Low-Resistance Meter
This project is a simple low-resistance meter. I used to call it a milliohmmeter but in fact it is good for measuring at best hundredths of ohms, and I'm not sure what you call that. I've seen it called a "low ohms meter" elsewhere. It's not as accurate as a commercial unit but should be adequate for hobby use. I used an AVR ATMega168, mainly because of its built-in 1.1 volt A/D reference. The LEDs are ancient MAN6740s from my junkbox; try to use something more modern that is brighter. Then you should be able to increase R9 from 100 to perhaps 470 ohms or more.
A LM317 is used as a 250 mA constant current source (U3). This is passed through Rx, the unknown resistance, and the voltage dropped across Rx is sampled by the AVR's A/D converter, using the internal 1.1 volt reference. Diodes D3, D4 and D5 are used to clamp the A/D input to around 2 volts, to prevent the AVR from seeing overvoltage when no resistor is connected.
The AVR program samples the input and calculates the resistance of Rx. The result is displayed as a 4 digit number.
There are a number of issues regarding accuracy. The problem is that very small resistors produce very small voltage drops across them, unless the current is high. At first I wanted to use a 1 amp current source, but heat in the power supply was a problem, and I ended up using 250 mA. This is a compromise which limits the lower end of the resistance scale that can be measured. Because of the small voltages presented to the A/D, resolution is a problem, and although the readout resolves to the millivolt, the lowest order digit is not reliable. To illustrate the problem, a count of 1 difference from the A/D may calculate to 5 or more milliOhms, so you can't use this as a milliohmmeter.