Hello all, I'm new to the forum. I will try to contribute as much as I browse more of your topics, but for now my first post is asking for a little push in the right direction.

I need to detect very small leakage currents, on the scale of 1/4 mA, without the use of a multimeter.

Basically, I want an LED to turn on when it detects a current above 0.5mA. Anything below that, I want the LED to be off.

I made something that somewhat works using a BJT. R2 would be the device that has the leakage current I'm trying to detect. It looks like this...

I realize this is not a very robust solution.

the difference between 1mA and 0.4mA seems to be enough to open and close the gate (base), but the only problem is the leakage current spikes sometimes and turns the LED on which I want to snuff out. I tried a 180uF cap but the spike lasts longer than I thought so it makes the LED flash more gradually.

Can anyone give me some ideas? I was thinking of using a uC, but I dont want to spend time on something that might not work (will it reliably distinguish such small currents?)

I'm looking for a simple elegant solution maybe even a way to change current threshold this circuit responds to.

 

To clarify, I am more interested in average current over, say, 10 seconds.

This might involve a uC....but if there is someway to go around it, that would be great.

 

I'd use a comparator, such as one of the four on a LM393 available at the Shack. You could set a variable reference voltage on one pin and look at voltage on top of your resistor (bottom to ground?) with the other. You could detect extremely small currents this way. For instance it's easy to detect a touch of your finger on a wire. Of course the comparator itself will draw more current than you're looking to detect. Maybe that rules it out? Oh, and putting a capacitor on the input will "integrate" the input over whatever period of time you want. Just choose a capacitance appropriate for the resistance involved, ie. so that RC equals about half the time you wan to integrate over.

 

Thank you I will try it