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Basic Electronics » Need help using a 5v relay to trip a 12v relay

October 26, 2012
by Steven
Steven's Avatar

What I thought would be a quick trip to Radioshack turned into three trips and much confusion.

What I'm trying to do is use the microcontroller to turn on and off a 120vAC device (bedroom lamp, Christmas lights, etc).

To make a long story short, the gentleman at Radioshack did not have a 5vDC relay that would handle >1A at 120vAC and suggested I instead use a 5vDC 1A at 120vAC relay to trip a 12vDC 10A at 120vAC relay. Made sense at the time.

So right now I have:

  • 5vDC relay, rated 1A at 120vAC (SPDT)
  • 12v relay, rated 10A at 110/120vAC (DPDT)

I bought a 10ft extension cord from Walmart that I cut in half; I want to wire the relays in the middle. So my question is: how do I wire these things up? Where am I getting the 12vDC to trip the second relay? Here's a picture of the two relays and their packaging:

Relays Click to enlarge.

October 26, 2012
by JimFrederickson
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Hello Steven,

While I don't want to "permanently discourage you", I do think you need to "slow down a little".

120VAC is not something to be taken lightly, and it definitely is something to be approached in a consistent and manner.

Your post doesn't seem to be "consistent".

"Radioshack did not have a 5vDC relay that would handle >1A at 120vAC" - Okay...

So you want to be able to use 5VDC to switch 120VAC, and they didn't have that available, I am good at that point.


"instead use a 5vDC 1A at 120vAC relay to trip a 12vDC 10A at 120vAC relay"


Wasn't switching 120VAC with 5VDC what you wanted?

You have that?
At least it seems you did when you said, "5vDC 1A at 120vAC".

I think, maybe, you meant to say "5VDC at 12DC relay to trip a 12VDC 10A 120VAC Relay"?


In any case, there is no need to use one relay to trip/close another relay.

The preferred method, if you are using a relay, would be to use a transistor to control the 12VDC Relay.

My suggestion...

Slow down a little. Search the forums here and elsewhere, and read-up alot!

Just here, in this particular forum just search for:
"switching 120vac"
(Just in case, without the "double quotes". Those are only there to delineate the literal search term...)

That alone will answer all of your questions.
(At least amongst the many posts that show up...)

Being a "little inconsistent", or wrong, with 12VDC may fry your Microcontroller.

The same if 120VAC may be much different/worse.

October 26, 2012
by Steven
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I definitely understand your concerns; I certainly know 120vAC is not something to play around with. I'm confused as to what you mean by "consistent," though. I certainly did slow down when I realized I didn't fully understand what the RadioShack employee meant; I'm obviously not going to plug in a cut-in-half extension cord without knowing for certain I have everything right.

The 5vDC relay they had does not have high enough amperage rating to use in my application. Using the 5vDC relay to "trip" the 12vDC relay (with the higher amperage rating) is a suggestion of the RadioShack employee I was talking to; this, using one relay to trip another, is what I don't understand.

After Googling the topic, there seems to be many different opinions on how to safely toggle 120vAC.

The point is, right now I have two relays, neither of which work on its own, and I'm wondering what my next step should be. Links to clear resources would be helpful, along with schematics or explanations.

October 26, 2012
by JimFrederickson
JimFrederickson's Avatar

I guess my mistake...

You did write:
">1A at 120vAC"

I didn't take the ">" as having the "greater than meaning".


More than 100 watts is what you want to switch?

Not a problem, but it is more than a single light!

Look at these links:
Nerdkits - Relay Question
Jumper One - Using Relays

These should answer all of your questions.

There are several examples/methods of controlling a relay that require a different voltage than the Microcontroller. (In this case 5v vs 12v...)

October 26, 2012
by JimFrederickson
JimFrederickson's Avatar

Problem with one of my links?

Strange, it worked in the preview but now. (Not sure if the "example part" was there before and ignored, or happened during the post...)

Nerdkits - Relay Question

October 26, 2012
by pcbolt
pcbolt's Avatar

Stephen -

I guess the big problem is the 12v question - namely;

"Where am I getting the 12vDC to trip the second relay?"

You could use a 12 volt power supply or battery to power the Atmega168 through the voltage regulator, and use the same supply to power the relay. Hopefully the relay won't pull too much current to reset the MCU. (You might have to put a capacitor on one side of the regulator). The coil current of ~90 mAmp is too much for an MCU pin so you will have to use a MOSFET like the 2N700's that are included in the Nerdkit to act as a switch. You may even find a power MOSFET at Radio Shack that would take the place of the first relay.

Another solution would be to use a Solid State Relay that is optically coupled to the AC switch. HERE is a unit that Grainger sells. It has all the specs you need and it is reasonably priced (for an SSR). Making an AC switch can get complicated if you make your own depending how you need it to respond (fast switching vs. PWM vs. continuous operation etc.), so it might be worth it to buy the SSR. If you want to go super cheap you can use some triacs but you really have to know what you are doing...not recommended first time out.

October 27, 2012
by Noter
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Maybe something like this ... comes from Wisconsin, probably can have it within a week after ordering.


October 27, 2012
by Steven
Steven's Avatar

Thanks for helping me out guys! I did some more research on using a transistor to switch the relay, and I ended up finding 5vDC relays that handle 10A at 120vAC. Good stuff! They'll be shipping soon and then I'll be able to get this working!

I guess I'll hang on to these relays I bought in case I need them (I'm sure the 12vDC 10A at 120vAC relay will come in handy at some point). If anyone wants the 5vDC 1A at 120vAC relay I'll be glad to mail it out to you!

November 18, 2012
by carlhako
carlhako's Avatar

Hi Steven

Recently I have been working on one of my projects to switch the fridge on and off via an MCU. I found solid state relays off ebay ($10 shipped) to suit my needs you can switch 110-240v directly from the MCU. The only problem with them is when your running higher wattage you need a heatsinc on them but couple of hundred watt produces nearly no heat.

The relay will probably work perfectly for you but I thought i would just throw this in as they work so well.. also no clicky noise :)


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