Dryer Outlet Grow Light Controller DIY

Hey guys, I am interested in building one of these "see the link I post below".

The site I believe is trying to front their overly priced power relay. $20 plus shipping. Radio shack has a bunch for about 7 dollars and lowes for int he 10+ range. I am sure one of those will work.

Does anyone know anything about electricity that can explain the specs so I can make sure I buy the right one?

They suspiciously dont mention the stats required on the relay so you have to buy the one from their site.. go figure.

http://www.hydroponics.net/learn/grow-light-controller.asp

Though this is old, I want to build the same thing. Only thing is that I have a 4 prong dryer outlet (hot/hot/neutral/ground) instead of the older style (hot/hot/ground). I'm wondering if you can just cap off the 4th(neutral) wire and do everything else the same.??
Anyone else done this yet?

The NEC started requiring the neutral conductor on all range & dryer receptacles in about 1996 because the controls inside the range/dryer run on 120v (and ranges usually have 120v "convenience ooutlets"). So rather than use the ground as the neutral, a neutral had to be used in addition to the ground. Otherwise, the ground becomes the neutral current carrying conductor for the 120v devices in these appliances. A dangerous condition especially when you take into account that water is nearby!
As far as you'r light controller, it is a straight 240v device & requires no neutral. The relay is powered by an external 120v source. If you were to add a 120v outlet or an onboard 120v timer & relay circuit, you would need the neutral.
So, if you were using a NEMA 14-xx series (4-prong) plug, you would not need the neutral wire & could simply cap it off with a wire nut.
As far as the $29.00 price for the relay -That's not so bad.
If you want a better relay for less money go to www.dxsoundco.com they sell a 40-amp two pole relay with 120v coil for $19.95 and a bunch of custom made controllers at about half the price of the others available. Great source for $99 ballasts too. You might have to email them about the relays.