In order to test medical devices, industry increasingly uses simulators closely reassembling the behaviour of physiological systems. In the context of respiratory therapy, most available simulators are designed based on a ventilated volume. This highly adjustable volume allowing for fast dynamical changes often leads to very costintensive test devices, particularly when incorporating realistic spontaneous breathing. Therefore, in this article we introduce a novel concept for a low-cost lung simulator, capable of mimicking the ventilation behaviour of the human lung at the Y-piece of a mechanical ventilator. The proposed design does not require any enclosed spaces to hold inhaled air nor expensive precise linear actuators adjusting its volume. Instead, the setup is designed based on the design of a mechanical ventilator, connecting the system with one port to the ventilator and then dividing the hose into two independent branches. Each branch has an integrated radial fan and a proportional valve, controlling the inspiratory and expiratory flow, individually. The mass flow and pressure are measured at the systems inlet port, representing the condition at patient airway. In contrast to existing setups, the proposed design is not limited by the physical properties of a volume such as fixed maximum size, allowing the simulation of various types of patients and conditions. Numerical simulations to evaluate this system design showed the ability to generate a realistic spontaneous breathing pattern. With a first experimental setup it was possible to prove the feasibility of this approach, by generating common flow curves during spontaneous breathing. Building on this design, the approach could eventually lead to a more accessible method for testing.