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pulse rate measurement

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sam288

New Member
I tried building a heart rate sensor by using red LED and LDR as well as IR LED and phototransistor pair.....and holding a finger between the LED and the respective sensor.....
In case of LDR i am not getting anything at the output....What could be the reason..?
In case of phototransistor the pulses are too many to be heart beats.....
I havent done any amplification or filtering as yet....
Can anybody suggest which method is better and provide me a ckt diagram if possible....thank you..
 

KMoffett

Well-Known Member
sam,

Could you post your circuits, that don't work?

ken
 

KMoffett

Well-Known Member
Are you testing figure 4.1-3 ? Where are you taking your measurement, and with what? Be aware that in all of these circuits, the light from the LED can be swamped out by room light, at the detector. You must shield the LED/detector from outside light sources.


i also tried the sensor of this ckt...(only initial part of LDR in vtg divider) i am grtting only a shift in dc level when i insert a finger between the led and ldr...

www.howareyouperceived.co.uk/images/Pulse%20Amplifier.doc
The pulse signal will be a very small variation (millivolts) in a large (volts) DC offset. Have you tried looking at the signal with an AC coupled oscilloscope set on a high sensitivity.? This circuit DC couples all stages so does not compensate for the initial offset.

This is a better circuit because the 1µF AC coupling capacitors should eliminate the DC offset and allow only the pulse signal to be amplified.

ken
 

sam288

New Member
yes i built the ckt in fig 4.1-3....i checked the output across the LDR....and did not use any shielding to prevent stray light....do u suggest i should try once more with shielding...also havent understood how they have calculated R1=10ohms...
 

KMoffett

Well-Known Member
Without shielding from ambient light you will probably never see the tiny pulse variation voltage. The 10Ω resistor was probably chosen to limit the current to a safe level for the LDR, if the LDR went to a very low resistance if exposed to a bright light.

I personally would not use either of your first two linked circuits, because of the DC coupling. It makes it very difficult to amplify the small pulse signal in the large DC offset of the LDR/resistor half bridge.

What test equipment are you using to look at the pulse from your detector?

ken
 

sam288

New Member
i am using a normal CRO...but i had seen high frequency pulses at the 3rd ckts output on a DSO...so should i go for the IR ckt...? and there is a mistake in the 3rd ckt that they have used HPFs instead of LPFs (capacitors and resistors exchanged)...maybe thats why i am getting high frequency pulses...
Also i dont know why they have used 2 filters when filtering and amplification can be achieved using a single filter...
 

sam288

New Member
I have been working on these circuits for quite a few days now...but i am not getting the desired results from any of the LDR ckts..The IR one works but the pulses at the transistor output are too many per minute...can anybody please give me a 'heart rate sensor' circuit which works and is simple to build...i would be very very grateful....thank you..
 

KMoffett

Well-Known Member
sam,

Can you post a photo of how you have your LDR/LED setup...and a schematic of exactly what circuit(s) you are using?

Ken
 

sam288

New Member
As of now i am doing everything on bread board. The phototransistor fixed on it and the led is wired such that it comes above it...and the finger is placed in between....in case of ldr similar arrangement was made....
I have tried the IR sensor from the 3rd link posted in 1 of the above posts by me.(tried only sensor because they have used HPFs which i dont know why...)
It was giving me some kind of high frequency square like wave at transistor output...
(wonder whether the transistor is properly biased??)

Then i tried building the 2nd order Butterworth LPF given in the pdf document of the LDR sensor ckt...so that i can give the IR sensor output to it but it is not acting as an LPF practically...it is passing all frequencies..

Should i use a simple RC filter to remove high frequency noise...remove dc by a capacitor and then simply amplify the signal....
 

KMoffett

Well-Known Member
Back to the input basics. What kind if a signal are you getting out of the LRD/LED pair? Do you have the finger covered from ambient light? Without the details, I'm just guessing.

Kenb
 

sam288

New Member
I am getting a periodic train of flat pulses of amplitude about 5v and frequency 100hz... between the C and E of the phototransistor(see sensor of 3rd link)... since it is an infra red led i did not use shielding from ambient light..
 

audioguru

Well-Known Member
Most Helpful Member
An old fluorescent tube light and an AC incandescent light produce IR at 100Hz if the mains frequency is 50Hz.
 
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