7 Segment display

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goujam
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Posted by goujam: Wed. Jul 9, 2008 - 02:54 PM
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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

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Learning and Information, General Electronics
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S-Sohn
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Posted by S-Sohn: Wed. Jul 9, 2008 - 03:02 PM
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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

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goujam
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Posted by goujam: Wed. Jul 9, 2008 - 03:14 PM
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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

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S-Sohn
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Posted by S-Sohn: Wed. Jul 9, 2008 - 04:09 PM
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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

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goujam
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Posted by goujam: Thu. Jul 10, 2008 - 04:09 PM
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Thanks Sebastian,

This is just the help i need, well appreciated.

thanks

James

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