I'm looking at an Optoisolator with a Logic Output..It's an optoisolator with a Schmitt Trigger on the output. I think the datasheet is a little bare for my comfort level, so I'm looking for some advice/confirmation on connecting these.

These are the devices I would like to test:

This is my interpretation of the datasheets. For devices that are "the same"(for my application) they have different hookups on the output. Also, I've tried to contact Fairchild, but have not heard back.


simulate this circuit – Schematic created using CircuitLab

Using Circuit 3 with 330 ohm pull up Output RED = FODM611 Output Input BLUE = 3.3v PWM Signal enter image description here

Using 100k pull up enter image description here


Both devices have the same type of output, and must be used in the same way.

Both need a decoupling capacitor between VCC and ground (both datasheets say so).

Both have an open-collector output (which can only pull the output down), so they need a pull-up resistor between the output line and VCC. The value of the pull-up resistor depends on the required speed; the datasheets have no switching speed/load resistor graphs, so you have to measure. (You get the speed specified in the datasheet with the specified load resistor; larger resistors result in slower switching, but less power usage.)

Do not use a capacitor on the output signal line. The datasheets show test circuits, which have a capacitor there to simulate stray capacitances in a real circuit. Adding a real capacitor to your circuit would reduce the signal quality.

  • \$\begingroup\$ Thank you for your clarity, but to be absolutely clear, I've updated my question with a third circuit at the bottom. Is this what you are describing? Where R11 needs to be adjusted for my circuit? \$\endgroup\$ – GisMofx Jul 28 '15 at 13:04
  • \$\begingroup\$ Yes, but you do not necessarily have to adjust R11. \$\endgroup\$ – CL. Jul 28 '15 at 14:01
  • \$\begingroup\$ I've tested the 3rd circuit in the question, as you suggest in your answer. It's working, but when the LED is switched on, the Vo line should pull down to ground, but it only pulls down a tiny amount. When I put a 10k pull-up resistor, The Vo pulls down to about 3volts. Any idea as to what's going on? I think something about gain, but I don't fully grasp that. \$\endgroup\$ – GisMofx Jul 31 '15 at 21:19
  • \$\begingroup\$ Is the left voltage source really 1V? \$\endgroup\$ – CL. Aug 1 '15 at 5:12
  • \$\begingroup\$ Are you referring to the isolated source? In my test cases, I'm using 12v, 5v, and 3.3v \$\endgroup\$ – GisMofx Aug 1 '15 at 12:06

Both devices have open collector outputs (the output transistor can only pull the output towards ground - when the output should be high, the output transistor just "lets go" of the output pin) so you do need a pull-up resistor, as in your first drawing, with both devices.

  • \$\begingroup\$ Yes, Thanks for pointing that out. I meant to have the PullUps . Circuits Updated. \$\endgroup\$ – GisMofx Jul 27 '15 at 15:33

First, the FOD needs a pullup resistor to +5 as well. Recommended values seem to be in the range of 350 - 1000 ohms. Go to the end of the data sheet and see the test configurations.

Second, be very sure that you are not getting any spikes/dropouts on your input, even very narrow ones. These are high-speed devices (1 MHz to 10 MHz) and will happily pass along input noise that crosses its thresholds. A small capacitor (say, .001 to .01 uF) across your input is probably a good idea for your application. A Schmitt trigger is good for eliminating smallish noise on slow transitions, but it's not magic.

  • \$\begingroup\$ I forgot to add the pullup resistors. I've updated the circuits. Thanks for pointing that out. The frequencies that I'm passing through are around 30-400Hz. What about the output side.. Both devices show different output connections and I'm looking to see if they are correct or they can use the same output. Why does the 611 need a cap and why does the H11L1 have the resistor? \$\endgroup\$ – GisMofx Jul 27 '15 at 15:37
  • \$\begingroup\$ Yes, they do have different pinouts, so you can's use them interchangeably. Both really ought to have a 0.1 uF cap between Vcc and ground. It's more important on the FOD because that is faster and more susceptible to high-frequency weirdness. No, they should not have 10k resistors, they won't work with them, and I have no idea why you've added them. They don't appear in any of the data sheets. \$\endgroup\$ – WhatRoughBeast Jul 27 '15 at 15:48
  • \$\begingroup\$ Mistake on my part. Fixed. Aside from pinouts being different, would the circuits be the similar for both devices? \$\endgroup\$ – GisMofx Jul 27 '15 at 15:56
  • \$\begingroup\$ In 15 keys, yes \$\endgroup\$ – WhatRoughBeast Jul 27 '15 at 16:08

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