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Guest spongeidiot

Help with Audio Guru's Stethoscope.

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Guest spongeidiot

Hello! I have just bread boarded Audio Guru's Electronic Stethoscope and I have been having trouble getting the heart sounds.

1. I recently just bought a mic with a minimum frequency of 20Hz which is enough to cover the heart sounds. However I still do not hear any.  This is the mic i bought. http://www.kingstate.com.tw/index.php/tw/products-ecm/products-water-proof/item/92-keeg1538wb-100lb and i am just using alligator clips to connect it to the circuit should I still connect this mic to a shielded cable even if it has wires already? I just dont want to risk overheating the mic and  cause damage to it.  ???

2. A dual LED is not available from where I am from. Right now iam just using 2 LEDs. When I turn it on, One is LED is steady and the other LED is not on. I assume that that is for the sounds and is supposed to be blinking when there are heart sounds. Am I correct with this one?

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Hello! I have just bread boarded Audio Guru's Electronic Stethoscope and I have been having trouble getting the heart sounds.

1. I recently just bought a mic with a minimum frequency of 20Hz which is enough to cover the heart sounds. However I still do not hear any.  This is the mic i bought. http://www.kingstate.com.tw/index.php/tw/products-ecm/products-water-proof/item/92-keeg1538wb-100lb and i am just using alligator clips to connect it to the circuit should I still connect this mic to a shielded cable even if it has wires already? I just dont want to risk overheating the mic and  cause damage to it.  ???

Your mic might be defective.
The datasheet does say it is an electret mic but I think it is. The polarity of its wires is not shown but I assume that red is signal and black is shield ground. Swap the wires to see if it works.
If the unshielded mic wires are too long then they will pickup mains hum.

2. A dual LED is not available from where I am from. Right now iam just using 2 LEDs. When I turn it on, One is LED is steady and the other LED is not on. I assume that that is for the sounds and is supposed to be blinking when there are heart sounds. Am I correct with this one?

LEDs 1 and 2 should blink: "1, 2 pause 1.2 pause" on and on where 1, 2 pause is one heartbeat.
Nothing in the schematic will cause one LED to light steadily and the other LED not to light. Maybe you have C2 connected with backwards polarity?
The output of the TL072 opamps should be 10mV (0.01VDC) or less. If the 741 opamp has its worst input offset voltage of 6mV then the gain of 71 times in this circuit will cause its output to be 0.4V which is not enough to light an LED.
Measure and report the output voltages of the opamps with no signal.

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Guest spongeidiot

The polarities were correct. I tried speaking into the mic but i had to speak close to it inorder for the LED to light up. While the other is steady. Does this mean its defective already because i have cut it short?

l have measured the output pins of the TL072. Pins 1 and 7 based on the datasheet with reference to the ground. What i got for the first op amp was fluctuatinf from 0.9V to 0.12V and 2nd opamp was from 0.16V to 0.20V. The output of the 741 was 3.74V. Not sure on how to measure the input offset.

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The polarities were correct. I tried speaking into the mic but i had to speak close to it inorder for the LED to light up. While the other is steady. Does this mean its defective already because i have cut it short?

l have measured the output pins of the TL072. Pins 1 and 7 based on the datasheet with reference to the ground. What i got for the first op amp was fluctuatinf from 0.9V to 0.12V and 2nd opamp was from 0.16V to 0.20V. The output of the 741 was 3.74V. Not sure on how to measure the input offset.

The wiring or the opamps are defective. Maybe you built the circuit on a solderless breadboard and its strips of contacts and connecting wires are picking up mains hum or causing high frequency oscillation.

The maximum input offset voltage for a TL072 is listed on its datasheet as 10mV. The voltage gain of the first opamp is 1 so its output should be plus or minus 10mV or less.
The voltage gain of the second opamp is a little less than 1.6 so its output should be plus or minus 32mV or less. Most opamps will have much less offset voltage.

The voltage gain of the 741 opamp is 71 so the output of the second opamp is wrong at 3.74V/71= 53mV.

Speaking 5cm from an electret mic produces about 20mV RMS which is 28mV peak. The input to the 741 opamp will be 28mV x 1.6= 45mV peak and its output will be 45mV x 71= 3.2V which will light a red LED but dimly light a blue or white LED. Maybe your voice does not produce low frequencies (are you a girl?). 

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