Muliple 7 segment display.

ZOR

Senior Member
I am doing a timer with a 14M2 and using a 4511 to drive 2 7 segment display LED'S. The idea to display 0:0 - 1:0 - 1:5 - 2:0 upto 5.
To conserve Picaxe ports I went for a 4511 device where a combination of 4 ports switching generated digits 0-9 on 1 7 segment display.

I then saw a circuit;

Multi7segment.jpg

I thought the transistor in each cathode of the display allowed you to individually set the value of each digit by turning on/off the display however I cannot seem to do it. Am I missing out something or does the circuit just turn of digits? Thanks


4511.jpg
 

hippy

Technical Support
Staff member
The circuit allows enabling each digit in turn.

Set the bits for the display directly or via 4511 for one digit, enable the display of that digit. After a while, disable that digit, set the bits for the other digit, enable that. Disable that digit, then repeat.

Do it fast enough and it will appear both digits are on at the same time.
 

westaust55

Moderator
The circuit with the transistors is for multiplexing.
You set up the 7-segment data for the first digit then turn on the transistor to display that digit.
After a short time turn off that transistor and set up the data for the second 7-segment digit finally turn on the second transistor for a short time.
Loop back to displaying the first digit.

This method relies upon persistence of vision so that after a risky is displayed, turning it off for a short time (while displaying the next digit) and finally back on, if fast enough our eyes / brain perceives the display was on continuously. If the driving program does not get back to the digits fast enough the display will appear to flicker.

Another option needing only 3 output pins is to cascade two 74HC595 chips.
This scheme continuously drives both digits and the PICAXE is not spending a lot of its time updating the display.
See this thread for a 3-digit version using 74HC595 chips
http://www.picaxeforum.co.uk/showthread.php?13687-3-digit-display-using-74HC595
 
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ZOR

Senior Member
Many thanks hippy, westaust55, I will try that out, also lookup the 74HC595 thread.

Edit: Thanks, bit flickery but not a major problem, using Pause 12 for visible display time, any more it hops back and forth too much, any less leaves digit elements on too long. It be perfectly acceptable for what I want.
 
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hippy

Technical Support
Staff member
Thanks, bit flickery but not a major problem, using Pause 12 for visible display time, any more it hops back and forth too much, any less leaves digit elements on too long.
Sounds like you might not be multiplexing correctly, perhaps not turning the digit strobe off before changing the digit to display, or not running the PICAXE fast enough.

Post your code and members can review what you have.
 

ZOR

Senior Member
Thanks BESQUEUT, I did not know of this module. Will buy for future.
Re my flicker, I was using a SerTxd command to check the ReadADC of a pot, forgot there and was slugging my delay times. Since removed, altered my pauses between setting digit properties, now unable to see flicker, thanks again.
 
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