When Waltham startup Affectiva announced a hackathon inviting local coders to try out the company’s emotion-sensing technology, the organizers were shocked at how quickly the roster filled up. The bigger surprise: half the developers who signed up were women.

Hackathons, like most tech events, tend to be demographically skewed: most speakers, panelists, and participants are young, male, and white.

“I was very vocal that we wanted gender balance,’’ said Rana el Kaliouby, who cofounded Affectiva with Rosalind Picard, her mentor at the MIT Media Lab. “I think that allowed us to be thoughtful about how we got there.’’

El Kaliouby and her team had a multistep plan to spread the message to women in Boston’s tech community — contacting women in leadership positions, advertising at events for women in tech. But they didn’t need the strategy. Turns out that their contact list, with a large number of women, was all they needed. By mid-January, the event was completely booked and half of those signing up were women.

Affectiva’s core technology is software that can deduce emotions from facial expressions. In its most basic form it can run as an app on an iPad. The app uses the built-in camera to scan the face in front of it and determines emotion in real time.

After bagging high-profile deals and partnerships with big companies like the advertising giant Millward Brown, Affectiva is looking to stay innovative by building relationships with Boston’s developers and young tech companies in the area.

The hackathon in March is a first step in that direction. Participants will have the opportunity to pair Affectiva’s software with devices like the Nest smart thermostat; Amazon’s Echo speaker, which recognizes voice commands; or Pavlok’s wristband, which helps you break bad habits by giving the wearer a mild shock.

For example, with a hack that linked Affectiva software with Amazon’s Echo, said el Kaliouby, “We could measure the sentiment of people in a living room that could drive the music in the living room, maybe turn the lights brighter.’’

In another effort to build a relationship with independent coders and young companies, Affectiva is releasing a software development kit online. Tech companies often release this set of tools online to give other potential partners a chance to interact with their software and build new applications.

Affectiva intends to have a free tier with access to the basic features, said Gabi Zijderveld, vice president of marketing and product strategy at Affectiva, to allow cash-strapped startups to get a taste of the software.

“We want to make it accessible to really, really small companies; I think that is where the innovation will come from,’’ Zijderveld said.

Nidhi Subbaraman can be reached at nidhi.subbaraman@globe.com. Follow her on Twitter @NidhiSubs.