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Current/Voltage drop when LED blinks

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.

Reply to
Wayne.
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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

Reply to
Andy Wood

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.

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?

Tried a DC bench supply?

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

What's a Wisher board?

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

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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Reply to
David L. Jones

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

Reply to
rebel

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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Reply to
Don McKenzie

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.

Reply to
Wayne.

Never did find out what blinky does with a wisher board

Reply to
F Murtz

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.

Reply to
Wayne.

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

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Reply to
Bob Larter

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