BCD to 7-seg driver IC

[NinjaKitten]

I've always been annoyed that the outputs of the 74247 and all other similar BCD to 7-seg drivers are inverted, meaning I can't directly drive a 7-seg LED display from it without messing about with transistors or inverters. I'm trying to build a clock and having 42 transistors switch the LED displays on and off is not a fun idea.

Any ideas for a more simple solution?

EDIT: I've tried using a common cathode LCD display, but all I get is wierd flickering in Multisim.

[audiosteve]

Those are open collector outputs.
Think of the outputs as more like solid state switches.
The NEG of the LED is tied to the output and when switched on, the current passes trhough the LED and the '247 to ground. When off the output is high Z and is not actually driven except by a weak internal pull-up. RTFDS.

[NinjaKitten]

So how do I drive the LEDs?

There's a diagram where they're driven straight from the chip here:
http://www.interq.or.jp/japan/se-inoue/e_ckt10_1.htm

And another here
http://tams-www.informatik.uni-hambu...7449-demo.html

But neither simulation works for me.

[audiosteve]

You'd use an 7-segment LED with a common anode.

Or you can use a common cathode with a different part like a 4511

This may help:

http://www.denizyildirim.org/mylibra...ay-drivers.pdf

[NinjaKitten]

I managed to fix the issue. I'd put a single resistor on the common pin instead of a res pack on the input pins. Switched that over and it worked fine.

Thanks for the help

[NinjaKitten]

Now I've sorted my circuit, I need to source the parts.

The 7402's and 7408's should be easy to find, as I can use any matching TTL or CMOS gates that run at 5V.

With the other two, I've been hunting around on Farnell with no luck. They don't seem to allow you to search for the old TTL chips as their number alone - 7448 and 74191 don't return anything useful.

So, anyone know where I can get the following ICs in the UK?

7448N - BCD to 7-Seg Decoder (2k pullup version)
74191N - BCD Up/Down Counter

[NinjaKitten]

Here's the circuit diagram just in case any of you want it:

http://www.ninjacodingmonkey.co.uk/w...9/05/clock.gif

[poynton]

Originally posted by NinjaKitten

So, anyone know where I can get the following ICs in the UK?

Try Cricklewood Electronics.

They are showing the 74LC.. versions of both.


Andy

[NinjaKitten]

The site has what I'm looking for, thanks.

The 74LS48s are really expensive though... it's gonna cost me £12 just for them.

[dmfraser]

For common cathode displays I use the CD4511 to drive them. Unfortunately, you do need a current limiting resistor on each pin. If you only use one on the common line the brightness will vary.