# Finding unknown resistance

by ee1215
Tags: resistance, unknown
 P: 29 If I have a circuit similar to this: http://i.imgur.com/JaUGb.png I am reading in voltage as an input from Vin, when switch is closed, and am wanting to find out the unknown resistance R. Similar to dmm...what would be the best approach? Reading the voltage, calculate current to find R?
Mentor
P: 37,602
 Quote by ee1215 If I have a circuit similar to this: http://i.imgur.com/JaUGb.png I am reading in voltage as an input from Vin, when switch is closed, and am wanting to find out the unknown resistance R. Similar to dmm...what would be the best approach? Reading the voltage, calculate current to find R?
Use a PIC uC with an on-chip ADC...
 Sci Advisor P: 3,956 The voltage at Vin will be proportional to the resistance R. So you can say Vin = R / (R + 1000) * 5 volts Once you measure the value of Vin, you just put it in the above formula and work out the value of R. For example, if the value of Vin was 3 volts, you would move the 5 volts and the (R + 1000) to the left of the equation and say 3 volts / 5 volts = 0.6 So, 0.6 * ( R +1000) = R so 600 ohms = R - 0.6R = 0.4 R R = 1500 ohms Now check, 1500 / 2500 * 5 volts = 3 volts So, the answer is right.
P: 29

## Finding unknown resistance

thanks vk, going to give it a try with my module.

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