Detecting low current

Like many in the Arduino world, I come from a programming background and am need of assistance on the hardware side of things.

I need my arduino to detect when a small amount of current (1.5 volts, 3 uA) passes through a wire. This wire is part of a simple on/off switch. When on, a battery powered clock unit counts up. When off, counting stops.

Is there a way to detect this current without interupting the functioning of the digital clock? I've looked into optocouplers, but it does not seem they will function with current this low. Any help would be greatly appreciated. Thanks!

How do u know is that very low low current level? did you measure it?

Chances are good that you really should be measuring a voltage and not a current. Can you give us more background/info?

-- The Gadget Shield: accelerometer, RGB LED, IR transmit/receive, speaker, microphone, light sensor, potentiometer, pushbuttons

If we presume minimum detectable voltage on analog input ~0.15 V (setting it higher than any noise it can pickup from a board), than R = 0.15 / ( 3*10^-6 ) = 50 k Ohm. Connect resistor in series with a switch, and take a voltage across it to analog input of the arduino. analogread will give you around 30.

You can't measure 3uA easily but you can measure 1.5V easily. Forget the current and measure the voltage on the switch.

Perfect, measuring voltage worked well. Thank you all for your responses.