Use the 12v source (before the diode & regulator) to bias another transitor after the regulator to turn on power to the LED supply --no bias (12 v) no power to led's. Say that 7 times really fast... :) ----- Original Message ----- From: Steve Kasm To: Sent: Wednesday, January 30, 2002 10:54 AM Subject: Re: [EE]:Battery Backup? > This does sound simple, but I have one problem, my circuit has 4 7-seg > led that are gated on from the pic chip using 4 PnP transistors. I will > need to blank these displays somehow during battery backup. Transisers > base to pic pins, collector to ground, emitter to common cathode. > > Regards, > > Steve > > -----Original Message----- > From: pic microcontroller discussion list > [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Eoin Ross > Sent: Wednesday, January 30, 2002 12:24 PM > To: PICLIST@MITVMA.MIT.EDU > Subject: Re: [EE]:Battery Backup? > > How about placing a diode in a trace from the 12V to the input of the > regulator ... and another from the battery to the input of the reg. > > When the the 12V drops the power will be supplied by the battery. > > Does anyone see a problem with the reverse current through the battery > diode damaging the battery? > > > >>> kcer@COGECO.CA 01/30/02 11:48AM >>> > > Can anyone point me to any design notes on a regulator or circuit that > will switch my 5volts to a 9v battery to power the 16f84 chip when the > incoming 12v power is missing? > > > > > > Regards, > > Steve > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body