working devices on the circuit and I have not been able to get anything to work on it. The relay does function as expected — the LED on the PCB blinks on when it is triggered and it does make the clicking noise from the circuit closing — so I figured that it had to be an issue with the load power.

As I mentioned, I’ve got a breadboard power supply attached that should be providing enough power to the circuit to light the lights, and I have tested the lights directly on the power supply and they work. This lead me to think that there must be something wrong with the relay. I tested the output voltage from the relay with a multimeter and I had to turn it all the way to the most sensitive setting to get any sort of reading at all, which turns out to be something in the range of .3 millivolts. Obviously much lower than the expected 5V that should be coming out of there. I checked the resistance through there and it was within expected values.

So my question is, what’s happening here? Why am I not getting power from the relay?

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    \$\begingroup\$ Welcome to EE.SE, Andy. I've broken up the wall of text into some paragraphs for you. You need to add a schematic and delete some of the text. There's a CircuitLab button on the editor toolbar specially for this. \$\endgroup\$ – Transistor May 27 '18 at 19:23
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    \$\begingroup\$ Can you link the exact relay? \$\endgroup\$ – Harper May 27 '18 at 19:35
  • \$\begingroup\$ .. and as well as a link to your specific relay (actually I suspect you are using one of the many "relay modules", which are not only a relay, but also include the driver transistor and LED) please also edit your question to add some in-focus photos of your actual hardware and wiring. "I figured that it had to be an issue with the load power" Although that's ambiguous, the answer is no, to either interpretation I have, based on you hearing the relay clicking and seeing the LED light on the module. I think I might have guessed your mistake, but you need to supply the schematic and photos. \$\endgroup\$ – SamGibson May 27 '18 at 20:49
  • \$\begingroup\$ Why am I not getting power from the relay? .... a relay does not supply power ... it is only a switch ... so you have to connect it to a power supply same way as you would connect a push button switch \$\endgroup\$ – jsotola May 28 '18 at 3:54
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On the relay boards I've seen, the relay contacts connect only to the terminal block on the board - there is no power on the contacts unless you provide it through one of the terminals.

You should connect the positive supply to the "COM" terminal of the terminal block, and your LEDs to either the "NC" or "NO" terminal.

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  • \$\begingroup\$ This was actually my main issue. I hadn’t run the load through the COM port. I kept thinking that the “JD-VCC” post had something to do with it, so I tried all sorts of configurations that involved that, a couple of which I think actually fried my relays. It wasn’t until I found a datasheet online that said, explicitly, “Leave the jumper on the JD-VCC and VCC pins for most uses,” that I realized I was running the load through the wrong thing, and I’ve been thinking about it wrong the whole time. \$\endgroup\$ – Andy Callander Jun 4 '18 at 5:39
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You did not say anything about the relay type you are using. That would be great to know.

Usually, a Relais needs about 40mA of current to turn fully on. What makes you think that the digital output of the Arduino can deliver this (rather high) current?

Eventually you need a relay driver circuit between your Arduino and the relay. (A simple transistor will do the trick).

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  • \$\begingroup\$ I’m not sure how to explain this any more clearly. Maybe I can post some pictures. The Arduino is providing the 5V of power needed to trigger the relay. It is working, as I can hear the relay click. The power for the device controlled by the relay comes from the breadboard power supply and is also 5V. The output as measured from the COM and NO pins is 0V when open and .03-.04V when closed. Why is it not 5V? \$\endgroup\$ – Andy Callander May 28 '18 at 22:47
  • \$\begingroup\$ @AndyCallander - Hi, "Maybe I can post some pictures" Yes please, I already asked for that (see the comments underneath the question). "The output as measured from the COM and NO pins is 0V when open and .03-.04V when closed. Why is it not 5V?" Why do you think that measuring between COM and NO should be 5V when the relay contacts are closed? Taken at face value, you have the wrong expectation, but in case you mean something slightly different, I (and others) asked for a schematic diagram (showing what you have connected to what), photos, details of the relay (module) etc. Thanks. \$\endgroup\$ – SamGibson May 29 '18 at 2:18
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Well folks, I figured it out. I definitely had it wired up completely wrong. Unfortunately, during the trial and error process that eventually led to my understanding, I think I have damaged my relay module, my Arduino, or both. As it is now, I can no longer trigger the relay at all, and certain configurations still get the LED to come on, but no click, regardless of how much voltage I try to put into it. It’s pretty much borked now, I’d say. But a learning experience! I now know how to wire a relay properly! Now I just need to order another module (I have a backup Arduino, so that’s good...) so I can actually finish this project.

I promised some pictures, and while nothing is actually wired right now, at least this will give some idea of what I was working with.enter image description here

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