Project 04 Color Mixing Lamp 10KOhm resistors

Official Hardware Kits Starter Kit
1

Complete newbie here just started doing the projects from the starter kit.

In the project 04 (color mixing lamp) it is said that 3x 10KOhm resistors have to be used after the phototransistors. The book says "The phototransistor will force a current to flow in the resistor leading to a voltage drop accross it". I didn't really understand this sentence or why we're using the 3 10KOhm resistors at all.

I get why we're using the 220 Ohm resistors on the RGB LED and why we used 220 Ohm resistors on previous projects - they're used to avoid LEDs from burning out cause they can't handle the voltage. At least that's how it was explained in the first project.
But in the project 04 it seems that we're measuring the voltage coming from the phototransistor, so what are the 10KOhm resistors for?

2

The sensors are not phototransistors. They are Light Dependent Resistors or LDRs. Big difference. A LDR needs another resistor to make up a resistive voltage divider in order to translate the light to a voltage.

I often will use the internal pullup resistor as the load resistor in the LDR circuit as it allows me to leave the external resistor off. It reverses the logic from that in the tutorial but the convenience is worth the trouble.

3

Thanks for the links, they are very interesting. But are you then saying there are mistakes in the book? It specifically says they are phototransistors in few places.

4

hi eightvs!
I believe that in any case these components are mentioned as "photoresistor" and not "phototransistor" forward us screeshoots in case!

thanks

5

I'm sorry for delayed response, but these are the photos of the physical book where it says phototransistor on project 4:

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6

There seems to be 2 versions of this project. One uses the photoresistors (LDRs) and the other uses phototransistors. They are actually wired the same and the code should be very similar.

The 10K resistors serve the same purpose in both circuits. That is, they are the "load" resistor in a voltage divider made up from the sensor (variable resistance) and a fixed resistor.