Wireless sensor networks (WSNs) are commonly considered as a key enabling technology for ambient assisted living (AAL) and smart home services. Essential requirements for this kind of applications are: small node size, long lifetime, little invasiveness and reliable communication over a range of few tens of meters even in presence of obstacles. For this reason, in this paper a low-cost WSN node architecture for body fall detection is presented. Our node is partially inspired to the well-known TelosB/Tmote Sky so as to maintain full software compatibility with TinyOS, but it is highly optimized in size, it is equipped with a chip antenna with better performance than the typical inverted-F printed one and it is powered by a compact high-capacity lithium-ion polymer rechargeable battery. Several experimental results confirm that the node lifetime may exceed one week of operation in the expected working conditions.