TTL BCD-7 Segment display driver IC, problem

Other Hardware LEDs and Multiplexing
1

Folks,

I am building a project and I want 2 x 7 Segment displays.
To minimise pin usage, I thought I would use a BCD-7 Segment display driver.

I want to use 5 volts, so I looked in my TTL cookbook. 7447. Ok, but it is for common ANODE displays.
It puts the outputs LOW to activate the segment. Ok. It also boasts that it can drive nixi and other kinds of displays.
If I want to use LED it is said to use 330 ohm resistors.

But looking at the 7 Segment displays they are common CATHODE!

So it won't exactly work.

How else can I do this?
Or what am I missing?

2

You can multiplex it with nine wires (including the decimal). Use two NPN transistors to drive the common cathodes, one is controlled from a port line via a 1k resistor, its collector is connected to the base of the second via two diodes in series with the midpoint of the two diodes, the anode of each, pulled up by another 1k resistor to 5V.

3

Gosh - if only there were a way that electronics concepts could be laid out graphically.

4

Yes, if only.

(I was hoping someone with that skill and enthusiasm might do it with lightning speed, as I couldn't think of how to just before bedtime.)

5

Chuckle.

Yeah, I am trying to get my head around what you said.

Sorry, but I am a bit confused.

The brightness will be adjusted by using a PWM pin as the common..... what ever.

Could I use a CMOS BCD-7 segment chip at 5V reliably?

6

lost_and_confused:
I want to use 5 volts, so I looked in my TTL cookbook. 7447. Ok, but it is for common ANODE displays.

What's on the next page, right after the 7447?

7

Paul__B:
Yes, if only.
(I was hoping someone with that skill and enthusiasm might do it with lightning speed, as I couldn't think of how to just before bedtime.)

Pencil, paper; take a picture; attach it.
Arduous.

8

(BIG BLUSH!)

My TTL cookbook is a 1982 version.

So a 7448 is not listed.

Thanks.

9

Get a 4511 (CD4511, et al.) or a different display.

10

Haha, is that Don Lancaster's TTL Cookbook? My copy (1976) doesn't seem to have it either. But my copy of TI's TTL Databook (1973) does.

7448 might be getting hard to find, but there is a 74248 or something similar, don't know if availability might be any better for that.

11

MAX7219, drives up to 8 common cathode displays with SCK, MOSI, SS from Ardunino.
Or LED library and any pins for bit-banged interface.
Or 2 cd4AC164 shift registers. Do the value o- segment mapping in software, just an array lookup:

SPI.transfer(fontArray[numberToDisplay[0]]);
SPI.transfer(fontArray[numberToDisplay[1]]);

or
shiftOut (dataPin, ClockPin, MSBFIRST, fontArray[numberToDisplay[0]]);
shiftOut (dataPin, ClockPin, MSBFIRST, fontArray[numberToDisplay[1]]);

12

Thanks folks.

Given the 7448 (or 74LS48) I found at Jaycar. Local electronics shop.

So now I have to do a bit or thinking about the sketch and "board" layout.