Electronic Stethoscope With MEMS Microphone and Recharchable LiPo Battery

This instructable describes how you can make a low-cost electronic stethoscope using a high-quality MEMS microphone, and a high-gain low-noise OpAmp circuit.

The design includes charging circuitry powered by a conventional micro-usb connector (from a phone charger or computer's USB port).

The project requires the following parts:

• Cheap acoustic stethoscope (~$15, try this, this, or this pink one!)
• MEMS microhpone breakout board ($5, try this)
• 12V step-up regulator ($4, try this)
• USB LiPo Charger ($1.50, try this)
• Breadboard (for prototyping, try this)
• Protoboard (for final version, try this)

We also need the following electronics components; all part numbers and links for the entire bill of materials (BOM) are included from a single source (Digikey). For all resistors, use "metal film" type for lowest noise and best results:

• Resistors
  • 2 x 47 K (Digikey 47.5KXBK-ND)
  • 1 x 1 K (Digikey 1.00KXBK-ND)
  • 1 x 10 Ohm (Digikey 10.0XBK-ND)
  • 1 x 200 K Potentiometer (Digikey 3306F-204-ND)
• Capacitors
  • 1 x 0.1uF Electrolytic (Digikey 493-10253-1-ND)
  • 1 x 10uF Electrolytic (Digikey 732-8856-1-ND)
  • 1 x 100uF electrolytic (Digikey 493-13464-1-ND)
• OpAmp
  • 1 x TL072 OpAmp (Digikey 296-1775-5-ND)
• Audio Jack
  • 3.5mm stereo headphone jack (Digikey CP1-3515N-ND)
• Power
  • SPDT Slide Switch (Digikey A107673-ND)
  • LED 3mm (Digikey 516-1310-ND)

The image above shows the schematic for the amplification circuitry.

Notes:

• Our OpAmp (TL072) is dual-channel; our design only requires one channel of the OpAmp. The amplified signal is sent to both the left and right sides of the headphone jack
• If you'd like a stereo stethoscope, just duplicate the parts of the schematic around the 1st channel of the OpAmp a second time; connect one output to the left channel of the headphone jack and the 2nd to the right channel
• The gain for this circuit is defined by the ratio between R2 and R1:
  
  Gain = R2 / R1
  
  Therefore, using the 200K variable resistor (potentiometer) as R2 and a 1K resistor as R1, we can get up to 200 times gain.

The image above shows the power and charging circuitry for our amplifier. Notice that the connections between this part of the circuit and the amplification portion (preview step) are made through the names of the signals. For example, all signals connected to "+12V" should be electrically connected to the "VOut" of our 12v-step-up module (middle portion of the image for this step).

(Left) Battery Connections and Charging: Our USB-LiPo charging module does several things: notice that our battery goes through this module. The module thereby regulates and protects the battery (from overdrawing power) and it allows charging with USB. This portion of the schematic also shows how a SPDT (single-pole double-throw) switch can be inserted between the ground of the battery and the "B-" input of the USB-charging module to act as a power switch.

(Middle) Voltage Regulation: Our OpAmp requires 12V while a conventional 1-cell LiPo battery (very similar to the ones found in mobile phones) provides around 3.7V. We use a 12V boost step-up module (middle picture). This module has 3-pins; VIN (from battery), GND (from battery and to the rest of the circuit) and VOUT (our +12V signal required to power the OpAmp)

(Right) LED Power Indicator: While this is optional, it's nice to have so you don't accidentally leave the circuit on and drain the battery.

There are many ways to put together a functional electronic circuit. A quick way to prototype a circuit board is to use a solderless breadboard. The picture above shows an annotated breadboard version of our schematic.

Notes:

• Our 3.7V LiPo battery is connected directly to the GND and VIN of the Step-Up module;
  The GND and VOUT of the step-up are connected to the top power-rails of the breadboard
• The GND from the 12V step-up module should be connected to all GND connection so the amplification circuitry
• A headphone (Tip-Ring-Ring-Sleeve) breakout from Sparkfun is used for the breadboard version; this component is not necessary for a printed-circuit-board version.

A printed circuit board (PCB) fabricated by a company like Oshpark, BasicPCB, or DirtyPCBs is very low cost and far more robust and reliable than breadboards and prototyping boards. You can use a design tool like Autodesk Eagle (free!) to layout your schematic and board, and to produce the fabrication files for the above companies. Some companies (like Oshpark) can accept Eagle .brd files directly, which simplifies the process even more.

As a reference, Oschpark can manufacture THREE of these boards for $8.05. All you need to add is the electronics components.

You can fine the Eagle design files for this project in this github repository; look for MEMS-preamp_v04.brd (the board) and MEMS-preamp_v04.sch (the schematic).

Happy close listening!

Notice the tiny hole in the metal capsule at the center of the MEMS microphone breakout module. This hole is how sound gets into the microphone. In order to channel the sound from the stethoscope diaphragm to the MEMS microphone, we must fasten the cut stethoscope tube to this area and direct the incoming air from the diaphragm (i.e. the sound) straight into the MEMS microphone.

1. Cut the stethoscope tube about 1" below where it splits to go to the ear pieces
2. Cut a hole into your Altoid-like box that matches the size of the tube; consider using a "step-drill" bit to make this easier
3. Cut holes for the audio-jack, power switch, and LED light in the Altoid-like box
4. Use hot glue to afix the amplifier board to the bottom of the Altoid-like box
5. Use hot glue (plenty of it!) to fasten the tube directly over the MEMS microphone

Once you have a working circuit on a solderless breadboard, try to adjust the placement of copmonents so that it uses as few rows as possible when making your breadboard prototype (e.g. limit yourself to the left half of the breadboard). You can use an online tool like Autodesk's http://circuits.io to make this step easier.

Once you have all the components fitting and connected correctly on about 20 rows of a bread board, you can transfer your prototype directly form the solderless breadboard, to the solderable prototyping board that has all the same internal connections as your breadboard.

Another option is to go straight to making a printed circuit board (PCB), which is even more robust and reliable. This is our next step!