Final Project - Revisited yet again

What are you going to make?

A Digital Stethoscope fabrication kit

What will it do ?

It will enable people in developing countries which lack medical professionals to build their own stethoscope,  which will record their heart beat and breathing. This stethoscope can then be connected to a PC or smart phone and the sound sample can be sent to doctors for remote analysis.

What questions need to be resolved?

  1. 1)What type of microphone will best suit this stethoscope (Pizoelectric, ribbon, etc)? Will I be able to fabricate it myself ?


After extensive research I have found that I cannot use a piezo microphone, but rather an electret. The piezo will have to be stretched upon the body in order for it to pickup the heart and this is not feasible. I also found that the smaller electrets cannot pickup the heart and I will require a Circular - 9.7mm Dia x 6.7mm H microphone with a 20hz-20Khz frequency range. These cost about $1.6 on digikey.

  1. 2)Enclosure fabrication: will this be a press-fit kit fabricated on the laser cutter or will this be printed on the 3d printer.

Answer - I have decided to use the 3d printer for the enclosure. The from will be smooth and curvy.

  1. 3)Processor type:  I  will probably use either an ATMEGA8535 or AT90USB646, the latter  containing a USB module, but it is more expensive. Therefor I will try to make do with the ATMega8535 and implement the USB via bit-banging. Both devices have a built in 10-Bit A/D converter for the recording and playback.

Answer - I will use either the ATMega168, AT90USB646 or the STM connected to a AT45DB161B SPI flash which will be used to store the audio.

  1. 4)Earphones: Will the device have earphones or will a speaker be sufficient to playback the recording. How will the earphones be constructed? Will the earpieces be molded rubber or 3d printed?

Answer: The device may have an earphone plug, but the audio will primarily analyzed on a computer.

  1. 5)Power Source: Will this be contain a rechargeable battery or other. What type of battery will suffice to power this device for a full day of tests


I will probably use a a Polymer Lithium Ion Battery - 1000mAh which weighs 22gr and costs around $10

  1. 6)Additional things I will need:

  2. -A quad op amp - will be used for microphone amp and low pass filtering

  3. -Better earphones - my IPOD earphone are worthless. The blocked out all of the low frequencies.

What is the Schedule?

Hopefully after the next 4 sessions (input devices, output devices, molding and 3d printing) by mid November, I should have all the information to answer the above questions. In the mean time I can start planning parts of the enclosure and the PCB.

(Very) Tentative Schedule:

Oct 25-Nov 1: PCB Planning

Nov1 - Nov 8: Enclosure planning

Nov 8: Nov 15: Laser Cutting / Printing enclosure

Nov 15-ongoing: Parts Procurement

Nov 15-Nov 22: PCB fabrication, Ear piece fabrication, Ear piece fabrication

Nov 22-Nov29: PCB Stuffing

Dec 1st-Dec8th: PCB Debugging

Dec13th: Project presentation

What is the budget?

Below are the required materials for this endeavor:

Microcontroller and Memory Circuit

10k Pull-up Resistor for DataFlash “Chip Select” Line

1k LED Resistor

Ω100 Analog Voltage Filter Resistor

LED Status Indicator

22 pF Clock Signal Circuit Capacitors

100 nF Analog Voltage Filter Capacitor

Oscillator Crystal 8 MHz Clock Signal Generation

DataFlash AT45DB161B 16-Mbit Serial Interface Flash Memory

AVR ATmega8535 Enhanced RISC Flash Microcontroller

Microphone and Speaker Circuit

10k Feedback Resistor for Microphone Amplifier

10k Offset for Microphone Amplifier

10k Offset for Microphone Amplifier

1k Microphone Power Resistor

12k Microphone RC Filter Resistor

5k Chebychev Filter Resistor

1k Chebychev Filter Resistor

470 Chebychev Filter Resistor

1k input Resistor for Microphone Amplifier

15k Chebychev Filter Resistor

12 k Earphones RC Filter Resistor

1 μF AC Coupling for Microphone

1 μF Chebychev Filter Capacitor

1 μF AC Coupling for Earphones

22 nF Earphones RC Filter Capacitor

100 nF Chebychev Filter Capacitor

100 nF De-coupling Capacitor

1 nF Chebychev Filter Capacitor

4.7 nF Earphones RC Filter Capacitor

2.2 nF Chebychev Filter Capacitor

LM324 Quad Op-amp

2 standard jack sockets 3.5 mm

2 Microphones - Electret

Earphones - Type to be determined

Enclosure type - TBD

My current best estimate for these materials including enclosure is ~ USD 50

Unrealistic !!!!!!

What is the Schedule?

a new and hopefully more realistic schedule (still very tentative Schedule):

Nov 21-Dec 4: Analog front end design and testing + FW design

Dec 6: Part list ready +  Procurement

Dec  7 - Dec 10: Board milling + stuffing + enclosure printing + FW implementation

Dec 10 - 12th:  Integration

Dec 13: D-Day Project presentation