http://www.turbokeu.com/myprojects/countdown3.htm

I'm looking at doing a similar project of making a count down timer only i'm planning on using the 16f628.

The project uses a 4543B BCD to 7-segment decoder-latch and i can't to find it from most suppliers i use, so i was wondering if there are any alternatives people could suggest?

thanks.

 

If you have 8 pins to drive the display then you do not need the decoder. If the 16f628 does not have enough then upgrade. Or use a shift register to get more output pins.

 

You could use a 7446 or 7447 instead; they're open-collector outputs can sink up to 40mA.

I'm not too crazy about how they wired up the PNP transistors to mux the individual digits. Since the PIC outputs won't go higher than 5v and they're using 8v for the LED's source voltage, you wouldn't be able to turn current to an individual display completely off, so you'll see "ghosting" on the displays.

You'd really need to drive the PNP bases using NPN's; the NPN base resistor should be around 1k Ohms. The PNP base resistors should be around 3.9k Ohms, and they should also have pull-up resistors from the bases to +8v.