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Heart Muscle (HeartBeat) Sensors

Fr. Tom McGahee says

There are several methods for detecting heart beat.
  1. Acoustical. Two years ago I had one of my students wire up a sensitive condenser microphone to a cheap stethoscope. A simple opamp preamp circuit was then used to feed both an audio amp and an oscilloscope. We had to use earphones so that we would not get feedback (squealing).

    Pros: Simple. Cheap.
    Cons: Requires contact with the body around the region of the heart. (stethoscopes should ALWAYS be kept in a refrigerator so that they are very cold when applied to the patient). Picks up breathing and many other background noises.

  2. Electrical. An "instrumentation" amp built from op amps can be used to directly monitor the small electrical signals from the heart. Besides the two differential input electrodes, a "guard" electrode at ground potential is placed between the two differential electrodes (which are placed on the skin, on either side of the heart). It is generally a good idea to follow the output of the instrumentation amplifier with a low-pass filter and simple opamp amplifier to boost the useable gain. The filter helps eliminate a lot of unwanted noise that is otherwise present.

    Pros: Can be made very selective by proper placement of electrodes.
    Cons: Requires that electrodes be attached directly to the skin surface using electrode sets that provide a firm electrical connection via a conductive gel.

  3. Indirect measurement of blood flow using Infra Red. When blood flows through the capillaries due to the pumping action of the heart, it has one phase where the blood is barely moving, and another where the blood is moving quite fast. In addition, the oxygen content of the blood is different in each of these phases. If an infra red beam of light is sent through or scattered back through tissue that has blood flowing through it, the amount of infra red that gets through or is scattered back is affected by the presence of the blood. In it's simplest form the detector consists of an infra red LED that generates an infra red beam, and an infra red photodetector such as a photodiode or phototransistor. Units attached to the finger tend to be the reflective type, whereas those clamped to the earlobe are transmissive types. Usually you have to pump a fair amount of current through the IR LED.

    The electrical signal derived from the photo detector assembly is then processed. There is usually a low pass filter followed by another stage of amplification.

    Pros: Easy to attach to humans. Can be attached to fingers or earlobes instead of having to be attached near the heart. Electrically isolated from the patient.
    Cons: The resultant signal is not as sharp as that measured by a type (2) device.

Practical considerations for type (3) devices:
It is useful to feed the IR LED with a constant current source so power supply voltage fluctuations do not affect operation. A variable constant current source is useful so you can adjust for skin thickness, etc.

If a comparator is used to monitor the output, it can be adjusted so that a nice clean digital outout pulse is created. Measure period betweeen pulses and use this do derive beats per minute.




See also:

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