I've got a higher voltage square wave input that I'm feeding through FOD817B Optoisolator. The square wave frequency is in the range of 30 to 300 Hz.

The Circuit is like this:


simulate this circuit – Schematic created using CircuitLab

I've tried to filter the Circuit as shown above, it works ok, but it's still quite noisy.

(Scroll Down To Bolded Area) Here are some screenshots from the scope:

No Filter Higher Freq [No Filter Higher Freq]Higher Frequency

No Filter Low Freq(noisier) [No Filter Lower Freq]

With Filter(low freq.) [With Filter Circuit]

Here is the 12v input Signal with no filtering Input Signal No Filter

After realizing my scope was not properly grounded to the circuit Here are the correct signals enter image description here On Occasion I would see the Optoisolated Signal create the similar/same sharp downward spike in the middle of the positive cycle of the square wave. Like the 12 Unfilitered Signal, but sharper.

  • \$\begingroup\$ First, reverse your diode. Second, what is the amplitude of your square wave? \$\endgroup\$ Jul 23, 2015 at 23:59
  • \$\begingroup\$ oops. The real circuit has the diode correctly oriented. Amplitute is 12V on input. \$\endgroup\$
    – GisMofx
    Jul 24, 2015 at 0:30
  • \$\begingroup\$ I see no decoupling capacitor. \$\endgroup\$
    – CL.
    Jul 24, 2015 at 6:56

1 Answer 1


Frankly, I suspect you have drive problems. I'd suggest monitoring the voltage across R2 as a measure of the current through the LED. If it's not perfectly clean (and I suspect it's not) that's your problem. I'd guess that your low phase has a major spike about a millisecond before nominal transition to low, and the high phase of the drive signal shows a major drop about 5 msec in, down to about a volt or less with a rather low rate of recovery.

If the drive signal is clean, try a new optocoupler. Every once in a while devices do very strange things.

  • \$\begingroup\$ Thanks, I will try another optocoupler. I also found some Optocouplers with built-in Schottkey Diodes. This might be what I need..There seem to be so many different types but all seemingly have the same specs... Here's one: FODM611 \$\endgroup\$
    – GisMofx
    Jul 24, 2015 at 2:42
  • 1
    \$\begingroup\$ @GisMofx - Frankly, given the regularity in timing, I'd really focus on the drive. \$\endgroup\$ Jul 24, 2015 at 2:44
  • \$\begingroup\$ I tried a new Optocoupler and same result. I scoped the input signal and added the screenshot to my question. It's pretty square but the falling edge has some rippling. \$\endgroup\$
    – GisMofx
    Jul 26, 2015 at 19:01
  • \$\begingroup\$ "It's pretty square" - not! \$\endgroup\$ Jul 26, 2015 at 20:14
  • \$\begingroup\$ @GisMofx - Aaannd we have a winner. The falling edge does not have ringing, it has the same sort or exponential-decay spikes you see on the first trace. It also has just the right sort of spike just before the trailing edge. Get a new driver. \$\endgroup\$ Jul 26, 2015 at 20:15

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