problem with the power source?

Using Arduino IDE 1.x
1

Hi everybody,

I've got an Arduino UNO and everything works perfect except when I try to connect the board with an external source power (6 batteries of 1.5V each one). It seems like there is no enough power to reach (for example) 5V of the 5V pin, and the LEDs on the board are clearly not brighter than if I connect directly the board with my computer via USB.

I have taken two pictures of my board with a red LED connected in the 5V pin, and these are the results:

Dropbox - Error - Simplify your life (connected with my laptop)

Dropbox - Error - Simplify your life (batteries)

The batteries don't have any problem. I've tested each one individually and they are all working fine.

I'm forgetting something or there's a problem in my board? I've tested with another batteries, and the result is exactly the same.

Thanks!

2

You're forgetting the resistor in series with the LED to limit the current draw.

3

I think that this LED supports 9v, and that won't solve the problem isn't it?

The problem is that connecting the batteries in the Arduino board still works, but with less power. The 5v pin gives less voltage (about 3v) and some sensors doesn't work under 4v.

Is this a problem of the board or am I doing something wrong?

If this is a problem of the board I'm planning to cut a USB cable and connect it to a 9v battery, giving the power via USB adapter instead of the battery adapter. Anybody knows if that could burn any component of the board?

4

adri_lag:
This is a problem of the board or am I doing something wrong?

You are doing something wrong. Put an appropriate resistor on that LED before you finish off the processor.

5

The beautiful thing about led's these days is that they add little load to the circuit, simply put, your short circuiting the arduino and shoving 50 ma down the gnd port. This would be a great idea if you wanted to see your board burn (to fix this put a resistor in between +5v and the anode of the led). Any ways I think your problem is that the AA battery's have dropped to 6v. The arduino uses a linear voltage regulator meaning as the input voltage comes closer to the output voltage, the output voltage will drop. Again simply speaking, don't put anything less than 6.5 volts in the voltage regulator.
Use a multimeter and see what voltages they are at.
-Luke

6

adri_lag:
I think that this LED supports 9v, and that won't solve the problem isn't it?

No your led doesn't.

LEDs only draw their forward voltage. Red LEDs drop around 1.6V. Once at least that much voltage is applied, the LED turns into a short circuit.

What you are seeing is that on USB, there is enough current to feed your shorted out LED.

On battery, the batteries are dropping their voltage due to the excessive load of the shorted LED.

You must use a current limiter when using LEDs, period. The simplest is a resistor. Use a 1K resistor in series with that LED and there may be enough life in your batteries for it to work.