On Fri, 4 Dec 2009 13:06:02 -0700, "YES NOPE9" said: > I looked at the circuit and I don't understand how the 2N3906 can turn > on when the negative terminal of the solar cell goes below zero. It > looks to me as though the negative terminal has to go below negative > 0.7 volts. Where did I go wrong ? No, only down to 0 volts, because it is acting as an emitter follower and only needs to pinch off the NPN (one diode drop). The only real drawback to the circuit(VS a diode) is the quiescent current drain. If you had it connected to a battery and took it out of the light for long enough, it would completely drain the battery. Not good if it's lithium-ion. Cheerful regards, Bob -- http://www.fastmail.fm - Access your email from home and the web -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist