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### Topic: Understanding Project 4 circuit design (Read 3341 times)previous topic - next topic

#### HeatsArdy

##### Jan 02, 2014, 08:20 am
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

#1
##### Jan 02, 2014, 04:50 pm
Can you post the schematic? I've not seen the book you are talking about.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### HeatsArdy

#2
##### Jan 02, 2014, 10:12 pm

Sorry about the quality, this is about as best I could do... here is a video of what the finished product looks like

#3
##### Jan 02, 2014, 10:19 pm
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.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### HeatsArdy

#4
##### Jan 02, 2014, 10:50 pm
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.

#5
##### Jan 02, 2014, 11:13 pm
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).

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### HeatsArdy

#6
##### Jan 03, 2014, 04:21 am
Oh, I get it now! Thank you so much, that was very helpful!

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