Electrical – Using an optocoupler to tap into digital PLC signals

Yes, you can certainly use a fast optocoupler for a slow signal.

One issue is that most 'digital' optocouplers do not have any hysteresis, so a small amount of noise on top of a slow-changing signal could result in a transition that is not clean. In some cases that could be an issue. Say you are counting the zero crossings for a real-time clock.

Here is what the typical transfer function looks like of a 6N137 (Toshiba). Normally you'd want to run the LED at a nominal current well in excess of that, perhaps 5 to 7.5mA, to account for temperature, aging and worst-case unit-to-unit variations.

To expand on your example a bit, at 50Hz 240VAC, the mains voltage crosses the zero at about 100,000 volts/second. (100\$\pi\sqrt{2}\times240\$), so it's not moving all that slowly. If you want to detect the zero crossing at, say, 10V +/-20% you have to detect the voltage with an error of 20usec or less. That's assuming no voltage error (say you are using a comparator on the mains side to provide a precise transition voltage). If you allow a volt for voltage error then you must not add more than 10usec of timing error.

If we use the 7.5mA example, the LED must have 7.5mA nominally at 10V, which would represent perhaps 250mA peak if you were just using (say) a resistor in series with a Zener- which would be a destructive value. So some circuitry is required to limit the current and provide a snappy switching action (minimizing variations with temperature etc.) Some hysteresis and pre-filtering can prevent noise from causing messy transitions.

If all you are interesting in is counting cycles you may be able to get away with a resistor and diodes, plus some digital filtering with a micro.

You have many options.

1. If you need to connect very few optocouplers, you can connect them directly to the GPIO of your microcontroller (through a resistor), provided that:

   • You do not exceed the GPIO output current.
   • You do not exceed the total port current.
   • You do not exceed the total gnd/vdd current.

2. If you need to connect more optocouplers, you can try to use low-current, high current transfer ratio optocouplers such as SFH618 (https://www.vishay.com/docs/83673/sfh618a.pdf), and connect them directly to your GPIOs (through a resistor).

3. Or, you can use a BJT or MOSFET (see schematics below). Some notes:

   • Remember to put the pull-down/pull-up resistor, which ensures that the MOSFET/BJT are OFF when the GPIO is not initialized yet (e.g. during reset).
   • Pull-up or down resistor might be omitted if your MCU has GPIO pin with pull-up/down always enabled during reset.
   • If using MOSFETs, remember to use logic level MOSFETs (e.g. BSS138).
   • If you use the active-low solution, make sure that the high level voltage of the GPIO is VDD. I.e. do not use a 3.3V-GPIO and VDD = 5V in the active low solution!.

4. Still, if you need to drive many optocouplers (e.g. 6) you can use the 74LS07 you mentioned, as it allows a 40mA per pin, and you'll have to mount only one component (instead of 6 BJTs/MOSFETs). Remember that, unlike CMOS, TTL ICs are instrinsically pulled-up! However, you might still want the pull-up resistor (the datasheet also recommends not to leave inputs floating). And, since '07 is not inverting, this solution will be active LOW. The 74ABT126 is CMOS so you MUST use anyway the pull-up resistor!

^{simulate this circuit – Schematic created using CircuitLab}