ESP8266 Community Forum

- Mon Nov 02, 2015 3:31 pm #32863 Thanks again for your time/thoughts!

To be honest I think before I did something like that I'd probably use a multiplexor like the mcp23017 (which was actually my original plan).

Honest question though, is there a reason you don't recommend using the ADC pin? Given your options seem very smart, and would certainly solve my problem, it seems that simply using the ADC pin would be a possibly "easier" solution?

- Mon Nov 02, 2015 4:24 pm #32866

Honest question though, is there a reason you don't recommend using the ADC pin? Given your options seem very smart, and would certainly solve my problem, it seems that simply using the ADC pin would be a possibly "easier" solution?

You can't measure capacitance with just a free running ADC.
Since F = C/V = A*s/V (see https://en.wikipedia.org/wiki/Farad), you need to know 3 parameters. The easiest way to do that, is by fixing 2 of them and measure the third.
You can do that with your ADC by charging the capacitor with a known current and measure the voltage after a known time. But since you need to switch between charging and discharging, you still need an additional digital I/O, so you won't gain anything by using the ESPs ADC.
Another method is to charge the capacitor through a known resistor (ohm = V/A, so F*ohm = s) and measure the time it takes for the capacitor to reach a certain voltage. This is what the techniques I described will do. In the single GPIO example the internal pull-up of an GPIO is not know, but it can be assumed to be constant, so you can still use it to measure changes in capacitance instead of an absolute value.

- Mon Nov 02, 2015 7:08 pm #32876

<...>
...the GPIO0 pin so I could switch it between 3.3V for programming and a GPIO pin...

You can still use GPIO0 to activate re-programming as the pin only needs to be Grounded at reset/poweron, after the bootloader starts loading, pin 0 can be lifted from Gnd potential... when the sketch starts, the pin 0 will perform its normal function. I do this with a pushbutton some times and sometimes with a 3-pin male header and a shorting plug.

Ray

- Mon Nov 02, 2015 11:58 pm #32897 Well, again, all I can say is thank you.

I promise I'm not dense, but honestly I don't have a good "feel" for most of this stuff yet. But you've given me a lot to go on. I think your solution of using a single GPIO pin and switching betwen input/output seems nice as it's only one pin.

I know it probably seems simple to you, but I was just hoping to get away from having to do any more figuring out with the pin situation. So now I guess I just need to decide if I switch GPIO 0 with something "fancy", or if I just go straight into using a multiplexor. (I know there is also the "charlieplexing" solution but for whatever reason that doesn't appeal to me as much..).

I'm going to think about this and read a bit more and bread board the first solution. If that works well and it's not a huge pain to use the GPIO0 pin AND program with it, then I'll probably just go that rought!

Again, thanks, and any other tips/advice is much appreciated