In practice, you’re feeding a negative voltage to the Arduino pin half the time. Arduino’s don’t like that...

Remember that the transformer in the diagram isn't a transformer. It's actually an AC/DC converter drawn as a transformer because TS couldn't find a suitable symbol for a converter.

 

@AlbertHall I believe you but I'm trying to understand why R2 needs to go back to ground on the Arduino. I thought the (-) of the ac/dc adapter would complete that circuit. Why doesn't it?

@djsfantasi My understanding from the datasheet is that this product should convert the signal to DC. I thought that meant there would not be negative voltage and that the signal would be (at least to a good degree), rectified. What am I misunderstanding? In regards to what it's for, the end goal is to power an ESP-01 at 3.3V with a max draw of 300mA. I'm using a LD1117V33C (800mA) voltage regulator to step down to the appropriate vcc. That said, while I was trying to build that circuit, I realized I didn't understand what was going on with my power supply. So, this question really is just to understand what is going on so I have a better understanding moving forward.

Thank you both for your help!

Why, it certainly does convert to DC! I looked at your schematic and only saw a transformer. But now, looking at the application circuit in the datasheet. Do you have the external components specified? Particularly the two capacitors.

Although the datasheet doesn’t graph the output voltage, the in-circuit requirements for the two caps implies that the output is unfiltered by the component alone.

Hence, the output will oscillate between ground and the specified component and my comments with regard to your sketch are valid. Over a sufficiently large period (any multiple of 17ms), the average voltage will be zero.

It’s peak may be higher than 5V, so the average voltage is 5V.

But if your using the recommended external circuitry, I can’t explain what you’re seeing.

Remember that the transformer in the diagram isn't a transformer. It's actually an AC/DC converter drawn as a transformer because TS couldn't find a suitable symbol for a converter.

@AlbertHall, yep! I get it now. But after reviewing the datasheet, I still have questions. At a minimum, his simulation needs to include a FW rectifier after the transformer output.

 

@AlbertHall That did it. I changed the circuit to the one below which connects R2 to Arduino GND and I'm reading between 2.51 and 2.52 volts at A0. Thank you so much! But I still don't understand what's happening here. It seems to me like the A0 pin should ground out through the Arduino's power supply's GND. Can you help me understand what's happening?

 

@djsfantasi It seems like the voltage output is fairly stable, but I will be adding in the capacitors and the fuse for the ESP-01 power supply. I don't understand why R2 has to be grounded to the Arduino board as I mentioned above, but that seems to have resolved it. Thank you for your help.

 

@djsfantasi It seems like the voltage output is fairly stable, but I will be adding in the capacitors and the fuse for the ESP-01 power supply. I don't understand why R2 has to be grounded to the Arduino board as I mentioned above, but that seems to have resolved it. Thank you for your help.

Would you expect the LED in this circuit to light or not?

 

Yes. I’m not near my equipment or I would test instead of asking. But does it not?

 

Yes. I’m not near my equipment or I would test instead of asking. But does it not?

It would not light.
Any current that flows through the LED goes to ground but then there is nowhere else for it to go. There is no connection back to the battery.

 

Joe,

Look at this article on AAC. Note the difference between rectified DC and filtered rectified DC.

Your sketch is measuring the voltage at many places of the unfiltered waveform. On average, half will be above 2.5V and half will be below. Hence, the average will be 2.5 volts. When you filter the output, add the caps, the voltage waveform looks different.

 

Thanks, @AlbertHall! That was the piece that I was missing.

Thanks for the article @djsfantasi!