Can anyone help me with the circuit pls (ECG circuit)

Thread Starter

BlackCat123

Joined Mar 14, 2023
19
I'm building a simulate ECG circuit with this link: https://www.instructables.com/Digital-ECG-and-Heart-Rate-Monitor/
However, I make some changes for the supply voltage: I connect Vee to the ground, Vcc is 15V and I apply the the pulse wave with 1Hz frequency, bandwidth 500ms, Vpp is 10V. However, when I use the oscilloscope to measure the waveform of the output of the Instrumentation Amplifier (pin 6 of the 3rd LM741 from the top to the bottom), I got nothing but a straight waveform. However, in the multisim. the output generate signal waveform. Can someone help troubleshoot my circuit?
Here is my circuit: ecg.jpg
 

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ericgibbs

Joined Jan 29, 2010
18,827
Hi BC,
Welcome to AAC.
The VEE of -15V is essential for that circuit to work, setting VEE at 0V will give you that problem.
E
 

crutschow

Joined Mar 14, 2008
34,390
Why would you change the supply voltages and still think it would work?
You multisim simulation apparently did not use a good model for the 741 if it showed it operating with a single supply.
The ancient 741 cannot operate down to 0V with a single supply (look at its data sheet).
It is designed for ±15V supplies as Eric noted.

You could go to a single supply op amp such as the LM324, but the signal would need to be offset so it never goes below 0V.
 

panic mode

Joined Oct 10, 2011
2,732
your schematics shows +/-5V while original shows +/-15V.
it is common for OpAmp circuts to use dial supply. this can still be powered from single 30V supply by creating virtual ground.
1678807803534.png
 

Thread Starter

BlackCat123

Joined Mar 14, 2023
19
your schematics shows +/-5V while original shows +/-15V.
it is common for OpAmp circuts to use dial supply. this can still be powered from single 30V supply by creating virtual ground.
View attachment 289724
Because from multisim, I saw it works and it has the nice waveform so I believed the actual circuit works also. I tried to make a -15V to supply Vee and when I connect to the Vee, the power supply drops voltage significantly and it sparks. I have separated -15V connection with GND already.
 

ericgibbs

Joined Jan 29, 2010
18,827
hi, BC,
Have you read this warning from that site.?
How do you plan to use this device.?
Moderation.

NOTICE: This is not a medical device. This is for educational purposes only using simulated signals. If using this circuit for real ECG measurements, please ensure the circuit and the circuit-to-instrument connections are utilizing battery power and other proper isolation techniques.
 

Audioguru again

Joined Oct 21, 2019
6,687
Why use the very old circuit you copied that has 55 years old 741 opamps and its patient is an antenna who picks up 50Hz or 60Hz from electricity in the air? The circuit has only one probe plus a ground to the patient instead of using two probes for a balanced heart signal plus a right leg connection that cancels any external interference.

The very old circuit uses a 60Hz notch filter to reduce the 60Hz electrical pick up. Guess what frequency is a normal resting heart beat? 60Hz.

An AD620 IC is a modern instrumentation amplifier that has a datasheet showing an ECG circuit with a balanced input and a connection to the patient's right leg to cancel interference.
 

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Thread Starter

BlackCat123

Joined Mar 14, 2023
19
Why use the very old circuit you copied that has 55 years old 741 opamps and its patient is an antenna who picks up 50Hz or 60Hz from electricity in the air? The circuit has only one probe plus a ground to the patient instead of using two probes for a balanced heart signal plus a right leg connection that cancels any external interference.

The very old circuit uses a 60Hz notch filter to reduce the 60Hz electrical pick up. Guess what frequency is a normal resting heart beat? 60Hz.

An AD620 IC is a modern instrumentation amplifier that has a datasheet showing an ECG circuit with a balanced input and a connection to the patient's right leg to cancel interference.
So what is the right solution for this?
 

Thread Starter

BlackCat123

Joined Mar 14, 2023
19
hi, BC,
Have you read this warning from that site.?
How do you plan to use this device.?
Moderation.

NOTICE: This is not a medical device. This is for educational purposes only using simulated signals. If using this circuit for real ECG measurements, please ensure the circuit and the circuit-to-instrument connections are utilizing battery power and other proper isolation techniques.
It's a school project and I just designed for completion
 

Thread Starter

BlackCat123

Joined Mar 14, 2023
19
Imagine this is the breadboard and the lab power supply with 3 terminal. The virtual ground that you mention is the ground of the entire circuit, right???
 
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