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London (CNN) -- For a few days, Lockitron looked like the little app that would, and should, but then couldn't.
Lockitron, which enables doors to be unlocked through a smart phone, was rejected by Kickstarter in October. It was turned away for being a home improvement product, which the funding platform does not support.
But then Lockitron went "DIY" on its own crowd-funding.
In one day, orders hit $500,000. They are now sitting at $2.2 million, the equivalent of more than 15,000 units.
Lockitron creators Cameron Robertson and Paul Gerhardt were both University of Colorado students -- studying history, economics and finance, and computer science respectively -- when they started playing with inventions.
When they came up with Lockitron, the response was enthusiastic. "People gravitated toward the doorlock [idea]. People lose their keys, keys are copied, you don't know if a number of keys are out there ...they are a lot of inconvenience," Robertson told CNN.
The two, both 25, had worked on a "shoestring budget" -- around $10,000 to $15,000 -- and a year's worth of time to fund their creation, which first released in May 2011. Looking back at that time, Robertson says they discovered that "oatmeal for breakfast is wonderfully filling, and wonderfully cheap." The two "kept living like college students, frugal college students."
Feedback was "incredibly positive" and when they showed the product off at a party full of Silicon Valley investors, the warm reaction provided market validation, he added.
They turned to Kickstarter as the "perfect venue" to fund the project. "We sent our application in, thinking that in short order we'd be approved but that's, of course, not how it turned out."
After being rejected "we went through the stages of grief and sadness pretty quickly because we didn't have time to deal with it," Robertson says.
Ten days later, the pair's own crowd-funding system was up. "We had no idea what to expect in terms of traffic," Robertson recalls. "[We thought] if we shot for $150,000 in 30 days we might be able do it ...and we had no other option."
He credits Kickstarter for ensuring Lockitron had a well produced video, needed as part of the application process, which became a YouTube hit. "It had just the right mix of excitement, and explanation of the product," Robertson says. "It captured people's imaginations in just the right way."
Angel investors injected a "few hundred thousand" into the project last year, allowing Lockitron to launch its second, more streamlined version.
The first version required the app, which needs to be signed into, to be tapped to open the door. The upgraded Lockitron senses the person's proximity to the door and unlocks it once you are in range. It also allows people to unlock their doors from afar, ensuring guests are able to get in. It works with wifi and bluetooth, sending a signal to a physical block attached to the Lockitron owner's door.
If the phone is lost, the owner needs to sign out of the app for security beyond the phone's log-in code.
It works on a conventional U.S. deadbolt, which make up 60% to 65% of the market, and sells for $179. Robertson said the company hopes to eventually expand Lockitron into UK locks and beyond.
"The timing was right for this sort of product because people have become so used to their smartphones and using apps, that it was a natural extension they should have an app for the key as well," Robertson says.
"A lot of people see the hardware and think that's what we've done but [Lockitron] is the combination of hardware and app and software," Robertson added.