Electrocardiographs (ECG) are medical devices that record the rhythm of the heart, particularly abnormal rhythms caused by damage to the conductive tissue that carries electrical signals, or abnormal rhythms caused by electrolyte imbalance. ElectroCardioGraph Simulators (ECGS) are devices that generate electrical signals that emulate human heart electrical signals so that the ECG recorders or monitors can be tested for reliability and important diagnostic capabilities. Modern ECG instruments with automatic wave recognition, measurement and interpretation would need for testing, a carefully selected set of test signals. The main objective of this project is to design a practical ECGS device which can be used to perform the calibration and testing of ECG recorders. Various beat-rate or ECG rhythm (60, 80, 120,150 and 180 bit/min) of normal ECG waveforms produced by analog electronics simulate the standard 10 electrode ECG record. Additionally a state of arrhythmia (random selection of preset ECG rhythm) is also incorporated in order to simulate abnormal ECG rhythms, which in turn should be detected accurately by the ECG recorder. The output of ECGS is monitored by an Atmel ATmega8515 microcontroller as far as the rhythm selection and the display of it, is concerned. An audible signal triggered by the R wave is also produced for easy monitoring of the pace. The produced ECG signals have been compared to human ECG recordings and the device has been calibrated accordingly in various ECG rhythms. In conclusion this ECGS device can be used as a testing device for 12 lead ECG recorders.