At 9/25/2009 14:08, you wrote: >So did I at first. > >Hope it works for him
The problem I see with this circuit is that there's nothing limiting current on the LED other than the transistor itself. The datasheet for the 2N3905 specs a MINIMUM current gain of 30 at 50 mA of collector current, but it can be as high as 150. Assuming a gain of 100, you're going to put 43 mA of current through that poor LED. It will light very brightly but only last a few months. Better to put a current limiting resistor in series with the diode & let the transistor go to saturation. Assuming 2 V drop across the LED, a 2.7k resistor will yield just over 10 mA through the diode, regardless of the transistor's beta. That way you can also use a far more common PNP transistor, the 2N3906, & not worry about its higher gain. Bob NO6B
