Thanks Brevor and Alec_T. I put a meter across the PT and exposed to light across the full range of intensity I expect during the use of the circuit. The max was 10vdc and the min was right at 6vdc. I also went through and traced the circuits on the board and decoded the bands on the resistors. I also learned after a bit of frustration that the pin in the middle isn't always the base on the transistors... Learning is good!

Anyway, this is the schematic with values. I'll try the advice of adding a resistor.

 

Something doesn't seem quite right with those figures. When there is 10V/6V across the PT that leaves 2V/6V across the 50k/500k resistor string and the first C1815 will always be turned on. So the relay should never be energised.

 

OK. Thanks. Alec_T, you are awesome for spending the time to help me. I can't thank you and the others enough for your support. I'm an expert in some other fields and I feel humbled trying to learn electronics in greater depth. Ultimately, the success of this project could be life changing for me and my family. I've had many successful inventions in the past but they made millions of dollars for my employers. This one is all mine. For once.

I probably got the values wrong or placed the resistors in the wrong positions while trying to analyze the actual board. The traces on the board are difficult to relate back to the simplified schematic. The components on the schematic were laid out so simply but the PC board layout was surely designed to save space and materials.

Sorry for confusion. I'll recheck. The vendor will supply a complete schematic with my next order. He's been very supportive too, so even if I need to modify the boards myself, I want to keep giving my business to him.

 

Adding a resistor Rx, as shown here, will reduce the voltage across the PT. The Rx value should be about 1.5 times R3 value to get the PT volts <6. You may need to twiddle the trimmer to adjust the light sensitivity.
View attachment 76295

Edit: Ignore the ground symbol below C1, it's just to keep LTspice happy.

 

Alec_T, I modified the appropriate trace on the board so I could install the correct resistor in that spot. 1.5 x the value of R3 turned out to be 15K, which I happened to have in my box o resistors. It worked pretty well. The light/dark voltage were 3v and 5.15V, respectively.

I also noted that you said "LTSpice." I was curious, so I looked it up and downloaded the program. Cool. I've already built my circuit in LTSpice, except for the relay. Still trying to understand how to import 3rd party components but you and the others have taught me a lot! Thanks for your help.

I guess I'm ready to finish field testing the invention to finalize the design. My path forward will be best if I could get customized boards and just solder the things up my self, now that I'm making so many changes. The vendor also sells them as a kit, so I'll have all the components. No idea where to start on getting custom boards made or making them myself but I'm sure I'll figure it out.

Thanks all.

 

Good to know it works as LTspice predicted!
The Yahoo LTS user group, and many component manufacturers, have freely downloadable 3rd party sim models. You can add the '.sub' or '.mod' files to your LTS/lib/sub folder, and add the '.asy' files to the appropriate subfolder under LTS/lib/sym.