I am building a simple resistor / capacitor in series (RC) circuit to measure resistance with my RPi A+.

## RC Design

![enter image description here][1]

This design is similar to the one used in [this tutorial](http://raspberrypi-spy.co.uk/2012/08/reading-analogue-sensors-with-one-gpio-pin/) as suggest on [this post](http://raspberrypi.stackexchange.com/q/5167/). This circuit works by measuring the resistance of R1 by measuring the time required to charge C1. I do understand that Python is not a great language for precision timing, but this doesn't need to be very precise. It's mostly a project to learn circuit analysis with the Raspberry Pi. My Python script I am using for this math is the following:
	
	# time how long it takes for C1 to charge thru R1
	GPIO.setup (8, GPIO.IN);GPIO.output (10, 1)
	start = time.time ()
	while GPIO.input (8) == 0: False
	end = time.time ()
	
	# divide duration by capacitance to get resistance
	capacitance = 0.47 * 0.000001 # 0.47 uF
	duration = end - start
	resistance = duration / capacitance
	
	resistance *= 1.85938    # <===== THE HORRIBLE MYSTERY COEFFICIENT
	
	print ('resistance =', round (resistance), 'ohms')
	
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## Math Error - Mystery Coeficcient

The above setup outputs (mostly) accurate numbers for the value of R1. The problem is the unexplained coefficient ~`1.85938`. Some possible error sources I've considered include:

* internal resistance from the GPIO itself (with the 3.3V, 16mA max output per pin),
* problems due to inaccuracies in my components themselves (the resistors have tolerances of 5%),
* a flaw in the universe of the kind that make [`1 = 2`](Math.SE) and [never-ending chocolate](Math.SE) possible,
* or some combination of the above.

Does any circuit analysis / Raspberry Pi expert why RC circuits built with the GPIO would be off by a factor of ~`1.85938`?

[1]: https://i.sstatic.net/sjDIA.png