July 1 didn’t just mark Canada’s 150th birthday — it also was the one-year mark until the nationwide legalization of cannabis.

Smoking accessory retailer Prohibition took the opportunity to have fun with the milestone. The brand placed an interactive billboard by the river in Gatineau during Ottawa’s Canada 150 celebrations that detected nearby marijuana smoke. When it sensed the smoke, it sent a tweet to an “influential Canadian” (with pre-selected accounts including Prime Minister Justin Trudeau, actor/marijuana enthusiast Seth Rogen and media brand Narcity).

The temporary installation was created with media and creative from Bleublancrouge (and was timed to go live at 4:20 p.m., for obvious reasons). The screen featured a message about the countdown to legalization, along with branding for Prohibition. It also invited passers-by to celebrate by blowing a puff of smoke and sending a “#WeedTweet.”

The tech was “actually quite simple,” said Marie-Eve Best, associate creative director with Bleublancrouge. The side attachments used basic smoke detectors to activate the tweets, which had different celebratory messages (such as “The end of prohibition is one year away. Way to go, Canada!“).

More than 100 “#WeedTweets” were sent.

The activation was a subtle and celebratory – rather than goofy – way of building Prohibition’s following, said Best. “Marijuana is becoming more and more mainstream, not something people are doing in dark alleyways,” she said. “We didn’t want something that would speak to 16- and 17-year-olds.”

Best said the smoke-sensing billboard may make a comeback for other prominent events, such as April 20, 2018.