Electronic stethoscope

The traditional medical stethoscope has no amplification, the sound is weak, it is uncomfortable in the ear, and it is also greatly affected by environmental noise. The electronic stethoscope introduced in this example uses a multi-stage low noise amplifier with adjustable output volume, good frequency response, low background noise, and LED display.
Circuit Operation The electronic stethoscope circuit consists of a pickup sensor, preamplifier, low-pass filter amplifier, buffer, audio amplifier and LED display circuit, as shown in Figure 9-136.
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The pickup sensor circuit is composed of microphones BM, R1, and the like.
The preamplifier is composed of an integrated operational amplifier circuit ICl and a resistor R2-R5.
The low-pass filter amplifier is composed of an operational amplifier integrated circuit IC2 and resistors R6-R8, capacitors C3, C4, etc., and its cutoff frequency is slightly greater than 100 Hz.
The buffer amplifier is operated by an integrated operational amplifier circuit IC3.
The audio amplifier is composed of a volume potentiometer RP, a low voltage audio amplification integrated circuit IC4, a resistor R13, capacitors C5, C6, and the like.
The LED display circuit is composed of a two-color light emitting diode VL, a driving amplification integrated circuit IC5, and a resistor Rg-Rl2.
The signal taken by the pickup sensor is filtered and amplified by IC1-IC4, and the earphone BE is driven to sound. The low-pass filtered audio signal through IC2 is further amplified by IC5, and the driving LED VL is synchronized with the sound in the earphone.
Adjust the resistance of the RP to change the volume of the headphones.
Changing the resistance of the resistors R5 and R6 changes the cutoff frequency of the low-pass filter, thereby changing the frequency response of the electronic stethoscope.
Component selection
Rl-R4 and R7-R13 use 1/4W or 1/8W metal film resistors; R5 and R6 use sealed variable resistors.
RP uses a small synthetic carbon film potentiometer.
Both Cl and C5 use aluminum electrolytic capacitors with a withstand voltage of 16V; C2-C4 and C6 use polyester capacitors or monolithic capacitors.
VL uses two-terminal two-color LEDs, and two φ3mm LEDs (one for red and one for green) can be used in reverse parallel connection.
ICl-IC3 and 1C5 select LM741 or μA741 type single integrated operational amplifier circuit; IC4 selects LM386 type audio amplifier integrated circuit.
BE uses high quality two-channel stereo headphones.
The pickup sensor can be made by itself: using a diaphragm head of a conventional stethoscope, a 3-5 cm long rubber tube is placed on the diaphragm ear handle, and an ultra-small electret microphone is mounted on the other end of the rubber tube. The microphone and the circuit board are connected by shielded cables. The microphone is reinforced with heat shrink tubing or taped to prevent noise interference during operation.