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This circuit is a pulse oximeter , it is a medical device to indirectly measure the amount of oxygen in the blood of a person . The circuit is very simple and inexpensive to build , the sensor to be monitored only need an oscilloscope , but the case does not have an oscilloscope , here an article on how to make your pc a Oscilloscope . The operation is simple , light from an LED shines through on one side of your finger and is measured across the photo-transistor, a clothespin wood will engage the LED and the photo-transistor as a way to mount and align the sensor with finger . The monitor circuit uses only a single IC LM324 operational amplifier . There are three basic steps , the input signal is decoupled before being pre- amplified then passes through an adjustable low-pass filter to help eliminate noise 60Hz AC and finally a further low-pass filter in parallel with amplifier. Pulse Oximeter Circuit Diagram