Boxford TCL125. (Fresh start )

[Doddy]

Hmmm, you previously linked a reflective opto-coupler, (EDIT: Ah, I see what you did there - the linked datasheet has the slotted version later in the document) these are slotted opto-couplers (same premise, just a different physical configuration) - these more common with slotted wheels as shaft-encoders, etc.

For LEDs which typically outlive the service life of the machine, wiring in series is perfectly fine, and common practice. As you probably guess, you just need to add the forward voltage drop across the individual LEDs into the standard calculation for series resistor (the advantage being that you only need one resistor).

Where this conversation doesn't need to go, but I'll mention anyway, the forward voltage drop is not absolute - it'll vary across devices, temperature, forward current, day-of-the-week, and if you look at it funny. What that means is, you always design (calculate the series resistor) based on nominal values - and the slight variances fall by the wayside... what you can't do is drive an Led without a series resistor of some sort (or rather, a current limiting device or circuit) - as any slight variance in the forward voltage drop would have a significant swing on the forward current and could result in the destruction of the LED.

So, given a wet finger of 1.8V Vf for an IR LED, at 12V drive voltage and an If of 10mA, you'd end up with a series resistor value or 840 Ohm, or closest value of 820 Ohm in the E12 range of resistor values. But, you're overthinking things a little - you can wet-finger resistor values given a safe working forward current. So, whether it's one or two LEDs, and 10V or 12V, you'd probably just pick the ubiquitous 1k resistor. It's close enough. There's plenty of other factors in the system that will impact the performance more than 10-20% accuracy of resistor values.

I'm convinced to within a gnat's arse that there will be some method of current limiting/control in the Boxford board, regardless of the schematics. It's the done thing. And the cheapest (and most reliable) method is a series resistor. It *could* be that the original opto-isolators had integral series resistors for 5V operation (after all, once upon a time 5V was common logic supply voltage), but it's almost irrelevant here - you know the opto's you're buying, you can find the datasheet, you can determine the Vf, and then you can determine the series resistor. But 1k will do.

Your schematic is functionally okay, but remember both the LED and the associated photo-transistor are polarised devices - you need to get the wiring the right way around.