Cardiovascular diseases are the leading cause or morbimortality in developed countries. Its early detection may allow this problem to be dramatically mitigated. In this context, wearable devices based on photoplethysmography (PPG) sensors could play a fundamental role for the analysis of heart rhythms at a massive population scale. This paper presents a wearable unit for the acquisition of PPG signals, aimed at affordably enabling this kind of analysis by providing accurate heart rate data. The main issues to be addressed are the minimization of data storage requirements and power consumption, as well as the removal of artifacts caused, among other factors, by user's movements. The proposed data acquisition system takes advantage of readily available MEMS devices, as well as of an associated custom firmware that allows artifacts to be removed and memory usage to be optimized. The system has been evaluated with real users, achieving more than 80% of usable data, peaking at 97.5% during sleep time, clearly showing its suitability for monitoring heart rates at the intended scale.