Post by thread:anode drivers for LED

Subject: Time: 3:11 PM anode drivers for LED 1/12/00 Date: I'm am trying to build a low power 2 digit clock-calendar using a PIC16F84 driving two common anode 7 segment LED digits. The PIC reads a DS1302 and spends most of its time in sleep. I'll use the WDT time-out to wake the PIC. I'm trying to keep the current draw as low as possible. I thought I would be real clever and use 1 NPN and 1 PNP transistor to switch anodes. This way I can get away with only one pin running through base resistors to each transistor. The problem is that one base is forward biased even if all segments are turned off and draws current all the time. I could go back to one output pin for each digit but I thought someone on the list might come up with a way to do it with one pin and draw very little current when the PIC is sleeping. I'm running out of pins, ( I have two pushbutton switches to read too). I need to switch about 100ma. Also, anyone have experience with super-red led displays. They seem to be about 3 times as bright as normal red LED's for the same current . Can I expect to run them at about 1/3 the current then and get about the same light output as the normal ones? Stan

> I'm am trying to build a low power 2 digit clock-calendar using a > PIC16F84 driving two common anode 7 segment LED digits. The PIC reads a DS1302 > and spends most of its time in sleep. I'll use the WDT time-out to wake > the PIC. I'm trying to keep the current draw as low as possible. Then why are you using LED'? I guess you must have the LED's off until you press a button? > I thought I would be real clever and use 1 NPN and 1 PNP transistor to > switch anodes. This way I can get away with only one pin running through > base resistors to each transistor. The problem is that one base is forward > biased even if all segments are turned off and draws current all the time. So use logic level N & P channel fets. No current draw at all, even when turned full on. > I could go back to one output pin for each digit but I thought someone on > the list might come up with a way to do it with one pin and draw very > little current when the PIC is sleeping. I'm running out of pins, ( I have Is ZERO good enough for you? > two pushbutton switches to read too). I need to switch about 100ma. Lots to choose from. Siliconix, Motorola, IR all make suitable devices. Take your pick. > Also, anyone have experience with super-red led displays. They seem to > be about 3 times as bright as normal red LED's for the same current . Can > I expect to run them at about 1/3 the current then and get about the same > light output as the normal ones? Yes. You can also try strobing your LED's with a much higher (8x) peak current with a 10% duty cycle. Our vision system only sees peak brightness, not average so the LED's will appear MUCH brighter and yet you won't fry them. There should be a spec for "max peak current" for the devices. We once had a normal clear red LED running at .001% D/C at 50 AMPS using a laser diode driver cct. It glowed white. Quite bizarre back in late 70's when the only color you could get was red. Now that we get blue LED's I wonder if they'd glow in the UV... spam_OUTRobert.RolfTakeThisOuTUAlberta.ca