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Project Help and Ideas » Valentine Heart - is it safe?

February 09, 2012
by BobBills
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Just wondering, how close is the heart project to damaging the atmega?

PC5 will switch rapidly between tristated, then source and sink for up to 10 LEDs concurrently (random). 2 parallel LED are enough to go over the 40mA pin limit...

10 parallel LED are enough to hit the VCC-ground 200mA limit for the chip...

The array/matrix project only ever has one LED lit, so these numbers don't apply to it.

How 'long' can you sit at or above the data sheets's absolute maximums?

February 09, 2012
by BobBills
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Correction - matrix has up to 12 LED lit at the same time... so 240mA through the row pin...

February 09, 2012
by hevans
(NerdKits Staff)

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Hi Bob,

The LED Array actually lights more than one LED at a time. Every row has up to half of its LEDs on at a time. The main reason we can get away with this on the LED array is because the LEDs are only on for a fraction of the time. With 5 rows and two directions per row it is only possible to have every LED on 1/10th of the time.

On the LED Heart we are using PWM to dim and brighten the LEDs (in addition to using a similar scheme to the LED array), so the fraction of on time the LEDs typically have is usually much shorter.

Humberto

February 09, 2012
by BobBills
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Hi Humberto,

In the matrix - if all LED are on, then there are 12 on at a time, so 240mA into the row pin at that instant...

So how do you determine that it's not going to not damage the chip? How do you know when shortPeriod * 240mA > sustained * 40mA ?

Thanks!

February 10, 2012
by hevans
(NerdKits Staff)

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Hi Bob,

In some devices you will see ratings for instantaneous current, or instantaneous power vs DC current. On many other devices though, its really just an educated guess. We know that by using PWM on the output the average current over more a few clock cycles is going be less than the max that the pin can drive.

Humberto

February 18, 2012
by Vanceslas1
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Do you have a LCD wiring simular photo simular to the one you have in the picture of the heart project?

February 20, 2012
by hevans
(NerdKits Staff)

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Hi Vanceslas1,

That LCD is an old LCD we used to sell with our kits. The controller that runs it is the same as the new one, the new one just has more characters.

Humberto

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