Resistor Capacitor ADC

Basic Resistor / Capacitor circit used to simulate Analog over Digital pin.

Wikipedia entry

As made popular by this Adafruit post as well as: this, that and the other

Concept

"[...]the capacitor acts like a bucket and the resistor is like a thin pipe. To fill a bucket up with a very thin pipe takes enough time that you can figure out how wide the pipe is by timing how long it takes to fill the bucket up halfway" - Adafruit

As noted in the link, this can only be used on for reads of resistor-like sensors (like our photocell).

While this method can be used to generate results reliably - system (code timeing etc) and setup (exact wire layout) introduce the need for compensation on the application side. Thus, it is recommended to use proper ADC in all(?) cases.

Basic usage

Setup of a photocell read on pin 17 with a 1uF capacitor and two base resistors: 20 KOhms, and a photocell that we expect ot max at 200KOhm.

 ResistorCapacitorADC.read(17, { rKOhms: 20, cuF: 1, maxKOhms: 200 }).then(result => {
   console.log('measure time (ms)', result.measuredMs);
   console.log('\test base charge time (ms)', result.baseMs);
   console.log('\testimate (Kohms)', result.estimateOhms / 1000.0);
 });

Returned result contains the following properties:

• baseMs expected milliseconds delay calculated from config (static for config)
• measuredMs raw millisecond count used by read
• scalledMs ± delta between baseMs and measuredMs scalled to 63% digital pin fill time.
• estimateOhms measured resistance reverse calculated from scalledMs time (most common externaly used value)

(currently negative scalledMs values result in negative est Ohm values)