[sdiy] Driving LEDs from CMOS

[Mike Granger]

I would try connecting the LED to V+ through a limiting resistor and to the
collector of the transistor. The base resistor should be much higher, I'd use 47K
to 100K ohms myself. Connect the emitter of the transistor directly to ground.
You are dragging too much current out of the cmos chip output (because of the 47
ohm resistor), and this is messing up the logic levels.

DCMagnuson at aol.com wrote:

> 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