heart beat counter,jus started,need help..

[tralne]

im starting a project on digital heart beat counter..
It has a sensor, a preamplifier,low pass filter,an amplifier+pulse indicator,
amplifier+digital counter..

im confused about the selection of sensor..

the sensor should sense the heart pulse(from wrist or finger),
and convert it to electrical parameter..

ma requirements are that the sensor should be

- cheaper
- noise should be less
- compact

do u have any suggestion?
pls help..
thanks:wink:

 

[John Connor]

Hi
My guess is that you are using IR or red light for detecting the heart beat.
the best sensor would be CNY70 it is a reflective optical sensor with a plastic enclosing which reduce noise coming from ambient light.
However you could also try L14F1 which gave higher higher sensitivity when I tested it.


http://www.vishay.com/docs/83751/cny70.pdf
http://www.datasheetcatalog.org/datasheet/fairchild/L14F1.pdf

 

[tralne]

k..
i ve been thinking of 4 sensors

-piezoelectric
-infrared
-microphone
-with led

i dont know much about the cost , sensitivity or accuracy of these sensors..

i ve heard somewhere, piezoelectric is very costly. is that true?

the other idea was that to use the stethoscope and to fit a microphone to convert it to voltage..but microphone will have noise problems and we need a
compact device also

If we are using Infrared source and a detector for this,will there be any effect of noise?

 

[ckshivaram]

these are implemented in circuits given by

N S K E L E C T R O N I C S

 

[John Connor]

Yeah piezoelectric sensors are costly....
The most compact and low cost you can get is CNY70 (7x7x6 mm) which includes both IR emitter and receiver so no more extra modules required.
you can use it on the skin to detect heart rate by reflective method.
The main source of noise you will get will be of 50 or 60 Hz household power line signal which can be removed by proper filtering.
The microphone method you suggest might not work (though I have not tried it) as microphones are manufactured for audible range of frequencies 20Hz to 20KHz and your working frequency would be near about 2-4 Hz. and also you cannot detect sound from the finger.

The use of IR or LED produces the same effect however better sensors are available for IR region than RED light.

 

[Kripton2035]

this project should help you a lot ... :wink:

Heart rate measurement from fingertip :Embedded Lab

regards,

 

[tralne]

.
The microphone method you suggest might not work (though I have not tried it) as microphones are manufactured for audible range of frequencies 20Hz to 20KHz and your working frequency would be near about 2-4 Hz.

thanks john

may i ask u,
are the microphone manufactured only for 20hz to 20khz?
wont we get microphone for lower frequency range?

 

[tralne]

k this s ma basic block diagram..


SENSOR--->PREAMPLIFIER--->LPF---->PULSE AMPLIFIER---->COUNTER





can any one tell me why we use preamplifier before the low pass filter??

 

[dr pepper]

The above thread on the optical heartrate counter is eactly what you need.
You dont have the microcontroller thats fine, connect the output of the circuit (first schematic on page 2) to your counter.
If your not using the micro connect the cathode of d1 directly to ground, so that the infra red led is on all the time.
This circuit is very similar to the ones you find in hospitals.

 

[John Connor]

Yeah, you can get those but they won't be cheap.
try crystal or piezo microphones those are less noisy, however they are prone to handling noise.

---------- Post added at 19:46 ---------- Previous post was at 19:41 ----------

The outputs from transducers are generally very weak and for subsequent signal processing some minimum amount of signal is required otherwise the signal will be attenuated beyond recognition and no amplifier will work. Therefore we use a preamplifier to get a nice amount of signal to work with.

 

[tralne]

https://www.vishay.com/docs/83751/cny70.pdf

i want to use optical reflective sensor.
the bar code reader has some thing like this right?

which would be best,
using a red light or infrared rays, to detect the pulse change in our body?:?

thanks;-)

 

[John Connor]

As you are using reflective method, red light might give you better reception as absorption of Redlight is low in oxygenated haemoglobin and hence more reflection. However availability of highly sensitive and compact receivers for red light is another factor...
CNY70 uses Infrared.
Apart from the present discussion, you may like to consider two more type of sensors.
1)EKG Type Electrode sensors placed at different points on the body. This might be the cheapest of them all, Just two electrodes.
2)Ultrasonic sensors based upon doppler effect.

 

[tralne]

i am designing the low pass filter part..
sallen key is being used..

the heart beat rate considered for the design , is from 40bpm to 120 bpm.

so the frequency of signal ranges from 0.667hz to 2 Hz..

should i take exactly 2Hz as the cut off frequency?
what all things should i consider for designing the filter?

thanks

 

[ama_trig]

The microphone method you suggest might not work (though I have not tried it) as microphones are manufactured for audible range of frequencies 20Hz to 20KHz and your working frequency would be near about 2-4 Hz

Im not sure, but I think the heart beats at 2-4 Hz, while the sound generated from it, is completely hearable via ear, thus making it hearable to microphone too!
record the sound via microphone, decide it your self, even it is so simple to check the recorded signal for measuring the heartbeat

should i take exactly 2Hz as the cut off frequency?

if I were you, I would graph the signal amplitude, then decide on cut off or even filter type.

 

[tralne]

Im not sure, but I think the heart beats at 2-4 Hz, while the sound generated from it, is completely hearable via ear, thus making it hearable to microphone too!
record the sound via microphone, decide it your self, even it is so simple to check the recorded signal for measuring the heartbeat

thanks..

i had first decided to select the IR sensor..since i got a piezoelectric sensor, i decided to do with it..
but piezoelectric sensor is having many problem. it detects even the slightest movement of hand along with heart beat pulse..

if piezo doesnt work , i ll ve to try my next choices..

record the sound via microphone, decide it your self, even it is so simple to check the recorded signal for measuring the heartbeat

thanks for this

---------- Post added at 19:04 ---------- Previous post was at 19:01 ----------

if I were you, I would graph the signal amplitude, then decide on cut off or even filter type.

sorry , im doing all these stuffs for first time..and have less knowlege..
but, why should we graph the signal amplitude?
i mean, how can we determine the cut off from signal amplitude?

 

[ama_trig]

sorry the post was like this

I would graph the signal, amplitude, ... then decide.
Graphing amplitude will give you good ideas on how for example moving hands may generate noise, How strong its going to be, How effective the filter would work and how much noise you might be faced with, ...