buddy, come on, seriously, how does it matter who designed it and whether its commercial or college?

It makes a huge difference. If this was a commercial product, as opposed to an amateur one, my recommendations would be very different.

So here goes, commercial product response:

To make this reliable, you need to modulate the signal and use digital signal processing, the way a TV remote works. This would allow it to work reliably over a wide range of ambient light and transmissibility of the tissue. In fact, you could probably use the emitters and receivers designed for TV remotes and get better results.

 

It makes a huge difference. If this was a commercial product, as opposed to an amateur one, my recommendations would be very different.

So here goes, commercial product response:

To make this reliable, you need to modulate the signal and use digital signal processing, the way a TV remote works. This would allow it to work reliably over a wide range of ambient light and transmissibility of the tissue. In fact, you could probably use the emitters and receivers designed for TV remotes and get better results.

Thank you for that answer bob.

I'll have to study what modulation is and how to do it, I have only heard of it in passing until now. It'd be nice if I don't have to go hunting for photo transistors. I'll try to implement a solution in the next coming days, hopefully the same one you suggest.

 

Thank you for that answer bob.

I'll have to study what modulation is and how to do it, I have only heard of it in passing until now. It'd be nice if I don't have to go hunting for photo transistors. I'll try to implement a solution in the next coming days, hopefully the same one you suggest.

It is a major project to do this. I thought about it some more and I realize you cannot use the TV remote type sensor, it only has a single bit on / off output, which is of no use.

Modulation is done by blinking the LED at a specific frequency. You then use a narrow bandpass filter to detect the signal at that frequency, ignoring anything else. TV remotes typically use 38KHz.

 

It is a major project to do this. I thought about it some more and I realize you cannot use the TV remote type sensor, it only has a single bit on / off output, which is of no use.

Modulation is done by blinking the LED at a specific frequency. You then use a narrow bandpass filter to detect the signal at that frequency, ignoring anything else. TV remotes typically use 38KHz.

I was doing some YouTubeing yesterday after ur message on modulation. YouTube says modulation is used with different carrier frequencies to be able to identify different signals in a single communication line. I guess that isn't our purpose, since we have only one signal, and honestly there is no problem with the clarity of the signal except the occasional motion artefacts which come with working with animals, which I guess would be unresolvable even with modulation. But we are getting distracted again.

Our main issue is the death of the transistor, and we still don't know why it is happening. I think I'll do an experiment even if its time consuming, I'll change the transistor with the same kind as before and remove the LED and leave it on and see after a couple of days whether it survives. We would know whether it was the direct incidence of light to blame or not.

up on the thread I was asked how I know the transistor is dead. Generally when the LED and transistor are just resting on top of each other in an on state, the final output is clean zero volts. But when the transistor dies there is strange voltage output even when nothing is there, and when even I place my own finger in between, the signal which earlier would go as much as 2.49V would drop in peak amplitude to as low as 0.117V.

So, I'll just start the experiment.

 

YouTube says modulation is used with different carrier frequencies to be able to identify different signals in a single communication line.

Then it must be true, and apply to the the problem you are dealing with, if YouYube said so. I guess all the manufacturers of IR remotes got it wrong. The fact that they work so well must be dumb luck.

Clearly YourTube is more help than I am, so I will bow out.

 

Then it must be true, and apply to the the problem you are dealing with, if YouYube said so. I guess all the manufacturers of IR remotes got it wrong. The fact that they work so well must be dumb luck.

Clearly YourTube is more help than I am, so I will bow out.

Hi bob. I was just mentioning it so you know what I am doing and where I am coming from and what I know so far, so I can be corrected if need be.
I confess I have no idea how tv remotes work, and I have very little idea what modulation is or its scope is.
Since you say modulation is not for my project I guess I'll stick to just the experiment? That's where we have arrived at, right?

 

I don't think it's been mentioned so far, but is your circuit built on a pcb or on a breadboard?

 

Since you say modulation is not for my project I guess I'll stick to just the experiment? That's where we have arrived at, right?

Where did I say that?

Actually, what I said was that, for a commercial product, modulation would be valuable. I know you are not doing a commercial product from what you have said. It is clearly a one-off project that you, or someone you work with put together.

I can still think of no likely mechanism for your transistor repeatedly failing. It becomes even more puzzling after your description of what happens. A failed transistor will be typically either shorted or open and show no response. What you seem to be seeing is a change in response, which indicates a damaged transistor, not completely failed.

Here is how modulation helps. The phototransistor is seeing multiple signals from your emitter and other IR emitters, like daylight and artificial lighting. These other signals vary all the time, and must be ignored. By blinking the LED at a specific frequency, you can then make an amp that amplifies only this frequency, ignoring all other IR sources reaching the detector. TV remotes use 38KHz because it is an unlikely frequency to be found in other environmental IR sources. Obviously 60 Hz would have been a bad choice (or 50Hz in some parts of the world).

I gave you two solutions, if the problem is that the phototransistor is being overdriven and damaged by the IR source, but I am not convinced that is the problem. To solve this properly my you are going to need to do more experimentation to determine the probable cause.

 

@Alec_t amp circuit is on a PCB. Everything soldered properly.
@BobTPH thanks for that elaboration, now I am beginning to get what modulation's use might be in this project. Please don't let my low knowledge background prevent you from offering best ideas like these (commercial ones), I don't mind going through a learning curve, I'd like to do my projects in the best way possible.

The first experiment I'll try to do is the one mentioned above (post 64). Lets see if it works. If the transistor still goes bad then we'll know something else is at fault. I doubt ambient IR in the atmosphere can be strong enough to damage anything, after all these sensors are designed to be open to the atmosphere.

 

@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 .

Your power supply will be isolated from ground. Your circuit will build up a static charge over time. When you handle the sensor or attach it to an animal, it will discharge rapidly. ESD can damage sensitive components like your photo-sensor. All medical equipment is grounded for two reasons. The first is patient safety in case of a malfunction and the second it to avoid equipment damage by ESD.
Connect the circuit common of your power supply to the domestic AC supply ground and see if your sensor fails again.

 

Your power supply will be isolated from ground. Your circuit will build up a static charge over time. When you handle the sensor or attach it to an animal, it will discharge rapidly. ESD can damage sensitive components like your photo-sensor. All medical equipment is grounded for two reasons. The first is patient safety in case of a malfunction and the second it to avoid equipment damage by ESD.
Connect the circuit common of your power supply to the domestic AC supply ground and see if your sensor fails again.

wow, thanks. I know that in three pin plugs the top pin is EARTH, and I always wondered why in circuits we take one of the other two pins as GND.

I need more clarity. What is circuit commons? What should I connect to AC earth? The GND?

 

Is the base of the photo-transistor not connected (true if the pin doesn't exist!)?

Is the power supply for the opamps +5 and gnd or +5 and -5? (hard to read the schematic)?

With no light the 3.3uF capacitor will charge to 5 volts. Sudden light might be exposing the transistor
to high current from the 3.3uF capacitor. Worse case could be amps (v = It/C -> I = VC/t -> 5*3.3e-6/1e-6 -> 16 A)
this is likely high as it ignores the resistance and junction drop inside the opamp input pin as the input pin
is pulled below ground.

A 200 ohm or so resistor in series with the capacitor would limit the maximum current into the 10s of mA range
which it's likely the photo-transistor could survive.

 

Is the base of the photo-transistor not connected (true if the pin doesn't exist!)?

Is the power supply for the opamps +5 and gnd or +5 and -5? (hard to read the schematic)?

With no light the 3.3uF capacitor will charge to 5 volts. Sudden light might be exposing the transistor
to high current from the 3.3uF capacitor. Worse case could be amps (v = It/C -> I = VC/t -> 5*3.3e-6/1e-6 -> 16 A)
this is likely high as it ignores the resistance and junction drop inside the opamp input pin as the input pin
is pulled below ground.

A 200 ohm or so resistor in series with the capacitor would limit the maximum current into the 10s of mA range
which it's likely the photo-transistor could survive.

Thanks for that math.
1. There are just two pins in the photo transistor.
2. Power supply to op amps is +5V and -5V. -5V is taken from a TC7660.
3. Ok so to sort out the current threat, I'll put a 1K resistor in series and change the capacitor to 1uF as has been suggested by others. That good?

and I didn't quiet get what @KeithWalker said, what is circuit common which needs to be connected to mains earth?

 

With no light the 3.3uF capacitor will charge to 5 volts. Sudden light might be exposing the transistor
to high current from the 3.3uF capacitor

No, look at the other side of the capacitor.

 

Thanks for that math.
1. There are just two pins in the photo transistor.
2. Power supply to op amps is +5V and -5V. -5V is taken from a TC7660.
3. Ok so to sort out the current threat, I'll put a 1K resistor in series and change the capacitor to 1uF as has been suggested by others. That good?

and I didn't quiet get what @KeithWalker said, what is circuit common which needs to be connected to mains earth?

Connect the negative rail of the supply to ground.

 

Since we can't seem to to pin down exactly what is killing the transistor, how about connecting a reverse biased diode across the collector to ground, just on the remote off chance that there's a spike getting in from somewhere. Just my WAG!

 

Since we can't seem to to pin down exactly what is killing the transistor, how about connecting a reverse biased diode across the collector to ground, just on the remote off chance that there's a spike getting in from somewhere. Just my WAG!

Can’t hurt. Perhaps one to the power rail as well, limiting the village to -0.6 to 5.6V.

 

Ok, so I am going to repeat everything, tell me if I got something wrong.
1. Connect a 5.6V zener diode in reverse bias between the collector and GND (and one to power rail and GND too).
2. Connect a series 1K resistor between collector and 3.3uF Cap.
3. Connect the GND power rail to mains earth.

I am going to do all three together. I got it right?

 

Ok, so I am going to repeat everything, tell me if I got something wrong.
1. Connect a 5.6V zener diode in reverse bias between the collector and GND (and one to power rail and GND too).
2. Connect a series 1K resistor between collector and 3.3uF Cap.
3. Connect the GND power rail to mains earth.

I am going to do all three together. I got it right?

No, as far as I, and I'm guessing BobTPH are concerned, we were suggesting ordinary small signal silicon diodes, rather than zeners. Schottkys could also be used.

 

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No, as far as I, and I'm guessing BobTPH are concerned, we were suggesting ordinary small signal silicon diodes, rather than zeners. Schottkys could also be used.

How does this work? please explain. I'm not getting it.

would 1N4148 do?

Also, the regular non-zener diodes, I had thought that if they conduct in reverse direction then they are broken forever.