Pulse Oximetry is a non invasive method of measuring a person's oxygenation level. It monitors the percentage of haemoglobin (Hb) which is saturated with oxygen. A probe is attached to the patient's finger or ear lobe comprising LEDs and photodetector. As the light from the two LEDs (red - 660 nm and infrared - 950 nm) pass through the body tissues to a photodetector, it is absorbed by blood and soft tissue. The light absorption rate at the two wavelengths by the hemoglobin is different and depends on the degree of oxygenation. The light level changes as the blood is pumped by the heart. As a consequence, the oximeter also measures the heart rate in beats per minute (BPM). In this application note, we create a heart rate monitor by using one IR LED & phototransistor pair and observing the waveform at the phototransistor output. This is intended for illustrating a typical light sensor application and not intended for actual medical use. The accompanying video also shows the typical low level signals involved in this type of circuit and how the dynamic range from a 24 bit ADC (EMANT300 USB DAQ) allows such signals to be observed without further signal conditioning.