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### Topic: measure a resistor? (Read 1 time)previous topic - next topic

#### cjdelphi

##### Sep 14, 2012, 04:17 am
Measuring how many ohms?

5v ~~~~ 1k ~~~~ A0

sure i get a voltage reading... but how many ohms? Cheers

#### James C4S

#1
##### Sep 14, 2012, 04:26 am
Ohms Law
Capacitor Expert By Day, Enginerd by night.  ||  Personal Blog: www.baldengineer.com  || Electronics Tutorials for Beginners:  www.addohms.com

#### Goofballtech

#2
##### Sep 14, 2012, 04:46 am
http://arduino.cc/forum/index.php/topic,21614.0.html

#### marco_c

#3
##### Sep 14, 2012, 05:56 amLast Edit: Sep 14, 2012, 06:16 am by marco_c Reason: 1
You already have the ohms written down as 1k ohm
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Parola for Arduino http://parola.codeplex.com
Arduino++ blog https://arduinoplusplus.wordpress.com

#### Goofballtech

#4
##### Sep 14, 2012, 06:09 am

You already have th ohms written down as 1k ohm

#### cjdelphi

#5
##### Sep 14, 2012, 07:35 am
Cool... a 2nd resistor's required.... cheers

#### marco_c

#6
##### Sep 14, 2012, 04:03 pm
How do you propose to use this second resistor to measure resistance?
Arduino Libraries http://arduinocode.codeplex.com
Parola for Arduino http://parola.codeplex.com
Arduino++ blog https://arduinoplusplus.wordpress.com

#### danb35

#7
##### Sep 14, 2012, 07:08 pm

How do you propose to use this second resistor to measure resistance?

Simple voltage divider.  5V -> R1 -> analog input on arduino -> R2 -> ground.  Given the voltage reading at the Arduino and the value of one of the resistors, the value of the other can be calculated.

#8
##### Sep 14, 2012, 09:04 pm
Voltage divider: Vin - R1 -Vout-R2- Gnd
Vout = Vin *R2/(R1+R2)
Do some rearranging:
Vout(R1+R2) = Vin*R2
VoutR1 + Vout R2 = VinR2
VoutR1 = VinR2 - VoutR2
VoutR1 = (Vin - Vout)R2
VoutR1/(Vin - Vout) = R2

Ex: Vout 3V, Vin5V, R1=10K
3*10000/(5-3) = 15000

check Vout = Vin *R2/(R1+R2)
Vout = 5*15000/(10000+15000) = 3V
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.

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