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Andrew Kesterson
2026-06-04 18:46:20 -04:00
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05-photoresistors/README.md
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@@ -49,7 +49,7 @@ On a 5v current, a 10k resistor should give us an output of about 2.5v, right? R
* Put your voltmeter red probe on the +5V rail side of the resistor, and the black probe on GND. You will see 5v.
* Put your voltmeter red probe on the downstream side of the resistor, away from the +5V rail. Put your black probe on GND. What do you think you will see? 2.5v? No. You will see 0v.
The downstream side of the resistor is physically attached to ground. Even though there are wires and breadboard ports between that leg of the resistor and "ground", it is physically attached to ground, and it is therefore ground. The river has met with the ocean at that point. There is no longer a potential difference between the river and the ocean, regardless of the space in between, so there is no current.
The downstream side of the resistor is physically attached to ground. Even though there are wires and breadboard ports between that leg of the resistor and "ground", it is physically attached to ground, and it is therefore ground. The river has met with the ocean at that point. There is no longer a potential difference between the river and the ocean, regardless of the space in between, so there is no current. The current does not have to flow from the downstream leg of the resistor to the ground port of the breadboard to reach ground. It is already at ground as soon as it reaches the downstream leg.
The 2.5v measurement we are looking for, if we could find it, is literally trapped somewhere inside the resistor. The resistor, being the bend in the river that applies friction to the current, does theoretically progressively limit the voltage, but only to the portion that has current moving through it. Since the downstream side is connected directly to ground, current only flows until it reaches the point of connection to ground at which point the potential difference drops to zero, and current no longer flows. So if you want to find the 2.5v, you need to put something else between that resistor and ground, and it can't just be a wire. It needs to be something else that presents a potential difference above ground.