ECG/Heart Rate Monitor

Thread Starter

mattyleeboy

Joined Jan 27, 2012
23
Hi i am trying to construct a heart rate monitor/ecg machine. Using the heart rate monitor circuit attached and an amplifying circuit from part of an ecg circuit diagram i have got a circuit that to my knowledge gave me some sort of readings. But now it does not give me any readings, can anyone see where i am going wrong?

heart-rate-monitor-circuit-diagram.jpg

IMG_0559.jpg

Amplifier.JPG
 

hgmjr

Joined Jan 28, 2005
9,029
If it is the second one, then it looks like you are trying to power it with a single supply. If you look at the first schematic, you will see that the designer took precautions to add a capacitor in series with the 100K resistor in the input stage. This was done to allow the use of a single supply.

hgmjr
 

Thread Starter

mattyleeboy

Joined Jan 27, 2012
23
so what you're saying is that it is not as easy as just implementing the 2nd schematic into the 1st schematic as i have done in the actual photo of my board? (2nd attachment)
 

hgmjr

Joined Jan 28, 2005
9,029
so what you're saying is that it is not as easy as just implementing the 2nd schematic into the 1st schematic as i have done in the actual photo of my board? (2nd attachment)
It is best that you build the circuit just as it is built in the original circuit first and get it to work. Then if you want to tweak the design to see what improvements you can make then you can do so secure in the knowledge that you started with a working circuit.

This technique can be very educational to a beginner.


hgmjr
 

MrChips

Joined Oct 2, 2009
19,383
Building and getting an ECG monitor to work is not an easy task. There are a few things that you need to know. For safety, all electronics should be battery operated and not connected to the AC mains.

You will need special electrodes such 3M™ Red Dot™ electrodes and proper leads. At the minimum, you will need three electrodes and leads, two for signal and one for ground. You can experiment with the placement of the electrodes, such as one on each arm and the ground on the left leg, or on the chest as shown in the diagram below.


The patient must be perfectly still otherwise you will pick up muscular activity.
The most difficult problem is reducing the common mode signal.
 

justtrying

Joined Mar 9, 2011
428
I would suggest you start by building an instrumentation amplifier with a gain of a 1000 (there are plenty of schematics out there) - it is basically what you have in 3rd attachment. Check if it works with 2 electrodes. Your next step would be to add what is known as a right leg driver, that is the ground and helps eliminate noise. But you should be able to pick up heart beat with the instrumentation amplifier although it will be noisy. Having good electrodes is very important for quality signal. Once you are past these steps, you can move on to next part(s).

And yes, as a beginner myself, I've been through this process and it is very neat. One thing I've learned, start small, make sure it works, then move forward.
 

Thread Starter

mattyleeboy

Joined Jan 27, 2012
23
I am trying to use a bright LED And an LDR and shine it through a finger or ear lobe to calculate the amount of oxygen flowing through the blood...is the electrode method better for what i want to do?
 

MrChips

Joined Oct 2, 2009
19,383
You have to make up your mind what kind of monitor you want to build.
Is it an ECG, pulse oximeter, or heart-rate monitor?
For a heart-rate monitor, the simplest solution is a stethoscope on the chest using a microphone.
 

Thread Starter

mattyleeboy

Joined Jan 27, 2012
23
Oh i see, the reason why i may be seeing any results on my oscilliscope is because im using and LDR and LED rather than a microphone and stethoscope?
 

justtrying

Joined Mar 9, 2011
428
if you do pulse oximetry, you still need to figure out how the circuit functions :) Start on the breadboard with simulations, then move on to fingers. You might want to try it on an earlobe as it is thinner. Microphone is a good suggestion as well, but in medical devices ECG (electrodes) and pulse oximetry are the more common methods.
 

Thread Starter

mattyleeboy

Joined Jan 27, 2012
23
How do u get an actual heart beat signal from this circuit? All i seem to be seeing on the osciloscope is the capacitor charging and discharging.
 

Audioguru

Joined Dec 20, 2007
11,251
The first circuit shines a light through a finger and causes the meter to pulse with heartbeats.

The second circuit and the photo of a person with 3 electrodes measures the electrical signal from the heart with an ECG unit that shows the waveform of the electrical signal on a screen, it is not an electronic stethoscope that uses sound.

My heart was tested with all the different ways. Another way my heart was tested was with ultrasound that showed my heart beating and played the heartbeat in a speaker. It even played blood flowing sounds.
 

Audioguru

Joined Dec 20, 2007
11,251
How does the LED cause the meter to pulse with the heartbeat?
The first circuit has an LDR (light-dependent-resistor) that is a high resistance when in the dark and is maybe 100 ohms to 2200 ohms when lighted. It forms a voltage divider with the resistor in series with it so the voltage between them goes up and down when blood (during each heartbeat) blocks the light. The up and down voltages are amplified by the opamps which drive the meter.
Since Infrared is not used and ordinary light from an LED is used then ambient light must be shielded.
 

Audioguru

Joined Dec 20, 2007
11,251
The first circuit has a lot of voltage gain so its many wires and connection strips will pickup main hum if you build it on a breadboard instead of on a compact pcb.

It is designed to pulse a meter, not to show a heartbeat waveform.
It does not have a gain adjustment for the amount of LED and ambient light, the colour of a person's skin or the variables in the circuit so its waveform will probably be just a square-wave. Here is where to connect a 'scope:
 

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