[sdiy] Driving LEDs from CMOS

[Antti Huovilainen]

On Fri, 16 Feb 2001 DCMagnuson at aol.com wrote:

> 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+

1) Never connect loads directly to flipflops. Always use a buffer (with
impedance being about same or higher as a logic input of the logic family
you're using).

2) 47 ohms is way too low. The rule I use to calculate the value is 
(V+-Vbe) * 50 / max_load_current_of_transistor.
50 comes from a conservative estimation of transistor beta. Remember that
you can approximate transistor as a current amplifying device, Igain being
beta (which is usually between 100 and 200). So for switching loads, just
calculate the maximum load current, divide it by the beta and then you
know how much base current you need.

Antti

"If you don't eat yer meat, you can't have any pudding!
 How can you have any pudding if you don't eat yer meat!?"
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