So this below pulse sensor circuit works just fine, but I need to experiment whether the HPF is really necessary as I have tested it with very low HPF values like 0.5Hz and still can see my pulse. Every time I try to remove the HPF by feeding the signal straight to Active LPF stage, my signal vanishes. Why is that? How do I get rid of it?

 

You may be saturating the circuit from the DC bias.
Where is the pulse coming from?
What is it's amplitude?

 

You may be saturating the circuit from the DC bias.
Where is the pulse coming from?
What is it's amplitude?

The pulse is from a finger. The amplitude is around 1.4-2 Volts right after the photo transistor. Interestingly, at very low HPF cutoffs (0.5Hz), I am experiencing 8-10 seconds of delay before the pulse starts materializing. Why could that be? I am using a MCP6004 btw, not the LM324 mentioned in the schematic.

 

hi Don
When you remove the HPF ,is C1 cap still in the circuit.?
E

 

Hi Don.
This is what LTS shows for your amplifier.
E

 

hi Don
When you remove the HPF ,is C1 cap still in the circuit.?
E

Hi Eric. No, I remove the capacitor and connect the photo transistor output straight to the resistor in the active LPF stage.

 

Hi Don.
This is what LTS shows for your amplifier.
E
View attachment 292162

Thank you for this. I had started using LTS recently but then got too busy (and honestly a little lazy) to learn enough about how to use it. I am guessing this graph is really useful in understanding this amplifier's response. Please explain to me what it is saying, I am not educated in electronics.

 

Hi Don.
The previous plot for the HPF section, showed lower and upper -3dB points at 0.5Hz through 10MegHz
This plot is for the full amplifier, 0.339Hz thru 4.72Hz.
Maximum, gain approx 45dB
What is the required BW .?

E

 

Hi Don.
The previous plot for the HPF section, showed lower and upper -3dB points at 0.5Hz through 10MegHz
This plot is for the full amplifier, 0.339Hz thru 4.72Hz.
Maximum, gain approx 45dB
What is the required BW .?

E
View attachment 292168

ideally I was trying 0.5Hz to 50Hz because I heard commercial manufacturers go in that range, but I'll be happy in 1.5Hz to 15Hz. I don't understand why I am not able to remove the HPF. And I don't understand why there is so much delay for pulse to show at low HPF. I even tried 0.15Hz HPF with 1uF and 1M Ohm resistor. I thought (in my limited electronics knowledge) that maybe 1uF was too large for the job and taking time to charge, so I even tried 0.79Hz with 10nF and 20M Ohm resistor. Hardly any change.

 

Just to explain the answer given in post #2:

Because putting DC 1.5 to 2V on the second opamp input drives the output to the negative rail . You need the capacitor to block the DC level.

Why do you want to take out the high pass filter anyway?

 

Just to explain the answer given in post #2:

Because putting DC 1.5 to 2V on the second opamp input drives the output to the negative rail . You need the capacitor to block the DC level.

Why do you want to take out the high pass filter anyway?

I thought if I had gone as low as 0.15Hz cutoff, what was even the need of a HPF. I thought I'd just go without it and see what happens, and what I saw was that my signal vanished.

So if its needed to remove the dc level, why does it need to be right after the signal origin? I just need to understand the logic behind it all. Can't the HPF be placed after the LPF? or after the last stage amplification?

 

Hi Don,
Do you have an example of the expected waveform from the opto-coupler at the input of the OPA?
Also, an example of the signal you expect to see at the output of the final OPA.?

E

 

Hi Don,
This is your circuit using a heartbeat wav file from the web.
R1 and R7 are a simple signal attenuator to reduce the amplitude of the wav file signal.
E
Updated plot, more points.

 

So if its needed to remove the dc level, why does it need to be right after the signal origin?

Because the second amplifier is a DC amplifier. You don’t want to amplify the DC level from the photo transistor. You need a capacitor in series so that only the change in level is passed.

Or look at it this way. The second amplifier stage has a DC gain of -10. If the pulse at the phototransistor goes from 2.0V to 1.5V, the output of that second stage would go from -20V to -15V, but your power supply does not allow that, so it just stays at -5V all the time and the pulse had no effect.

 

In real life, in the absence of a very strong signal you need a lot of AC amplification. Why do commercial IR receivers use carriers in the 38 kHz range.

Maybe a commercial IR receiver is what you need. I bought a bunch of them new from an authorized distributor for about U.S. 15 cents each. They have excellent sensitivity and good selectivity.
Easy to used, something like this:

http://www.es.co.th/detail.asp?Prod=062100023

 

Thanks all, I think I'll go by @BobTPH 's advice and just stick with the HPF.

@ericgibbs , your charts are honestly going over my head, but I understand they are useful and I'll go through them again. If I need more clarity I'll come back. Thanks.

 

Hi Don,
Please confirm.
I assume when you say 'pulse' you are referring to a human heart pulse detector.
Which clips on the finger or ear lobe in order to detect the light transmission through the finger tissue and so enable measurement of the heart beat etc..????

E

 

Hi Don,
Please confirm.
I assume when you say 'pulse' you are referring to a human heart pulse detector.
Which clips on the finger or ear lobe in order to detect the light transmission through the finger tissue and so enable measurement of the heart beat etc..????

E

yes, I am using my finger to detect the pulse here. Apart from my desire to experiment with removing the HPF (which I have abandoned now), I was baffled when the pulse took so long to show up when I reduced the HPF and increased the LPF to 50Hz. I think I'll skip this experiment all together, but I'd like to understand the probable reason for such delay.

 

Hi Don,
Please confirm.
I assume when you say 'pulse' you are referring to a human heart pulse detector.
Which clips on the finger or ear lobe in order to detect the light transmission through the finger tissue and so enable measurement of the heart beat etc..????

E

Also, I remember there were crazy RC curves (I presume it was RC curves) forming. Sudden rise, then slow decline, happening again and again when I was playing with the ultra low HPF and 50Hz LPF. I am sorry I did not take a screenshot, but if you can imagine that could you explain the reason for all these anomalies?

 

Hi Don,
Please confirm my assumption on the details of this project, as per post #18?

The last plots I posted were from a human heart beat recording.

E