How not to burn finger
Do not place finger over the flash, it can burn your finger.
Heartbeats capturing using only camera requires good light condition thus flash near camera can provide significantly better signal and more accurate analysis. Flash can become very hot thus try to find the best position for stable holding of the telephone while finger cover camera completly.
For best signlal finger should cover camera lens completely (not only flash).
The first error is placing finger over the flash, this can burn your finger and there will be no real ppg signal for heartbeat detection.
Second error is partly covered camera. This will result with signal with many artefacts which can degradate the analysis.
Third situation is the best position where finger cover camera lens completlly and finger is close to the camera which will result with clear PPG signal.
During capturing, camera preview should be red and real time graph should be quasi periodic like it is shown on the following screen shot from the app.