#nodemcuesp8266
I'm using this ESP8266 board, which has a Vin pin.
This link – says specifically that Vin get a direct 5V from USB's DC. But when measuring it using a voltmeter, I get 0.1V (2 boards were checked and got the same result).
Can someone explain what is true? (My goal is exactly to get a direct 5V from external USB).
My guess is the \$ Rds_{ on }\$ for the internal mosfets are not matched which is the case for most GPIO's, they have wide tolerances on the GPIO circuity and since the ESP8266 doesn't have any documentation or testing (its a Chinese product and 95% of them have poor documentation and testing).
I'm assuming your using the GPIO for a low side switch, if you are, I would buy some mosfets and use them as a low side switch in the place of the GPIO ports so you know what your \$ Rds_{ on }\$ is going to be.
Either way I know that the conductivity of a liquid is going to be from 1 Ohm to a milli-Ohm, and if you put a mosfet in series with that that has 10's of milli-Ohms of resistance (and almost certainly varying from port to port). Since this is an unknown, why not buy some cheap mosfets and make it a known source of error.
So try this:
Make Vin go to your gpio, get some mosfets with the \$ Rds_{ on }\$ that you require and make sure the voltage of the GPIO (probably 3.3) will turn them on. Your load would be the connection to the water. You may need a dual stage with your 12V to get mosfets with a low enough \$ Rds_{ on }\$ for your application.
I would also look at the tolerances on your resistors, 330Ω at 1% would be 3.3Ω. So if your plugging in 330Ω in your software, the actual resistor could be anywhere from 333.3Ω to 326.7Ω. Same thing with the capacitors as some ceramics have 10% tolerances.
Its hard to say what current a cheap power supply from aliexpress can supply safely. But if the rating says 2A and you only need 1A it should be fine. From your description i assume that the device is not getting too hot to touch for a longer time, which adds to the assumption that 1A load current should be fine. If you want to try a somewhat experiemental approach you can stress the device with say 1.8A (Keep an eye on the output voltage aswell) and monitor its case temperature over a longer duration (until thermal equilibrium is reached). If you are still below or equal 70°C (@20°C ambient temperature) you are totally fine (only valid if the case is non metallic). Have a look at IEC 60950 if you want more detailed Information about how hot the surface of electronic equipment housing is allowed to get.
Best Answer
Yes, it does say that. However it shows only one type of NodeMCU board. Not all NodeMCU boards are exactly the same.
I believe that your "problem" NodeMCU board is slightly different than the "working" version which is shown on that web page.
Looking at photos of the "problem" board from your supplier's link, I noticed this part of the PCB is different than the "working board". See the 2 diodes in the red ring:
(Image source - Vendor "ModuleFans" on AliExpress)
I suspect these 2 diodes are forming a Diode-OR for the two possible 5V inputs - the USB port and the
Vinpin. This will prevent backpowering whichever of those two power supply sources is not used.That means you cannot use
Vinpin as a power output on that version of the board.The photos of the "working" board are a little smaller, but in the equivalent part of this PCB, I think I see only 1 diode in the red ring:
(Image source - Losant Blog)
This is probably only protecting the USB Vbus pin from being backpowered, if power is being supplied via
Vin.If my hypothesis is correct, then that board would allow power out to the
Vinpin, when power is supplied via USB.You would need to find and compare the schematics of the two boards (or reverse-engineer that part of the two boards yourself) to be completely sure. However my hypothesis, and seeing the different components on those two boards, support and explain the test results you have seen.