 # Understanding Project 4 circuit design

Hi everyone. I am trying to get a better understanding of the circuit in project 4. In particular, the part of the design I do not understand is the 10 kilo-ohm resistors which are plugged into the ground. I'm trying to visualize how the electricity is running through this circuit but I just don't understand those resistors. I feel very confident in my understanding of the circuits in the previous projects(where I feel I would be able to replicate the project without referencing the book), but I do not feel that way about this part of project 4.

Thanks for any help :)

Can you post the schematic? I’ve not seen the book you are talking about. Sorry about the quality, this is about as best I could do... here is a video of what the finished product looks like http://www.youtube.com/watch?v=4SgLTb_XeOc

The photoresistor and the 10K make a voltage divider which the Analog input reads. Analog voltage= ~5V*10000/(photoresistor + 10000) and Analog Reading = ~Analog/5 * 1023

Without the 10K, the photoresistor just becomes a variable value pullup resistor that would read as 5V/1023 all the time.

Sorry, I’m a complete beginner with electronics and I don’t really understand. This is the first time in the book I’m seeing one of these dividers. I think I understand the 5V*10000… 5 volts from the board, 10k from the resistor because Ohms law says volt s= current * resistence, but why are we dividing by (photoresistor + 10000)?
Again, I apologize if these questions seems silly, I’ve literally never worked on any sorts of electronics before. I am a programmer and my only experience with circuits are of the logical variety.

Ok, so V=IR, and V/R = I. Ohms Law. There will a current flowing thru both resistors. V/(photoresistor + 10000) = Itotal The voltage across each resistor is then V1= Itotal * photoresistor, Vout = Itotal * 10000, Vin = V1 +Vout Put it all together, do some algebra, and you get Vout = Vin * R2/(R1 +R2) (and Vin = V1 + Vout, or Vin - Vout = V1) R1 is the resistor between Vin and Vout (photoresistor), and R2 is the resistor between Vout and Gnd (10000).

Oh, I get it now! Thank you so much, that was very helpful!