Layout doubts on prototype project

[BlackCoffee]

Thinking outloud… Consider setting all of the above well thought considerations aside, and restarting afresh with ‘clarity and focus’ on what I’d call a Minimal Baseline working circuit.

Reading these posts, brings to mind a need to focus on getting a single-signal from a single sensor and trying to use it with moderate success. After that, the rest is about implementation and dialing-in opamps, capacitance, resistance, voltages…etc and you can piece-meal a circuit together, one item at a time…

Since this appears to be a ‘Self-Educational’ project, I pulled out some of my early prototypes (basically, the very first one’s) and thought this may help:

Video showing LM358N, two Resistors, Trim-pot, Cap (and Nano for Serial/other, otherwise the Nano serves no purpose as power is brought in independently).

The LED blinks from the LM358. Original prototype used IRsend/rcv Leds but I replaced them with a $3 finger sensor (and, didn’t want to bother tuning Cap for this). The sensor is fussy about finger placement and pressure, especially in under the desk lamp but, it’s good enough to demonstrate. The Nano’s LED blinks upon receiving the same signal.

ADDED: The screenshot is Not a PCB/circuit - it’s only purpose is to show the Part connections (as if it were a hand-sketched schematic).
I did Not clean up the Breadboard circuit after removing the IR/led’s, resistor and unconnected wires shown in video…

EDIT: I cleaned up the mess of wires, added a light-shield (paper tube) and re-tuned the input voltage via the trimpot - now, it’s less sensitive to ambient light and finger pressure.
I replaced the video with new one showing finger in/out a few times but always getting the bpm signal… (Note: there’s a totally independet #2 circuit with LM741 that’s not being used as the LM358 is my preference for this simple circuit…

[CrazyScientist]

Strange circuit. Why bias the circuit to ½ supply it will mean significant offset bias across several amplifiers

The output filter should have much higher resistors and lower caps as thd op amp will not be happy driving a capacitive load that big ( the data sheet says 100pf)

[Jonathan_Haas]

@CrazyScientist

The output filter should have much higher resistors and lower caps as thd op amp will not be happy driving a capacitive load that big

Yeah, the rest of the circuit looks kinda reasonable, IMO, not that I’m an expert, but 0.27 to 1uF output capacitance looks way to high considering the low currents/voltages I’d expect here. it will just smooth out the signal until nothing is left and it might overload the OpAmp.

Did you intent to put the capacitor into the output path as a DC filter instead? IMO, that would make much more sense for an audio style output.

[Piotr]

@CrazyScientist

The output filter should have much higher resistors and lower caps as thd op amp will not be happy driving a capacitive load that big ( the data sheet says 100pf)

I see nothing specially wrong in this filter. 1k seems for me typical at sound input/output connections.
100pF limit is only about directly loading the opamp output. After 1k you can use even 1F.
I have opened Diodes TL072 pdf (first that I found). The char for peak output voltage starts from 0.1k load.
The higher R load the higher peak output voltage. But until opamp can set voltage as it wants the output voltage will be the same (feedback sets it). From information given here it looked that opamp output need not to get close to power supplies so increasing peak output voltage (by replacing 1k with higher values) will have no influence on signal. But the output is then divided with 1k and sound card input impedance so increasing R in output filter we reduce the output voltage.

[Piotr]

@Jonathan_Haas

it will just smooth out the signal until nothing is left and it might overload the OpAmp.

1k 1uF low pass filter has a pole at 160Hz. What we want to see are 1Hz pulses. Of course to not see only 1Hz sinus we need some harmonic but 160 of them should be enough.
1k will not overload TL072 until you need not to get close to rails with the signal and we have here 9V battery (I suppose) so ± 4.5V supply and sound card input probably max is something around ± 1V.
First thing I have ever done (being in secondary school) with opamp was a voltmeter that was powered from 4.5V battery. LM741 is certainly not rail to rail amplifier and I had to divide 4V5 not into 2 x 2.25V but rather to something like -2.2V and +3.3V as I needed output from 0V to 1V. If I were dividing supply into 2 x 2.25 I would have accessible at output negative voltage that I didn’t needed and I would not get +1V that I needed.

[Jonathan_Haas]

@Piotr

1k 1uF low pass filter has a pole at 160Hz. What we want to see are 1Hz pulses.

In that case I didn’t say anything. I expected the output resulution to be in the kHz range from the description.

[Piotr]

@Jonathan_Haas

I expected the output resulution to be in the kHz range from the description.

Based on picture in post 13 I assume that ECG pulses are what is expected to see. I’m not sure if somewhere there the description said something more.

[John_Pateman]

Although the fundamental frequency of the ECG is around 1Hz, interpretation of the waveform requires a significantly higher resolution as the components (p, qrs and t waves) have a specific temporal and frequency relationship. Some filtration is acceptable but too much will remove or blur the important inflexion points. For instance, ST segment analysis needs to determine the J point with some accuracy as determination of ST segment elevation or depression is very clinically significant . I think this project is purely interested in displaying a non clinically interpretably ECG so it probably doesn’t matter but for a clinical device, filtration and frequency response is absolutely crucial.

[mikeparkin7557]

You are using a 10M resistor with a bipolar op amp !
It has no chance of working.
Use a modern op amp like TLO71, tlo61 or tl081 with JFET inputs.