7 Segment display

Log in or register to post comments
Go To Last Post
5 posts / 0 new
Author
Message
goujam
goujam's picture
Level: Wannabe
Joined: Thu. Mar 22, 2007
Posts: 58 View posts
#1
Posted by goujam: Wed. Jul 9, 2008 - 02:54 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi,

Sorry if this is a silly question but i am struggling to think straight.

I have a 7 seg display common anode, with the +Vcc connected to a boosted 5V supply. The avr is running at 3V, this is giving me a problem that the LED's are slightly illuminated when they should be off(due to the potential difference). To turn the LED's on i just turn the AVR output low then back on to turn LED's off.

What would be the best way to run the 7 segment with this configuration, so i get the LEDs off when they should be.

thanks

James

Tags:

Learning and Information, General Electronics
Top
S-Sohn
S-Sohn's picture
Level: Posting Freak
Joined: Sun. Aug 22, 2004
Posts: 1615 View posts
Location: Germany
Posted by S-Sohn: Wed. Jul 9, 2008 - 03:02 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

You could use NPN transitors to drive the LEDs (or a driver IC with open collector outputs like ULN2803). Or you could simply connect the common anode to 3V.

Regards
Sebastian

Top
goujam
goujam's picture
Level: Wannabe
Joined: Thu. Mar 22, 2007
Posts: 58 View posts
Posted by goujam: Wed. Jul 9, 2008 - 03:14 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

The reason i have the common anode at the boosted 5V is that i am using batteries to run the device. The AVR runs down to 1.8V but the LED's will go dim. I have to use 5V as i am running a 5V IC aswell from the booster.

I did not want to use 8 Transistors but ill look into the uln2803.

thanks

Top
S-Sohn
S-Sohn's picture
Level: Posting Freak
Joined: Sun. Aug 22, 2004
Posts: 1615 View posts
Location: Germany
Posted by S-Sohn: Wed. Jul 9, 2008 - 04:09 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

You could control the common anode via a PWM, measure the battery voltage and adjust the PWM duty cycle according to the power supply. This way you could avoid the step up converter. The application won't work down to 1.8V but approximately down to 2.5V.

You'd have to calculate the resistors for the min power supply:
Vcc = 2.5V, V_LED = 1.8V, I_LED = 10mA

(2.5V - 1.8V) / 0.01mA = 70 Ohm (Next E12 resistor is 68 Ohm)

So if the battery voltage is 3.5V, then you'll have to set the duty cycle to:
(2.5V - 1.8V) / (3.5V - 1.8V) * 100 = 41%

I recommend to use a FET to control the common anode.

Regards
Sebastian

Top
goujam
goujam's picture
Level: Wannabe
Joined: Thu. Mar 22, 2007
Posts: 58 View posts
Posted by goujam: Thu. Jul 10, 2008 - 04:09 PM
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Thanks Sebastian,

This is just the help i need, well appreciated.

thanks

James

Top