Low Pass Filter For Square Wave Through An Optoisolator

Question

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 []Higher Frequency

No Filter Low Freq(noisier) []

With Filter(low freq.) []

Here is the 12v input Signal with no filtering

After realizing my scope was not properly grounded to the circuit Here are the correct signals 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.

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.