[sdiy] Driving LEDs from CMOS

[danial stocks]

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?
CMOS chips do drop o/ps quite heavily when you load them up a lot.. but this 
driving led from CMOS o/p is fine.. note however you use a 47 ohm res to 
drive the trans and a huge 680 ohms to drive the led.. you are sending more 
current out of the flop into the tr's B-E junction than the led itself uses 
by a factor of about 10! transistors are supposed to amplify... a 10-47K res 
driving the trans should be fine, or you could even drive a led direct with 
these vals if you use the high brightness ones [the blue leds I've been 
using lately are a comfortable brightness for indicator use with 100k to 
9V!]
Cheers,
Dan
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