The Smart Wireless Heart Rate Sensor is used to measure the cardiovascular pulse wave that is found throughout the human body. This pulse wave will result in a change in the volume of arterial blood with each pulse beat. This change in blood volume can be detected in peripheral parts of the body such as the fingertip or ear lobe using a technique called photoplethysmography. 


The device that detects the signal is called a plethysmograph (or ‘Pleth’ for short).


The Pleth consists of:

  • An infrared LED which illuminates the tissue and
  • An infrared LED which illuminates the tissue, and a light sensitive detector (LSD), which has been tuned to the same colour frequency as the LED and detects the amount of light transmitted from the tissue. 


The Pleth supplied with this sensor is a transmission mode plethysmographic signal (PPG) device, which uses transmitted light to estimate absorption. The infrared LED and the light sensitive detector (LSD) are mounted in a spring-loaded device that can be clipped onto the fingertip or ear lobe. 


The infrared light emitted by the LED is diffusely scattered through the fingertip or ear lobe tissue. A light sensitive detector, positioned on the surface of the skin on the opposite side, can measure light transmitted through at a range of depths. Infrared light is absorbed well in blood, and weakly absorbed in tissue. Any changes in blood volume will be registered, since increasing (or decreasing) volume will cause more, or less absorption. Assuming the subject does not move, the level of absorption of the tissue and non-pulsating fluids will remain the same. 


The amount of light that can be detected by the light sensitive detector (LSD) will vary with each test subject, and as to whether the clip is attached to a fingertip or ear lobe. 


When attached to a finger:

  • Position the finger lobe clip so that the light sensitive detector is on the fleshy side of the finger. 
  • Fingers should be clean.
  • Nail varnish may cause falsely low readings.
  • Values should not be affected by skin colouring.
  • Some subjects may have poor peripheral circulation (the extent to which the blood vessels
  • in the fingertip are filled with blood), in which case another subject should be selected.
  • If the heart rate does not seem to settle, try warming the hands by rubbing together to increase the blood flow. 
  • If readings are lower than expected, try repositioning the clip to make sure firm contact is obtained.


When attached to an ear lobe:

  • Remove any earrings before attaching the finger/ear lobe clip to the ear lobe.
  • The clip can be made more secure by hooking the wire round the back of the ear, or by using the slide on the lead to attach it to the subject’s clothing. 
  • If the heart rate does not settle or if readings are lower than expected, try repositioning the clip to make sure a firm contact is obtained.


Each time the finger/ear lobe clip is attached to a fingertip or ear lobe, wait until the signal stabilises before starting to record data - the initial unstable signal will be due to compression from the clip being attached.


Stay still while recording data. Movement e.g. raising and lowering a hand, will alter the pressure that the finger exerts on the clip, whilst simultaneously causing a change in venous blood that will affect light transmission through the tissue. 


It is possible to alter heart rate by simply decreasing respiratory rate. Encourage the test subject to breath normally.


This sensor is not intended for medical diagnosis. Do not be alarmed if results do not correspond to quoted numbers. These numbers represent typical averages, and many healthy hearts have data that falls outside these parameters. It is quite normal for the heart to occasionally miss a beat. 


The number of beats per minute is calculated, by the Heart Rate Sensor, by timing the width of a pulse and ‘scaling up’ to a rate of beats per minute. For example, a recorded wave form of a 0.7 second duration would become (1 ÷ 0.7) x 60 = 85.7 beats per minute. Sporadic short or longer beats will produce artificially high or low readings. This should not present a problem as trends in heart rate are studied, not absolute values or rapid changes. 


The heart rate is averaged over every five beats to achieve a smoother reading.


If the finger/ear lobe clip ‘loses contact’ with the pulse, the sensor will attempt to re-establish communication. This may result in the reading momentarily dropping to zero. .


The light sensitive detector (LSD) in the clip can be sensitive to high levels of ambient infrared light e.g. strong sunlight. If this is the case, try excluding the light, by covering the clip with some dark fabric e.g. by placing your hand inside a cloth bag. 


With the beats per minute range (bpm) of the Heart Rate Sensor selected, the fastest speed that can be used to capture data is 50 Hz (20 ms). If an inter-sample time of less than 20 milliseconds is selected, then the values obtained will either default to the lowest reading, or the set-up will be rejected by the software. This fastest speed for the Waveform range (mV) is 1ms. 


It is possible for the dicrotic notch to be recorded as a separate pulse, and so will give higher readings than expected. The effects are more noticeable if the clip is attached to the finger; try using it with the clip attached to the ear lobe. 


Heart Rate (Notes)

Blood passes through the heart in two phases that alternate continuously.


The blood forced into the aorta during systole, moves the blood in the vessels forwards and sets up a pressure wave that travels along the arteries. This pressure wave expands the arterial walls as it travels and is felt as the pulse. 


The pulse may be felt at points where arteries pass over bones, known as pressure points e.g. in the wrist and neck. Pulse rate is a direct measure of the number of heart cycles in a minute (heart rate).


In normal healthy individuals the heart rate, and thus the pulse rate, varies with the phases of respiration. Irregular changes in heart rate occur in all people. Heart rate may be increased by exercise, nervous excitement, stress due to mental effort, by adrenaline entering the bloodstream, or with increase in temperature caused by fever. The heart rate decreases when asleep and some medical conditions may also cause a drop. 


Following exercise, it takes a while for the heart rate to return to resting level. This is the recovery time and gives an indication of the fitness of the individual. Recovery time may vary from less than five minutes in very fit individuals, to around 15 minutes. 


Safe Pulse Rates

During experiments, the safe heart rate should not be exceeded. The safe level is given as the maximum heart rate for age, minus 20 beats per minute 

Maximum heart rate = 210 – (0.65 x age) 


Waveform (Notes)

The heartbeat recorded by the Smart Wireless Heart Rate sensor is smoothed by the passage of blood through the capillaries, and the pressure signal does not resemble the pulse seen in an electrocardiogram (which is used to record the hearts electrical activity). However, the periodicity of the signal is unchanged, and the Heart Rate sensor can be effectively used to detect changes in heart rate. 


The upstroke, called the anacrotic limb, is abrupt and is due to contraction of the ventricle (systole). The downstroke is more gradual and corresponds to the elastic recoil of the arterial walls. The downstroke regularly shows a fluctuation known as the dicrotic notch. This is due to vibrations set up when the aortic valve snaps shut – it has no major significance as an indicator of health.


For a healthy person at rest, the heart beats on average at about sixty beats per minute or one a second i.e. around one Hz. This frequency is not constant (even in a healthy individual at rest).


It takes approximately one second to record a complete wave, so it is usually best to select a short recording time.


This Sensor is not waterproof. Clean the clip by wiping with a cloth that has been rinsed in disinfectant.