Electronic Stethoscope

electronic stethoscope

Create an electronic stethoscope giocoattolo pondering how to turn it into a professional with small simple construction changes. Given the fundamental absorption is less than 4uA standby time and ease of use.

After my nephew (4 years) saw his mother visited the doctor for a simple sore throat was thrilled by stethoscope. After a few attempts of parents with toy stethoscopes and not volendone buy a real one asked me if I could do anything I. After I responded that I do not do the magician I agreed to two conditions, both times the safety of the child.

  1. Use removable caps instead of ordinary earphones to avoid damaged eardrums due to mishandling.
  2. The size of the container must be such that it can not be swallowed.

    With these conditions, among other things, common sense, I started studying for the design.

    Eliminated the idea of ​​mini penlight batteries because it is easily reachable by the child opto for a CR2032 button cell directly mounted on the small printed.

    This solution forces me to think about circuits that do not absorb current absolutely unnecessary for two reasons, One is that they can not change batteries all the time and the other is that very often is forgotten turned on.

    After some research decide for operational MCP6042 with quiescent current of 600nA to the operational amplifier and a bandwidth product for amplification of 14KHz, actually it is not much but, if you want low current consumption and output rail to rail economic it is not found then very.

    Reasoning bandwidth begin to do immediately reverse calculation, admitting a frequency of 180 beats per minute would have a frequency of 3 Hz, Also using a 300Hz bandwidth would have an amplification more than enough for every single stage.

This simple circuit allows you to listen to the heartbeat. The transducer used to detect the pulse is an electret microphone. The model used is equipped with two terminals (polarized). As usual with this type of microphone, It works through a series resistor, R4. The potential drop across this resistance is applied to op amp through C7.

The first operational It has many functions:

The presence of C1 makes the unitary amplification for low frequencies, its value is chosen in such a way as not to restrict in any way the passband down.

C7 together with R8 form a high-pass with a lower cut 1Hz (60 beats per minute), the slope of this filter is slightly marked and therefore also pass safely lower frequencies with a minimum attenuation.

C6 and R1 form a low-pass with 330Hz frequency and this applies also to the speech that pass anyway higher frequencies with a minimum of attenuation, then even if the same operational her decreases its amplification wineskin said limit.

R1 and R6 securing the pre amplification of step a 46

The bandwidth of this stage before the amplification is reduced is set to 14,000 / 46 = 300Hz well above the incoming signal limits, this wide excursion outgoing calls is used to detect pathologies "puffs", certainly they do not need a child but, with a careful implementation to mitigate to the maximum noise by rubbing may be modified to professional implementation.

The second operational It has adjustable amplifier functions:

the resistors R5 and R7 together with P1 determine the amplification that can go from 0,97 a 45 with a total amplification of the chain which may go from 0.97×46=44 a 45×46=2070 (66dB amplification are more than sufficient in any case)

Even this second stage has a bandwidth of (14000/45=311Hz), the sum of several cascaded limitations allows to have a large slope without the use of complex circuits.

The third operational at the bottom creates the virtual mass:

R2 and R3 of identical value bisect the supply voltage while the capacitor C2 makes sure, with its inertia, this value is not affected by abrupt changes due to the signal present in the load, the two resistors are of high value to have a fixed absorption 1,5uA, further down it would have been counterproductive because then the current loss of the capacitor and the operational amplifier input resistance would compromise the stability of the reference.

The operational in Questin is biased in buffer configuration, we will find at its output the same voltage present at the entrance but amplified in current.

The fourth operational creates a push-pull amplifier:

For low signals work in class at fueling headphones via the resistor R10, as the power increases the potential drop on the latter exceeds the conduction threshold of the transistors that are responsible for providing the necessary power to operate in the headphone output.

Such a system is also served to reset the stadium absorptions at rest because the transistor in the absence of signal are interdicted.

Therefore, up to that in caps flows a current less than 3mA the two transistors do not intervene and works in class A over that threshold working in class B.

Some might argue that it is not the most suitable configuration for an audio amplifier but, must be considered the fact that the operational amplifier which operates the two transistors is in buffer configuration, and therefore will vary its control output to make sure that the output of the transistor, reported in input from R9, dia a voltage identical to that of input reducing to negligible values ​​the distortions.

The capacitor C8 is only meant to cancel any auto-oscillations of the stadium.

Construction and Calibration:

Having at hand a rectangular box roughly the size 6cm x 3cm I developed a printed that there entered comfortably, The following are some pictures of the project, Note that the battery is slightly smaller container. This container has closing interlocking and serves a certain force to reopen, which it is good for the safety of the child.






The most observant will have noticed the presence of a built-in 6 pin into the printed not listed components, Well it is not a built-in is the only thing that I found at once that he had the same impression of the power button.

For the children there will be a box to be placed on the heart and with the data from ear headphones listen to the heartbeat, constructively quite similar to a real stethoscope.

I just hope that the child will be happy when Santa Claus will hand this object of his desires stethoscope.

4 replies
  1. Avatar
    maxmazzinghi says:

    Hello nice little project that I'd like to replicate it…are not very “ferrato” with SMD, I'd rather do it even a little larger but with standard components.
    Everything ok for integrated and other components but the BC807 and BC817 transistors are only SMD. With what can I replace them?
    Thank you.


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