We all know that the hottest tween accessory is a cell phone, but how can outdoor ad campaigns use this tool to reach the wireless generation? Cambridge, England-based tech firm Hypertag may have found a way to make the connection with a new high-tech approach to poster messaging.

The company has pioneered the development of a small chip designed to be imbedded in posters or billboards in major city centers. Upon passing one of these ads, passersby can activate the chip’s infrared technology with their cell phone to receive more information about the product, enter a contest, download a free ringtone or receive a coupon.

The first wave of these mobile-enabled posters dotted the urban landscape of London, England in July 2003 to advertise and disburse coupons for Nintendo’s Pokémon: Ruby Edition video game. Since then, Hypertag has added Proctor & Gamble, Hewlett Packard and feature film distributor United International Pictures (for the U.K. release of horror/comedy flick Shaun of the Dead) to its client roster.

In January, the company had just wrapped a two-week campaign with publisher Pan MacMillan to promote Teen Idol, a new book for tween girls from Meg Cabot about a teen who falls for a young movie star when he visits her small hometown on a shoot. Pedestrians who pointed their cell phones at posters touting the novel won prizes ranging from phone screensavers to a US$750 shopping spree.

Hypertag also hooked up with mobile phone company O2 last year to give away musical ringtones to Party in the Park concert-goers who aimed their phones at a series of posters in O2’s on-site tent. In all, 25% of folks who visited the tent interacted with the tags to download the freebies. ‘It’s the immediacy of finding out if you are a winner in front of the poster that appeals,’ says Jonathan Morgan, director at Hypertag.

It takes Hypertag roughly three weeks from the time the campaign is booked to design, tech up and hang the posters. The company will work with clients on coming up with the best giveaway for their goals, be it a free screensaver to promote a TV show or vouchers to drive sales of a product. ‘But we haven’t done enough campaigns yet to say that there’s a particular favorite download,’ says Morgan.

Campaigns like this do cost a little bit more than an average outdoor plan because the poster manufacturer will usually charge a higher fee in order to recoup what they’ve spent on licensing the Hypertag technology. At the end of a typical two- or four-week campaign, partners receive a report breaking down how many people used the service, but to respect consumers’ privacy, the tags do not store cell phone numbers. ‘That’s something advertisers like because they can put it on the poster: You won’t get spammed by using Hypertag,’ says Morgan.

Next up on its to-do list, Hypertag is looking at widening its services to include Canada, the U.S., South Africa and Australia by the end of the year.