There is not enough resistance in the ground path on that small, low current board for the current to produce significant voltage changes when the load changes. The LM78L15 has one ground and you will need to give it one or two capacitors to keep it from oscillating. Still, the number of grounds is irrelevant unless you make quite an effort to make the ground traces long and narrow.

If you place the BPX61 directly across pins 2 and 3 of the amplifier, there is no ground circuit involved for an error to be amplified.
MrChips speaks of a, "star" ground where every part which needs a ground has its own conductor to one point, but you have already demonstrated that you have very few problems without knowing of this method. You can re-design it with a star ground shape, but I am hard pressed to understand why you need to fix what is not a problem.

MrChips also speaks of signal to noise ratio. I was under the impression that you are only discovering, "if" or, "if not". Zero or one. You might make this point clear to us.

Yes, I am only doing a If/If-Not scenario. I'm not particularly interested in the light's intensity or really the variability of the source as long as it triggers and stays high as long as the light source is present or until I direct otherwise. I've redone the schematic with the LM78L15 device...

 

Your schematic is drawn badly. The +24V arrives at pin1 of the LM78L15. The capacitor is from pin1 to pin2 and the other capacitor is from pin3 to pin2 in the physical world and the 3 of them get one ground line. This keeps the capacitors very close to the regulator to do their job with little or no effects from inductance in the circuit board wiring.

Here, let me fix that for you:

 

There is not enough resistance in the ground path on that small, low current board for the current to produce significant voltage changes when the load changes. The LM78L15 has one ground and you will need to give it one or two capacitors to keep it from oscillating. Still, the number of grounds is irrelevant unless you make quite an effort to make the ground traces long and narrow.

If you place the BPX61 directly across pins 2 and 3 of the amplifier, there is no ground circuit involved for an error to be amplified.
MrChips speaks of a, "star" ground where every part which needs a ground has its own conductor to one point, but you have already demonstrated that you have very few problems without knowing of this method. You can re-design it with a star ground shape, but I am hard pressed to understand why you need to fix what is not a problem.

MrChips also speaks of signal to noise ratio. I was under the impression that you are only discovering, "if" or, "if not". Zero or one. You might make this point clear to us.

@#12 is reading in between my lines. That's ok too. I don't mind.
I didn't mean a "star" ground point. I just wanted TS to know that GND is necessary when asked for.

Re: binary vs analog output. I think TS is asking for binary output. TS still needs to establish a threshold level that is above ambient level and below signal level. How much wriggle room is available has not yet been established. Perhaps an analog comparator is called for?

 

@#12 is reading in between my lines. That's ok too. I don't mind.

I hope I didn't do it too badly.
Unfortunately, I have to be somewhere in 10 hours so it's time to sleep. See ya' later.

 

View attachment 137016 Your schematic is drawn badly. The +24V arrives at pin1 of the LM78L15. The capacitor is from pin1 to pin2 and the other capacitor is from pin3 to pin2 in the physical world and the 3 of them get one ground line. This keeps the capacitors very close to the regulator to do their job with little or no effects from inductance in the circuit board wiring.

Here, let me fix that for you:

DAMN, that was stupid.... that's what I get for drawing on the fly. What a moron; of course C1 was wrong. Sorry. Lost a couple hundred credibility points there.

 

DAMN, that was stupid.... that's what I get for drawing on the fly. What a moron; of course C1 was wrong. Sorry. Lost a couple hundred credibility points there.

Meh - I think most of us are pretty lax about credibility points here. Acknowledging and correcting your mistakes (and learning from that process) is the key. Everybody makes mistakes. It's how you handle them that defines you.

...and one layout error in a schematic redraw is pretty minor in the grand scheme of things anyway.

 

Thank you for your courtesy, but I don't have any suggestions. More like admiration for a person who stretched the limits of practical application. I am educated in this subject. I have seen Teflon standoffs, air built circuits, circuits built on glass, and guard rings designed to make this sort of circuit work. The fact that you could do this is remarkable to the point that I wonder if a contributing factor is that nobody told you how difficult it is. You are working in a range where humidity is a factor and the circuit might not work properly in a month if you fail to protect it from contaminants in the air coating it. I suppose I just made two suggestions: Protect it from air born contaminants and be aware of humidity.

At the risk of opening a can of worms, can you or someone on the forum explain why having the joint where the TLC271, resister and capacitor not touching the PCB made such a difference? Is this an air-built circuit? I discovered this only by chance while tinkering trying to get it to work. But I have no idea why it worked; in fact, it was really an accident. It seems completely counter-positive to do such a thing on purpose. And maybe why making other side of the same connections made no difference?

 

Leakage currents.

R1 = 10GΩ

Most experimenters will never see one of those in their lifetime.
Did you remember to handle the 10GΩ resistor while wearing cotton gloves?

 

can you or someone on the forum explain why having the joint where the TLC271, resister and capacitor not touching the PCB made such a difference?

When you connect a wire to any solid, that substance can have its own resistance and it can become coated with another substance which has conductivity.
When you connect a wire to air, where is the leakage path? If air was a good conductor, vacuum tubes would never have been invented. They would just be jars full of Earth atmosphere containing the physical structure of a heater, cathode, grid, and plate.

why making other side of the same connections made no difference?

They all go to low impedance points where a nanoamp or a microamp, give or take, makes no difference.

Is this an air-built circuit?

Pin 2 is. The rest of it isn't.

ps, I find that MrChips presents his ideas in ways that are unfamiliar to me, but he is entirely competent. Sometimes he connects well with the questioner, sometimes I do, and sometimes both of us seem to connect well. That is one of the strengths of this Forum. Several competent people participating almost always means somebody connects well with the questioner.

 

When you connect a wire to any solid, that substance can have its own resistance and it can become coated with another substance which has conductivity.
When you connect a wire to air, where is the leakage path? If air was a good conductor, vacuum tubes would never have been invented. They would just be jars full of Earth atmosphere containing the physical structure of a heater, cathode, grid, and plate.

They all go to low impedance points where a nanoamp or a microamp, give or take, makes no difference.

Pin 2 is. The rest of it isn't.

ps, I find that MrChips presents his ideas in ways that are unfamiliar to me, but he is entirely competent. Sometimes he connects well with the questioner, sometimes I do, and sometimes both of us seem to connect well. That is one of the strengths of this Forum. Several competent people participating almost always means somebody connects well with the questioner.

VERY much appreciate your reply. And now a really "Sticky" question. I'd like to pot the small PC board that this circuit will be on. The only thing showing would be the glass and slight edge of the sensor and the power ingress and egress points. Based on the previous conversation, do you think this will be possible? I don't want to ruin components if anyone feels there isn't a chance in hell of it working.

 

You could consult with a manufacturer of potting compound. That's what I did when I needed a bulk resistance of 100 Meg on a surface mount board.

Another way might be to use a container that can be sealed against air pressure and fill it with dry nitrogen. That's how we kept condensation out of some laser optics.