Collecting data from smartphones can provide useful information for health research, but developing mobile health (mHealth) apps requires extensive resources and implies often overlooked regulatory and privacy-related requirements. Mobile health "aggregators" like HealthKit or Google Fit and specialised platforms may allow reducing costs in mHealth research. Current platforms, however, do not efficiently exploit mHealth aggregators and provide little attention to regulations. We propose an open-source platform that can be used by health researchers without the need to develop custom apps. The platform has a strong focus on regulatory compliance, patient consent and transparency, and allows collecting data through electronic surveys and querying aggregators. We tested the feasibility of our platform in a pilot study with 18 healthy volunteers. The results show that the participants' app is usable and well-accepted and is able to frequently collect data about physical activity from both phones and connected wearables. Some limitations were identified regarding data loss because of insufficient connectivity and the impossibility of extracting data from wearables that are not compatible with aggregators.