[sdiy] Driving LEDs from CMOS

[DCMagnuson at aol.com]

Hi everyone,

I've been messing around with some logic circuits to convert a momentary 
switch to a DPST switch with a schmitt trigger and D flip-flops.  The core of 
the circuit is simple.  Switch feeds the inverting schmitt trigger, which 
then gets sent to the flip flop.  Using a logic probe, everything is fine.  
Pushing the switch will alternate the Q output between high and low.

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