Since this is a demonstration, I recommend you not to set a fixed limit in the code, but to provide a manual "calibration". I.e. by pressing a button Arduino reads the current values from the sensor (averaging them over a period of 3-4 seconds) and uses this as a reference value: the threshold value will then be an increased percentage of the detected value (e.g. if 150% is ok, put "int thresh=avg*1.5;").