New to relays

I would like my arduino to flip some switches around the house. It looks like I can do that with this -

Am I correct in understanding that my uno board, despite being 5v, will not flip that relay over? I heard I need a transistor to increase current (how the heck does that even work?!) . If so, will this one do the trick?

Thanks!

blah44:
(how the heck does that even work?!)

It's the most fundamental use of a transistor: as a switch.

A simple google search will show you plenty of results.

The arduino will not supply enough current to flow through the coil to actually the relay.

You are better off buying a gadget like this, which effectively has the transistor built into it to amplify the logical signal output
from the arduino, into the current required to drive the actual relay solenoid.

michinyon:
The arduino will not supply enough current to flow through the coil to actually the relay.

You are better off buying a gadget like this, which effectively has the transistor built into it to amplify the logical signal output
from the arduino, into the current required to drive the actual relay solenoid.

5v DC 1-channel H/l Level Triger Optocoupler Relay Module for sale online | eBay

While that does look nice, can I accomplish this with the little transistors I posted?

Have a look at this tutorial, which uses exactly that transistor to switch a relay.

JimboZA:
Have a look at this tutorial, which uses exactly that transistor to switch a relay.

Thanks a ton JimboZA!

It may go without saying, but I will say it anyway: Be careful with the legs of the relay. With the Arduino and the 3904 you are at 5v and safe. On the business side of the relay, if you want to switch lights you are likely at 120AC. Soldering that relay into a little protoboard along with the transistor makes something that requires safe handling (those traces can get you!) and a little protection.

bigread1212:
Don't assume he is in the US, he might be switching 240 ACV! Your warnings still apply, of course.

Keith:
Yep. Could be 240, but I guessed US and you guessed 'he'. So we're both big guessers. LOL.

I use "he" as a generic pronoun.

And I only said "likely" at 120 if you want to get technical about it. Meant to be humorous.

bigred1212:
It may go without saying, but I will say it anyway: Be careful with the legs of the relay. With the Arduino and the 3904 you are at 5v and safe. On the business side of the relay, if you want to switch lights you are likely at 120AC. Soldering that relay into a little protoboard along with the transistor makes something that requires safe handling (those traces can get you!) and a little protection.

Working with mains voltage is dangerous, and you should know what you are doing.
What bugs me is that board you can buy ready-made is extremely dumb in construction. First you have an opto-isolator for nothing, it's not needed when it's operating a relay. Also, the board layout is dangerous, there is almost no isolation between the high- and low-voltage parts (see picture)