# Photoresistors

This is another analog to digital converter project using a photoresistor like I used a potentiometer in the last project.

# Lessons Learned

- Resistors in a circuit don't work the way I thought they did

# Resistors

I always thought resistors worked much like a hose reducer. Say you have a 2" hose with a 1 gallon per minute of water flowing through it. Now you reduce that hose to 1" with a 2" to 1" adapter. The orifice is physically 50% smaller so now you can only fit 50% as much matter through it in a given period of time. Electronics tutorials often use streams of water as an analogy for electrical current, and this makes easy sense. 

So when I looked at the circuit diagram, I wondered, "why are we measuring the voltage on te circuit UPSTREAM from the photoresistor?" Wouldn't it stand to reason that you would witness the impact of the resistor *downstream* of the resistor? For example it doesn't make sense to measure the impact of a `2" -> 1"` hose reducer on the `2"` side, right?

The answer is because, as it turns out, if you take the measurement downstream of the photoresistor in this particular example, you will always read 0v. You will never see a change in the output voltage from the photoresistor. In order to see the impact of the photoresistor in the circuit, you need to measure upstream of the photoresistor.

<center>
<img alt="wot" src="wot.jpeg"/>
<br/>
*The face of a man who realizes he has failed*

*to grasp something very fundamental*
</center>

If you remember back to [tutorial 4 with Analog Digital Conversion](../04-adc/README.md#measuring-potentiometers), I talked about potentiometers, and how they are a `voltage divider`.

> Potentiometers are adjustable resistors (just like a rheostat), but with 3 pins, they're acting as two resistors in series like a voltage divider.

I used the term voltage divider there not because I actually understood what it was doing, but because that was part of the terminology used in the material I used to better understand potentiometers. And while I understand the moving the wiper changed the resistance on the output tap, and I understood that this could be described as a voltage divider, I didn't really understand what a voltage divider was, what it does, and why it does it.

A voltage divider does exactly what it says: it creates a point in the circuit at which the available voltage is equal to some portion of the input voltage, produced by dividing the input voltage across a couple of points of resistance.