Actually, the LED's are on 1/3 of the time, not 1/12. So far, I've found that muxing (with driver transistors) at 20mA is bright enough. And actually too bright at night. In this case, it should be the same, but I thought if it got to be a problem, I could always drop to some 10mA LED's etc. Cheers -Neil. -----Original Message----- From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Tan Chun Chiek Sent: Friday, June 14, 2002 11:13 AM To: PICLIST@MITVMA.MIT.EDU Subject: Re: [PIC]: Component reduction... Do note that power source/sunk by PORTA and PORTB is limited to 200mA, shouldn't cause much of a problem if you are driving the LED within specs. Since the LEDs are muxed, it's only switched on 1/12 of the time, you might like to put a buffer and drive them harder to achieve the same brightness as driving them directly. Regards, Tan CC ----- Original Message ----- From: "Pic Dude" To: Sent: Friday, June 14, 2002 11:57 PM Subject: [PIC]: Component reduction... > Been trying hard to squeeze my PIC boost gauge (from some months > ago) into a round gauge enclosure, and that means elimination of > as many external parts as possible. So I'm looking at every > component and thinking of why I really need it. Converted the > circuit from 10 direct-driven LED's to 12 MUX'ed LED's (3x4 config) > and then this occurred to me last night... > > http://www.avn-tech.com/stuff/mux_no_transistors.jpg > > Not shown in the pic is that all LED cathodes are to the left. > Also, the dashed line represents the division between the main > board and the display board (yes, I need 2 round PCB's). > > I could possibly eliminate the driver transistors since I have > extra ports to generate enough current. The reason of tying the > lines together is that I can reduce the number of wires going > between the main board and the display board. > > I think I'm not violating any PIC rules here: > - I can keep the tied-together lines at the same logic-level by > keeping them on the same port. > - By multiplexing, only one set of 4 is on at any point in time, > so I'm within the port's total max current. > - Also within the chip's max current. > > BTW, I need the 16F872 since I need an A/D. Nothing smaller w/flash. > > I guess what I'm looking for is the experts' blessing on this. Is > there something wrong here that I am missing? Or shall I start > my PCB layout....? > > Thanks, > -Neil. > > -- > 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