This paper presents the design and implementation of an electrocardiography (ECG) simulator for educational purposes. The representation of the ECG leads is carried out using Fourier series instead of most common options such as look-up tables. The device is designed to produce simultaneous simulation of 12-lead signals and can be connected to any commercial ECG or vital signs monitor. All the design is open-source and open-hardware, and the total cost of the components is significantly lower than commercial alternatives. The device's performance was validated, obtaining a mean and maximum relative error of 1.8% and 3%, respectively, and an uncertainty of ±1 at 50 bpm and ±3 at 250 bpm. This design could help biomedical programs to have more availability of simulators to facilitate hands-on learning for students in biomedical instrumentation courses.