 # Rudimentary Resistance Meter

Hi everybody, I'm trying to use an Arduino as a relatively simple go/no-go resistance meter, to measure, for example, if a given circuit is either 2kOhm, 4kOhm, or significantly higher. My question is, can I do this by just sending the 5V through the circuit, and then returning it to an analog input and reading the voltage, which could then be compared to reference measurements?

An acquaintance suggested I'd need to use a voltage divider with resistors roughly matched to the resistance being measured, but I can't quite wrap my head around why that would be necessary in this situation, since I'd only be feeding the regulated 5V max to the input (if it were a dead short). Would that serve to narrow the range/increase the accuracy of the measurement? I almost want to say that it would decrease the accuracy, since less than 5V would be hitting the input at full, so it would never read full, right?

Am I missing something, or was he incorrect?

The resistor by itself does not change the voltage, only by looking at the whole circuit can you tell what the voltage drop is. There is a resistance in the analog input that you have to consider.

This is more or less what the circuit you are describing will look like

A/D converter
unknown | 100Mohm
+5 ----^^^^^-----±—^^^^^^------

According to the atmega168 datasheet the input resistance on the analog input is 100Mohm. So the input acts like there is a 100Mohm resistor between the a/d converter and ground. if you have a 100Kohm resistor with on end connected to the analog input and the other to +5 then the voltage at the input will be 4.999 volts. if you use a 1Kohm resistor then it will be 4.99999 volts. That wont give you much range at all on your reading.

by using your unknown resistance as part of a voltage divider you will get a wider range of readings. if you want to read between 1K-10K ohm, put a 4 k resistor in series with it

Arduino input
unknown | 4 kohm
+5 ----^^^^^-----±—^^^^^^------

This will give a range of voltages between 4V and 1.43V. Without the divider you would get a range 4.99999V to 4.9999V. The divider is a much better solution