Cardiac coherence biofeedback training consist on slowing one's breathing to 0.1 Hz in order to simulate the baroreflex sensitivity and increase the respiratory sinus arrhythmia efficiency. Several studies have shown that these breathing exercises can constitute an efficient therapy in many clinical contexts like cardiovascular diseases, asthma, fibromyalgia or post-traumatic stress. Such a non-intrusive therapeutic solution needs to be performed on an 8 to 10 weeks period. Even if some heart rate variability based solutions exist, they presented some mobility constrain rendering these cardiac / respiratory control technologies more difficult to perform on a daily used. In this paper, we present a new simplified smartphone based solution allowing people to process efficient cardiac coherence biofeedback exercises. Based on photo-plethysmographic imaging through the smartphone camera, this sensor-less technology allows controlling cardiac coherence biofeedback exercises through a simplified heart rate variability algorithm.