Does anyone know the minimum detectable current for the input pins on the Electron, particularly for the digital pins? I am hoping to minimize power loss by using the max size of resistors when using sensors such as voltage dividers and could not find this info in the Electron Datasheet despite searching high and low.
The digital inputs on Particle devices are CMOS so they switch based on voltage and the only current going into or out of a pin is the leakage current which is spec’ed at +/- 1uA.
You won’t save any power by using large resistors on the inputs and in fact you can cause some problems by choose a value that puts the inputs in the linear region of operation and the input oscillates between high and low.
If you are instead asking about the analog (ADC) inputs, that is a different story, but you said digital inputs above.
Thanks for the info @bko. I understand now that no current goes into or out of a digital pin (other than leakage current). However, the size of my resistors in a voltage divider (for example, a 10 K ohm in series with a 3.9 K ohm) will affect the power loss through the voltage divider when using a 3.3V pin to determine the on/off state of a 12V power supply). Do you know how I could determine the max size of the resistors in this configuration that would still be able to sense whether the power supply is on? For example, could I use a 10 M ohm resistor in series with a 3.9 M ohm resistor instead of the 10K/3.9K option?
(I know this question is different than the first one I asked but based on your answer to my previous question, I realized my real question changed.)
I would say that for safety, you want to dominate the leakage current by a factor of 10, so your 13.9M ohm total resistance on a 12V supply will have 0.863uA flowing and the leakage current can still dominate, so no go.
If you bring your resistors down by 10, you are closer to safety but down by 12 would be safer. That would be around 1.16Mohm total, so I would pick 820kohm and 300kohm. The total current in the divider will be 12V/1.12Mohm or 10.7uA–very small.
I moved the smaller resistor down to limit the nominal voltage below 3.3V. Your original ratio has that voltage at 3.367V which is high. With the new ratio, that voltage is 3.21V at a nominal 12V.
You should also know in advance if you 12V is really always going to be 12V–in automotive systems, 12V is usually anything but nominal! A zener diode can be very useful in these systems.
Just what I was looking for. Thanks for the great help @bko.
Thanks for the help!