Current/Voltage drop when LED blinks

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I am doing the traditional "hello world" with an Atmega8.

The setup: LM7805 with a status LED, Atmega8, and LED with 220ohm
resistor on PB0.

When I set PB0 low, the LED illuminates, but the status LED dims for the
duration.

I am showing a range of 4.7 to 5.3v, even at rest.  I have also changed
out the voltage regulator; LM7805T, LM7805L, LM317.

The voltage is far more stable using the LM317, but I still experience
the dimming of the status LED, even though the voltage is rather
constant (4.9 to 5.1).

It is almost like there is a short.  I have expanded the circuit to use
3 Wisher boards: 1 for the voltage regulator, 1 for the blinky, and the
3rd for the uC.

The LEDs require a peak of 20ma.

Any ideas would be appreciated.

Re: Current/Voltage drop when LED blinks




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What is the input voltage to the regulator? How stable/current
sensitive is that?

Does the current drawn by the whole thing increase by the expected
amount when the LED is on?


Andy Wood
snipped-for-privacy@trap.ozemail.com.au

Re: Current/Voltage drop when LED blinks


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Is the status LED directly across the regulator or another micro output pin?
If it is directly across the supply then it shouldn't dim, as the LM7805 has
a low output resistance and can supply ample current.
If it's on another output pin then the ground of the micro would be suspect.
Do you have a photo of the actual construction?, this could provide vital
clues.

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Sounds like it's oscillating perhaps.
Has your 7805 got proper bypass caps on it?
What is the 7805 input voltage?
Looked at it with an oscilloscope?

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Tried a DC bench supply?

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It shouldn't change at all, something is wrong.

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What's a Wisher board?

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What's this "blinky"?, the status LED I presume?
What's the circuit?

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Most LED's do not need to be driven at 20mA to be usable, 5mA or so is
usually plenty.

Dave.
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Re: Current/Voltage drop when LED blinks



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What sort of bypassing on the reg?  Have you got (access to) a CRO to look at
your 5V rail?

Re: Current/Voltage drop when LED blinks


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if you can scribble down the schematic, scan it, and throw it up on the
web somewhere, it may help.

Cheers Don...



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Re: Current/Voltage drop when LED blinks - Solved


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It was a dodgy controller.  I swapped out the installed atmega8 and
replaced it with another, and it worked fine.

I noted that without blinky, the circuit used 50mA: status LED, uC, and
7805.

When blinky fired, the current usage jumped to 80mA.

But in both cases, voltage remained the same.

Re: Current/Voltage drop when LED blinks - Solved


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Never did find out what blinky  does with a wisher board

Re: Current/Voltage drop when LED blinks - Solved


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Wisher boards are the solderless breadboards.

The way I had the circuit setup: one board had the voltage regulator
with the status LED, the seconds had the uC, and the third with the LED
and resistor for the blinky hello world.

Re: Current/Voltage drop when LED blinks


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Add two 100nF ceramics to the 7805, one between input & ground, & the
other between output & ground.

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