Spindle Index Noise

Wondering how I remove it? Standard cheapo Chinese BOB with the 12v opto inputs there are about 5 inputs. Ive wired up a photo sensor to the 12vdc and the output directly into the BOB input. The index indicator goes on and off with the single tab that runs through the sensor, its sitting on the main chuck shaft.

So it works of kind. I expect at speeds it might be accurate. What I dont understand is why the spindle speed DRO shows a positive figure when the machine is not running. Currently it sits at about 84rpm to 140rpm when the chuck shaft is not moving. I have considered putting another opto in the circuit. But that seems wrong. Debounce - makes no difference. My lead from the sensor to the BOB is shielded, and the shield is earthed at one end only. Its weird and I am out of ideas. I guess, in a perfect world, the DRO would sit at zero when there is no movement, and rise when there is actual speed. Has anyone acheived that or is this just a poor part of Mach 3 I need to accept?

I know this will be common but was also unable to see any fixes on here. I do plan to thread with this machine.

You need to measure the actual voltage going into the BOB. It may be hovering right at the witching threshold.
Electrical noise is probably also a problem.

My Mach3 spindle DRO works fine.

Cheers
Roger

Im not sure where to continue to be honest. I have done a bit of work around this and with my basic meter have zero to 8.4v on the on/off input to the BOB, I even thought about installing a second PC817 prior to the input to the BOB. My current background is medical electrical so the machine is earthed to the moon.

My machine is made up of parts I have found lying around in my workshop so there are lots of different voltages in my cabinet to allow for the different coils in the contactors etc. I did need both 24vdc and 24vac, found that a rectifier I had sits nicely into 4 din connectors so am using the output of that to run the switching side of my BOB. Using that raw 24vdc I thought was the problem but when I changed that to a decent switching power supply there was no difference.

I even put a cap between the output of the sensor and the ground - but that actually increased the DRO speed whilst not turning. Hey, I am not electronics so am sure that was a big no no. I was wondering about installing a small resistor between the output and, I think, the positive power supply to the sensor. When its supposed to be zero maybe that will drain the circuit - is that what a pulldown resistor does? Thats about the end of my skills to sort this problem.