Re: [Openeeg-list] Confused--Please help!

[Peter S. <pet...@ar...>]

Ray:

There is something very odd about your LED. In normal operation there
is usually a voltage of 1.5 through 2 V across an LED's terminals,
depending on the color for the most part and a little on the current.
9 Volts must fry it for sure--though there might still a be short
flash of light during its accelerated death. I have never seen any-
thing written on an LED. You will probably have something different
than a pure LED, maybe an array of several, a combination with a
resistor, a blinking LED with an additional electronic circuit inside,
or an incandescent lamp, but whatever it was--it seems to be defective
by now. And of course, as it is a light emitting diode, the current
must flow through it in the correct direction, which is the direction
of conduction, not reversed. As long as the voltage and current are
limited, it does not harm an LED if reversed, it simply doesn't shine.
But never attach a pure LED directly to a power source, the current
must be limited. This is mostly done with a simple resistor.

If I remember correctly you also asked how to attach the LED to the
entire circuit. If you have the ModEEG, then there are terminals on
the digital board, where the LED must be connected directly. If you
want to want to use an LED as a power indicator and want to power
it on your own, then you need a current-limiting resistor in series
with the LED. LEDs can usually stand 20 mA, so design it for something
like 10 through 20 mA. Calculating the supply voltage minus the LED
voltage (say 2V) and dividing the difference by the intended current
gives the value of the series resistor. That combination must then
be attached to the DC power rails, that is in parallel to all other
circuitry.

Regards, Peter


Ray Cole wrote:
> 
> Thank, Oliver!   Your information was very helpful, and I appreciate it
> so much.  Now I can continue on, and complete the whole thing within an
> hour's time...except for attaching the electrode discs to the ends of
> the cables, and they should be here, tomorrow.
>  
> Yesterday, I was just playing around with the power set-up, and I
> couldn't get the LED to light up.  It's a fully-charged 9v battery, and
> the LED has "9v" written on it, but I couldn't get it to light up, even
> if I put the battery voltage straight across the LED with wires.  Is
> there something about LED's that I don't know?  It's a brand new LED,
> but I suppose it could be a bad one; sounds a little doubtful, though.
>  
> Vielen Dank!
>  
> --Ray Cole