Once again, I have never seen the TS drawing of the circuit being built, I have only seen what I find at the end of a link.
So I have no way to verify that a circuit on a PCB is the same.
Next, I have no way of looking at an actual PCB and seeing if the conductors are adequately sized.
I have not seen the circuit with the voltages relative to the supply terminal negative shown. AND I still have no clue about the values of those resistors. And my ability to see what other folks are thinking inside their heads is worse than poor!

Next, consider the possibility that in copying the drawing shown in that link, that an error happened. And has been duplicated several times! And consider that some voltages on the version that works are not the same as on the versions that fail to function correctly. How can I know that???

 

Once again, I have never seen the TS drawing of the circuit being built, I have only seen what I find at the end of a link.
So I have no way to verify that a circuit on a PCB is the same.
Next, I have no way of looking at an actual PCB and seeing if the conductors are adequately sized.
I have not seen the circuit with the voltages relative to the supply terminal negative shown. AND I still have no clue about the values of those resistors. And my ability to see what other folks are thinking inside their heads is worse than poor!

Next, consider the possibility that in copying the drawing shown in that link, that an error happened. And has been duplicated several times! And consider that some voltages on the version that works are not the same as on the versions that fail to function correctly. How can I know that???

Once again, I have never seen the TS drawing of the circuit being built, I have only seen what I find at the end of a link.
So I have no way to verify that a circuit on a PCB is the same.
Next, I have no way of looking at an actual PCB and seeing if the conductors are adequately sized.
I have not seen the circuit with the voltages relative to the supply terminal negative shown. AND I still have no clue about the values of those resistors. And my ability to see what other folks are thinking inside their heads is worse than poor!

Next, consider the possibility that in copying the drawing shown in that link, that an error happened. And has been duplicated several times! And consider that some voltages on the version that works are not the same as on the versions that fail to function correctly. How can I know that???

A-Here is the circuit I built.
B-one version on strip board
C-another version on strip board
D- the layout of the brown version
E-The working bread board/ an LED instead of coil and LED/s

https://drive.google.com/file/d/1JnQ5P2fexE8M2GJslqgvtddIEXUoQ7cA/view?usp=drivesdk

https://drive.google.com/file/d/1BChAg2B09_u33DMEJHPcAes54CEWQ4qo/view?usp=drivesdk

https://drive.google.com/file/d/1WwS9Vv_UD-KO-ovHhdJTzx7M4vpxPN8e/view?usp=drivesdk

https://drive.google.com/file/d/1dE8awOU7-i4W-P7vpBnE9HhtYjVIzd-4/view?usp=drivesdk

https://drive.google.com/file/d/1q3sCKv7DVp6OwM19SeL9elp033ZtLJWB/view?usp=drivesdk

I’ll stop belaboring my issue now and keep trying to figure out the issue

Thank guys

 

Here is a suggestion for a logical diagnostic sequence: Start with the one that functions as you want, and with a schematic, measure and record the voltages at each point bet6ween components. Between the photo-transistor and the resistor, at the junction of the three resistors, at the drain of the mosfet. For the mosfet to fail it must have excess heat or excess current. Excess heat will be from being into conduction for excess time. Excess current would be from that 1N5821 diode being backward or failed short. My guess is that the gate of the mosfet is at a voltage that enables conduction in the linear portion of the mosfet response, and that causes excess heating.
Then check those same points on a failed build and the cause should be possible to determine .
My first guess is that the photo-transistor is not conducting adequately when the light beam is hitting it.

 

Here is a suggestion for a logical diagnostic sequence: Start with the one that functions as you want, and with a schematic, measure and record the voltages at each point bet6ween components. Between the photo-transistor and the resistor, at the junction of the three resistors, at the drain of the mosfet. For the mosfet to fail it must have excess heat or excess current. Excess heat will be from being into conduction for excess time. Excess current would be from that 1N5821 diode being backward or failed short. My guess is that the gate of the mosfet is at a voltage that enables conduction in the linear portion of the mosfet response, and that causes excess heating.
Then check those same points on a failed build and the cause should be possible to determine .
My first guess is that the photo-transistor is not conducting adequately when the light beam is hitting it.

Well I took your advise and checked every component and took voltage readings at every point possible. I discovered that the voltage at the mosfet gate when IR light was present/no block between IR led and photodiode -was as specked (2.4-2.7v) and when the sensor pair was blocked the gate voltage was 12-12.5/7. Now both voltages are as should be. Low at gate when no sensing(no conduction), high when sensor blocked(full conduction). The mosfet conducts all the time. I did this exact thing with 3 other board designs, same schematic. Same results. I thought to myself this is nuts so I dug around in an old parts box and found a new old stock mosfet. Plugged it into one of my boards and it worked normally!! Plugged it into my other boards and every one worked. THE ONLY PART I ASSUMED WAS/COULDN’T be bad, WAS! I wasted a ton of time. So much for cheap easy to find parts on Amazon. Returning them and sourcing DigiKey parts. Gave the seller a few nice words too.
I think this will solve my issue.
Thank you all

 

Interesting indeed!! I wonder now if that was the only one with that problem.
Probably a test setup for those mosfets prior to installation would be useful.

 

Interesting indeed!! I wonder now if that was the only one with that problem.
Probably a test setup for those mosfets prior to installation would be useful.

I built a tester after I found the issue. I bought 20 at a time and the last box was still on my bench unopened I tested them all. Out of twenty 4 worked intermittently. I’m returning all of those plus as many as I can pull out of previous fails before I started to use heads for fast change out…

 

I built a tester after I found the issue. I bought 20 at a time and the last box was still on my bench unopened I tested them all. Out of twenty 4 worked intermittently. I’m returning all of those plus as many as I can pull out of previous fails before I started to use heads for fast change out…

I wonder if they were all part of a batch of production rejects, those that did not quite meet all of the specifications in the QC test that is done for product grading.