Current/Voltage drop when LED blinks

[Wayne.]

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.

 

[Andy Wood]

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.

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
[email protected]

 

[David L. Jones]

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.

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.

I am showing a range of 4.7 to 5.3v, even at rest.

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?

I have also
changed out the voltage regulator; LM7805T, LM7805L, LM317.

Tried a DC bench supply?

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 shouldn't change at all, something is wrong.

It is almost like there is a short. I have expanded the circuit to
use 3 Wisher boards:

What's a Wisher board?

1 for the voltage regulator, 1 for the blinky,
and the 3rd for the uC.

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

The LEDs require a peak of 20ma.

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

Dave.

 

[Don McKenzie]

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.

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

Cheers Don...



--
Don McKenzie

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[Wayne.]

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.

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.

 

[F Murtz]

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.

Never did find out what blinky does with a wisher board

 

[Wayne.]

Never did find out what blinky does with a wisher board

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.

 

[Bob Larter]

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.

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