74LS47 drives 4 7segment display.

[ah_Hui]

Hi guys, currently working on a small project.
I'm looking for a driver/decoder to drive 4 common cathod 7segment display?
Is it possible to use 74LS47?..if not, any recommendation what driver to use?
perheps can show me the circuit as well...
thanks guys..

 

[Sebi]

I don't know driver IC for common cathode display in LS-serie, but in difficult situation use the shunt-methode.The outputs are open-collector so when saturated, the segment not lit. Also need inverting the BCD inputs. Another way: additional PNP transistors for each segment output...
So i recommend to buy a common anode display, it's cheaper.

 

[k7elp60]

common cathode display

You could use a CD4511 with 150 ohm current limiting resistors on 5V to drive the display directly.

 

[panic mode]

You could use open collector inverters but it's much simpler
to obtain 74LS48 instead of 74LS47.

 

[Hivolt]

You can use a 74LS48, but remember that is it open collector, therefore you must tie resistors from each segment to +5v instead of series resistors as with the 74LS47. A 7 sip style resistor network works great, it can be placed along side the 74LS48 connecting from pins 9-16, just watch for the pin 1 mark on the sip and make sure it connects to pin 16 of the 74LS48. A resistor value of 150 ohms works well for high efficiency displays.

 

[panic mode]

Actually 7447 has open colelctor, 7448 has pullup resistor built in.
Check the lower right schematic ('48 output)

 

[Hivolt]

Yes you are correct, technically the 74LS48 is not open collector, however the 2 k pullup is useless to supply drive current to an led display ( very dim, and as far a drive current goes I consider it equivalent to o/c) the use of the sip I mentioned gives sufficient current for a viewable display.

Just for arguement sake, the on chip 2k pullup resistor will give a drive current of : 5v(vcc) -1.6v (assumed display forward voltage)/2000 ohms = 0.00156A or 1.56mA , you see, not much for driving a display. It gets even worse if you use a display with a higher forward voltage such as green ,yellow, or blue.

The use of a 330 ohm sip will render a drive current of about 12 mA.
( assuming a 1.6v display forward voltage and taking the 2k on chip pullup into account ).