Photoresistor Circuit: How it works (PicoScope 2204A & Snap Circuits).

[00:00:02]Okay, let's take a closer look at how this photo resistor circuit works. So, there's basically two resistors in series. This LED we could consider a fixed resistor. This photo resistor we could consider a variable resistor. And this resistance is going to change with light. So the circuit when it's on, there's going to be a voltage drop on this resistor and on this resistor that's going to equal battery voltage.

[00:00:31]So the question is how much is happening here versus there that can vary based on whether we change the resistance of this. So if I go ahead and put some light on this, you'll see that that LED lights up.

[00:00:50]So, we just went from a situation here where the voltage drops are roughly equal about 1 and 12 volts on each resistor, right? And the battery voltage is about 3 volts. And then when we put this light here, we're actually letting more voltage go across that LED. It's lighting the bulb. So, let's investigate this further by connecting it up to the scope, which I have it connected now to my PicoScope. And the connections here are going to be different than you might expect because this is a what's called a um shared ground. Meaning that when I connect one ground up, for example, this one, the other one's going to automatically have a ground as well.

[00:01:34]They're all going to be connected. So that means I have to connect this up in a in a sort of different way than usual.

[00:01:41]So, normally if you had a floating ground, you could just connect up this is your ground here and your plus for that resistor. And then this would be your ground and your plus. But with a shared ground, I have to have the same potential for both grounds on these channels here. So, what I'm going to do is I'm going to make this the ground and then this is going to be going to the positive of channel A for the photo resistor. That's going to give me a positive potential and this is going to give me the potential across the LED which is going to read as a negative voltage. And we'll just flip that on the scope. So let's investigate this further with the scope and go into screen capture mode resistor.

[00:02:27]Start our record. I'm going to turn on the circuit and you'll see red is the voltage drop across the LED resistor and blue is the voltage drop across the variable photo resistor. They're both now hanging right around 1 and a half volts. The if I cover the photo resistor, you can see the voltage increase because I increased the resistance by reducing the light.

[00:02:50]Let me take my finger away and we're back to where it is at ambient light level. I'm going to go ahead and put some flashlight light on this.

[00:03:01]And you see, look what happens to the to the blue trace, right? It the voltage drops on it. That's the photo resistor.

[00:03:09]And that's because the resistance is is reduced from the light. You'll notice the LED goes the other way. It actually increases because now it's consuming more of the voltage.

[00:03:23]So, we can modulate that by just, you know, moving the light away or back again. In fact, I can even go close enough to go a little bit over range because I'm only at 2 volts on the LED.

[00:03:36]So, I thought that was pretty cool. Just wanted to show that to you guys. I'll take the light away and we're back to ambient levels. I'm going to go ahead and turn the circuit off. And so, that's the whole thing in a nutshell. Thanks for watching, you guys.