Web3.js – Resolving ‘Web3 is Not Defined’ Error

I'm trying to create a npm module with functions for interact with the ethereum blockchain, for the interaction i'm using the web3js API and i will put this module inside the my new module.

In my packaje.json i put this:

"dependencies": {
"web3": "^0.18.3"
}

I'm installing the module with the option –save and in the folder node_module i have all the modules required for web3 and this one installed.

So when i try to test the functionality of my new module i obtain this error:

web3 = new Web3(new Web3.providers.HttpProvider("http://ropsten.infura.io/")
);          ^
ReferenceError: Web3 is not defined

Why is happening this if i have installed the module? Some idea?

More information:

I call my new module with:

 var ether = require('./lib/newmodule.js');

Inside the newmodule.jsi have this for the conection:

if (typeof web3 !== 'undefined') {
    web3 = new Web3(web3.currentProvider);
} else {
    // set the provider you want from Web3.providers
    web3 = new Web3(new Web3.providers.HttpProvider("http://ropsten.infura.io/"));   
    if(!web3.isConnected())
        console.log("not connected");
    else
        console.log("connected");
}

and this for use the module:

module.exports = methods;

And inside methods one methode for print the data and test if work or not the js. In a other Dapp project the newmodule.js works fine, but the web3js is installed by meteor.js so i think's i'm not doing something well with the web3.js installation or building the structure of the module.