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post #8 of (permalink) Old 03-01-2016, 12:50 AM Thread Starter
Algae Grower
Join Date: Feb 2016
Posts: 13
Discussing this with coworkers today, we basically concluded that the AC signal described in previous posts was probably the scope reading AC noise/leaks coming from the wall wart and that there was likely no signal coming off of those pins, and that those pins might not even be connected to anything in the wall wart (or, maybe, to ground), who knows.

So, previous posts = garbage.

Anyway, I went and got the wifi controller instead.

And aha! Now we're talking!

After setting up the device, loading up the application and making sure I could adjust the lighting levels, I disconnected the lamp and hooked up the scope to the 0V and the two mystery pins.

I adjusted the brightness via the application, and looked at how the signal changed on the scope. And now, it the waveform certainly looks like a PWM signal at 500 hz and at 3.3 VDC. The only weird thing about it is that it seems to be negative voltage, going from -3.3v (LOW state) to 0V (HIGH state). I'm not sure why at this point, but the scope doesn't lie, and I tried reversing the scope's ground/measurement tips to see if I'd get a different polarity, and the signals went all wonky on me.

I added pictures of what I saw.

On the first image, you can see the regularity at which the scope sees the many cycles of what appears to be a PWM (I set the light at 50%) going from -3.3v to 0V. The image tells of a frequency of 250 hz, but I think that's because of how "zoomed out" I was on the time scale. When zoomed in a little bit on the individual cycles, the scope clearly shows a solid and steady 500 hz.

The other images below show the scope's results based on the settings of the wifi module/app.

10% brightness, 50% brightness, 90% brightness.

You can see a very constant rising of the wave, then jumps to top voltage for the % of the duty cycle programmed by the app.

So, now that I know what to look for and what to replicate, my next steps are to try to come up with code and circuitry based on an arduino that reads similar wave forms on the scope at the same frequency. Then, hook up the arduino to the lamp and see what happens.


Just tested a couple of things with an Arduino.

1) The good news is that the default PWM frequency is.... 500 Hz.
2) I can also convert a 5V PWM signal to 3.3v using a simple level shifter like 74AHC125.
3) I can use an ICL7660 to negate this voltage (which I don't have right now)

Looks more and more like a piece of cake, but I'm bothered by the fact that the scope is reading a negative voltage for those pins.

But I'm beginning to form an idea as to why that might be, but anyone cares to offer hypothesis?
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Last edited by Darkblade48; 03-01-2016 at 08:06 AM. Reason: Please use the edit function for back to back posts
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