[sdiy] Driving LEDs from CMOS

[Dana.Scott]

47 ohms?  That's way too low a value. Try something around 27K instead.
-Dana

> My problem comes from trying to drive an LED from this Q output.  I went
from
> the Q output through a 47 ohm resistor to the base of a 2N3904.  Emitter
to
> ground and collector through a 680 ohm current limiting resistor, then to
the
>  LED.  The other LED leg goes to V+
>
> When this portion of the circuit is connected, the LED appears to work
fine,
> but measuring the Q output now shows the logic high state at only 1.6V,
> rather than 5V.  Apparently when the LED is lit it drags the voltage down
at
> the Q output of the flip flop.  Are there any work-arounds for this?  This
> 1.6V logic high won't properly drive the next stage of the circuit.  When
I
> disconnect the LED, everything works fine.
>
> Is there a better method of driving the LED than the 2N3904 method I used?
>
> I could upload a schemo if anyone needs to take a peek, but it's a pretty
> standard LED driver scheme.  Thanks in advance
>
> Dave Magnuson
>