Logic gate into boolean equation

Fred Marshall wrote:

(snip regarding a seven segment decoder)

> Well, yes except that the whole equation is negated.

Most logic outputs are better current sinks than sources.   The 7447 has 
open collector outputs that sink current only, and stand 15volts when off.

I once made a counter with each segment made of four discrete LEDs. 
Four LEDs in series, appropriate resistor and a 12 volt power supply 
driven by the 7447 outputs.

-- glen