photoresistors

[MangoBot]

What value of resistor should be used for these photoresistors? Thanks for the help, Ask any questions and i will try to aswer them.

[kpmcgurk]

What will they be used for? I am assuming you mean the $50 robot... I think any will work as long as they are the same.

[SmAsH]

if you mean the resistor that goes with the photoresistor  then it is 1.62kohms.
if you mean the photoresistor itself then any value should do fine as long as they are both the same.

[Razor Concepts]

1.62k ohms is for admins specific photoresistors, they still can be used but they won't be the optimal value. Check out the photoresistor tutorial, where you can calculate the exact resistor you need for each photoresistor. You don't need two photoresistors of the same value as long as you calculated the resistor properly.

[MangoBot]

thanks for the help guys

[spizzak]

Whats that 1.62k ohm resistor for anyway?

[SmAsH]

im not sure if this is the exact reason but the placement of the resistor can determine whether the voltage increases or decreases with light... but it may also be a p-up or p-down resistor... but i don't know...

[Finnik]

Let me try to explain it...

First, here's the picture of the photoresistor circuit:


Now, for a start, imagine the side-branch to V-out isn't there.
As you probably now, voltage in series adds up, so V-1 + V-2 = V-total
You have a total voltage of 5V between V-in and ground, so the total voltage across both the variable resistor and the fixed resistor together is always 5V.

When the resistance of the photoresistor changes, so does the division of the voltage between the two resistors.

e.g. If for a certain condition the photoresistor has a resistance of 1.6 kohm, and you have fixed resistor of 1.6 kohm then there would be an equal voltage of 2.5V across both the resistors. Together this would make 2.5 + 2.5 = 5V total voltage.

Another example, if the photoresistor has a resistance of 4.8 kohm, with the same fixed resistor, then you'd have a division of 3,75V across the photoresistor and 1,25V across the fixed resistor. Together always counting up to the total 5V.
It is all relative, you take the total resistance as 100%, across which there is a 5V voltage. If the photoresistor has 75% of total resistance then it has 75% of the 5V, so 3,75V. Same for the fixed resistor.

So now that you now about the division in voltage, here is where the side-branch comes into play.
Of course you want to measure the voltage across the photoresistor to determine how much light there is.
The side-branch brings out the voltage that's left after the photoresistor to V-out.

Across the photoresistor you have for example a voltage of 3V, then there is a voltage of 2V left over at the junction. Seeing as V-1 = V-2 = V-total in parallel (the voltage in all the branches is the same as the total voltage). So the voltage of 2V is lead to V-out whilst still also being across the fixed resistor.

Now, you can see why the V-out increases with light. This is because the photoresistor decreases in resistance with light and thus has a lower voltage across it, resulting in a bigger part of the 5V being left over after the photoresistor.

Your choice of the fixed resistor depends on the range of values that the photoresistor has for the different light conditions you want the sensors to operate in.

Hope this helps you.

[SmAsH]

also, this tutorial explains a bit http://www.societyofrobots.com/schematics_photoresistor.shtml

[Finnik]

Yeah, that's where I got the picture from, it however does not explain the workings of a voltage divider circuit and the use of the fixed resistor.

Also, I assume MangoBot read this tutorial as the building of the photoresistor sensors is not explained in the 50 dollar robot tutorial itself, but you are pointed to that tutorial, which explains how to make it and how to choose the resistor

[MangoBot]

so would a 1.6kohm resistor work instead of the 1.62kohm resistor?

[SmAsH]

yes, you can always change the code a bit if they are too sensitive...
heck, i used a 1.5k...

[MangoBot]

thanks for the help