I mentioned this problem in post #46. Attached is how I would deal with the drift. I don't remember exactly what your range of pulse widths is. I put in a couple of single-pole filters with ≈1mS time constant. You might have to change those. Also, you can change the ranges on gain, threshold, and hysteresis.
I did not attempt to filter out 120Hz, so if you have not solved that problem, it would still have to be dealt with.
I included the .ASC file, in case you want to simulate it on LTspice.

Hi Ron H,
For some reason the .asc file shows a different circuit that the .png file. Which is the correct one?
Thanks
Neptune24.

 

Hi Ron H,
For some reason the .asc file shows a different circuit that the .png file. Which is the correct one?
Thanks
Neptune24.

The .PNG is the correct one. I went to LTspice on my computer, and the .ASC version was not there! The old .ASC schematic (the one you have) came up. I did a Carbonite file restore (first time I had used it), and got the correct version. Not sure how all that happened.
Anyhow, it opened in LTspice, but I kept getting a "time step too small" error, so I put in several initial conditions, and it ran. It is attached below.

 

The .PNG is the correct one. I went to LTspice on my computer, and the .ASC version was not there! The old .ASC schematic (the one you have) came up. I did a Carbonite file restore (first time I had used it), and got the correct version. Not sure how all that happened.
Anyhow, it opened in LTspice, but I kept getting a "time step too small" error, so I put in several initial conditions, and it ran. It is attached below.

This is simply awesome!
I will implement the circuit after figuring out how it works.
Could you also make up a circuit that will analyze my results and write the paper?

Merry Christmas

 

This is simply awesome!
I will implement the circuit after figuring out how it works.
Could you also make up a circuit that will analyze my results and write the paper?

Merry Christmas

Do you want a brief description of how it works?

 

Do you want a brief description of how it works?

That would be great!
As you probably noticed, I am far from being an expert in this field.

--N

 

That would be great!
As you probably noticed, I am far from being an expert in this field.

--N

OK, here goes.
R1 and C3 filter high frequency noise from the power supply, providing a quiet reverse bias voltage for the photodiode.
I modeled the photodiode as a pulsed current source (I1), with the quiescent current due to laser illumination being 200uA. The pulse amplitude is -6uA, with 1mS rise and fall times, and a duration of 10mS at half amplitude. if your pulses are different, edit the parameters of current source I1.
R5, the 10k load, gives us +2VDC, with 60mV negative pulses, on the anode of the photodiode (the node named "in"). This voltage is uncertain, due to temperature, diode-to-diode variations, laser variations, atmospheric attenuation, etc.
C6, in parallel with R5 (and R1 and R3), form a 160Hz lowpass filter (10%-90% risetime=2.2mS, not including the risetime of the current source).
C1 is a DC blocking capacitor, because we don't want to apply the uncertain bias voltage on node "in" to the amplifier.
R1 and R3 bias the input of op amp U1 to vcc/2 (2.5V in this example). C2 is another DC blocking capacitor. U1 and the feedback network R1 and U2 form an AC amplifier with a gain of (1+U2/R2). Due to the blocking capacitor C2, the DC voltage at the output of U1 is also vcc/2.
The voltage at the wiper of U4 attenuates (divides) the signal at the U1 output over a range of about 90% to 100%, depending on U4 wiper rotation.
R6 and C4 form a lowpass filter that removes (most of) the pulse, resulting in a DC slicing level to pass on to the Schmitt trigger comparator.
U5 is a voltage follower (gain of 1 amplifier), which provides a low output impedance to the hysteresis network consisting of U6, R8, and R9.
R7 and C5 form another lowpass filter for the pulses, basically identical to R5 and C6.
U3 is a comparator which slices the amplified pulse from U1, at the threshold level set by pot U4.
The hysteresis voltage range is approximately Vout*R9/(U6+R8+R9), where Vout is about 5V p-p.
Unfortunately, all 3 pots are somewhat interacting, so you will need an oscilloscope to set them to their optimum values.
The good news is that, once set, performance should be relatively independent of ambient condition variations.

 

OK, here goes.
.

I could not expect better assistance
Thanks a LOT!

--N