Two more things,
1. When the power is switched on and off, there is a spike in the output from the photo transistor. Why should it be so when power is coming via a linear regulator and there is a HPF to block all DC.
2. I had accidently supplied 7.13V to the system once. Could that have started the slow death of the Photo Transistor? I had actually connected the linear regulator in reverse and it had gone toast.

 

Yes it could be a quality issue. 7V shouldn't have harmed anything.
Could it be a mechanical issue, due to fracture of a connecting wire to the photo-transistor as a result of frequent bending ?

 

Yes it could be a quality issue. 7V shouldn't have harmed anything.
Could it be a mechanical issue, due to fracture of a connecting wire to the photo-transistor as a result of frequent bending ?

I doubt there could be a mechanical issue, in the past when the photo transistor has gone bad I have tried resoldering fresh wires, didn't help.
I searched on the net whether anyone else has had a problem with TCRT5000, I couldn't find much complaints.
If I chose to replace this photo transistor, can you guide me how to know which transistor is good quality and which bad?

 

As I see it, the transistor is exposed to current sources either from the power supply or from the cap, so one of them is to blame? If the power supply was to blame then the LED should have gone off too, but only the transistor is going bad, and only the cap end has no protection. So worth trying the current limiting resistor?

 

Guys I'd like to stick with the 3.3uF. Would a 1K in series suffice for current limiting purposes?

 

Guys I'd like to stick with the 3.3uF. Would a 1K in series suffice for current limiting purposes?

Huge mistake.
Now we know the actual application, a high pass filter is the wrong approach. You would be better to use a DC coupled amplifier with the DC offset removed by subtraction. Believe me, I have seen many students struggle with your approach and fail in the end.

 

Huge mistake.
Now we know the actual application, a high pass filter is the wrong approach. You would be better to use a DC coupled amplifier with the DC offset removed by subtraction. Believe me, I have seen many students struggle with your approach and fail in the end.

Hi MrChips, please elaborate.
1. What is the mistake? Sticking with the 3.3uF or the HPF in general?
2. When you say subtraction you mean differential amplifier or actual mathematical subtraction?
3. Which failures are you thinking of when you say you have seen your students fail. Are you saying you have seen the transistors go bad?

The thing is, not being an electronics engineer, I don't know why simulation by @ericgibbs is not likeable, but in my application it works just fine. The only problem I have is with this transistor dying. @ericgibbs had said the transistor might be fine and the circuit might be faulty, but if that's the case why would a new transistor work? Would changing the approach help the transistor?

How about I remove this HPF stage? There is another HPF/ac coupling in the end after all.

 

Hi MrChips, please elaborate.
1. What is the mistake? Sticking with the 3.3uF or the HPF in general?
2. When you say subtraction you mean differential amplifier or actual mathematical subtraction?
3. Which failures are you thinking of when you say you have seen your students fail. Are you saying you have seen the transistors go bad?

The thing is, not being an electronics engineer, I don't know why simulation by @ericgibbs is not likeable, but in my application it works just fine. The only problem I have is with this transistor dying. @ericgibbs had said the transistor might be fine and the circuit might be faulty, but if that's the case why would a new transistor work? Would changing the approach help the transistor?

How about I remove this HPF stage? There is another HPF/ac coupling in the end after all.

1. Using HPF.
2. Subtract using difference amplifier (look up summing amplifier).
3. Failing because the low frequency cutoff needs to be very low, below 0.1Hz.

We don’t know why your transistor is dying.
You cannot use the output HPF alone because the input amplifier has already saturated.

 

How large is the resistor that sets the transmitter's IR current?

 

I am struggling to understand how an LED pointed directly at the detector can sense the heartbeat of an animal. Is some part of the animal placed between the two? If it is done by reflection, they should be at an angle to each other for the best signal.

Also, since the detector you are using is intended to detect reflections, it is probably too sensitive to have the LED pointed directly into the detector. I now think that is the cause of the failures.

 

Is the photo-sensor connected directly to the amplifying circuit or is it connected by a cable?
Is the supply connected to earth ground?

 

Hi Bob,
There is a similar device that works through a human finger to measure the blood oxygen levels.
That version has two emitters of different IR frequencies, one that does not get affected by the blood colour level [the reference absorption] and the other which does.

This TS is trying to measure the blood absorbance of the IR, as the heartbeats, so that he can determine the pulse rate.

E

 

@BobTPH yes the tissue matter is supposed to go in between the emitter and photo transistor. But most of the time when the thing is just on there is nothing in between and they are just facing each other. I thought so too, that maybe something which is designed to detect reflection is being tasked with something more.
If I procure a different transistor, how do I know that it is up to the job? What should I check in the data sheets?

@KeithWalker there are wires, I think 24 gauge, which connect the transistor to the amplifier circuit. And the power is coming from a 2 pin adaptor, 12V, stepped down to 5V eventually .

 

Hi Bob,
There is a similar device that works through a human finger to measure the blood oxygen levels.
That version has two emitters of different IR frequencies, one that does not get affected by the blood colour level [the reference absorption] and the other which does.

This TS is trying to measure the blood absorbance of the IR, as the heartbeats, so that he can determine the pulse rate.

E

That is exactly what I was thinking when I asked.

Okay, we have the answer from the TS, it is passing the light through some tissue (the ear would be my guess.) Perhaps the solution is to have a pushbutton and light the LED only after putting it in place. Or better yet, run the LED at low current until the detector notices large drop, then up the current until the signal goes too high again. A micro could do this easily.

 

Hi John,
If you consider it maybe be due to continuous detection when there is no tissue matter, between the Emitter/Detector that is causing the failure
Have you considered some form of miniature switch which switches off the Emitter power, when the jaws of the flesh clip are fully closed?

Do you follow?
E

 

Hi John,
If you consider it maybe be due to continuous detection when there is no tissue matter, between the Emitter/Detector that is causing the failure
Have you considered some form of miniature switch which switches off the Emitter power, when the jaws of the flesh clip are fully closed?

Do you follow?
E

Yes, that has come to mind, my only problem with that is that I have no mathematical way of knowing how much direct light intensity is safe intensity? How do I know that even with tissue matter in between, the intensity of direct light would not be too much for the transistor. And this tissue matter, unlike human finger, can be actually quiet thin, anywhere between 10mm and maybe even 2 mm.

 

Hi,
Consider controlling the Emitter current, what is the present value of the series resistor?
E

 

Hi,
Consider controlling the Emitter current, what is the present value of the series resistor?
E

Its 440 Ohms but that's not in our control, signal quality decides that.

There have to be photo transistors out there designed for direct incidence, that way we can't blame this alteration anymore for the trouble.

 

Its 440 Ohms but that's not in our control, signal quality decides that.

Hi,
With due respect, may I ask who designed this project and is it intended for commercial use or a college assignment?

What do you mean by signal quality decides that.?

So far, we have not seen any oscilloscope images of the signal coming from the Emitter or the final amplifier.

Please post images of the failed and working transistor detector outputs, with and without a flesh sample in the project.

E

 

Hi,
With due respect, may I ask who designed this project and is it intended for commercial use or a college assignment?

What do you mean by signal quality decides that.?

So far, we have not seen any oscilloscope images of the signal coming from the Emitter or the final amplifier.

Please post images of the failed and working transistor detector outputs, with and without a flesh sample in the project.

E

buddy, come on, seriously, how does it matter who designed it and whether its commercial or college? I'm sorry I can't answer questions which I just don't find useful and relevant to the problem, I can just hope you'll understand.

I don't have an oscilloscope but I'll be happy to share images from an arduino plotter, it'll take some time though as I'll have to replace my transistor now, and I'm not going with the same transistor again, I'll have to find a substitute. Would be most happy if anyone can give suggestions of which transistor I should go for.

By signal quality I mean the pulse signal on the plotter, of course the intensity of light decides how clear the signal is.