Finding unknown resistance

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?

berkeman

Use a PIC uC with an on-chip ADC...

vk6kro

The voltage at Vin will be proportional to the resistance R.

So you can say V_in = R / (R + 1000) * 5 volts

Once you measure the value of V_in, you just put it in the above formula and work out the value of R.

For example, if the value of V_in 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.

ee1215

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