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schematic
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schematic

by failed on Mon Nov 05, 2007 4:42 pm

ok, this is perhaps a basic question: why did you add to every ir led a transistor? wont it save space if you use just one?
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by montag on Mon Nov 05, 2007 4:45 pm

If you loaded 1 transistors with all the LEDs it would not switch as fast and would not modulate the codes properly.
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by maltman23 on Mon Nov 05, 2007 8:19 pm

The more current we can pump through an emitter, the brighter it shines. And we want them to shine!

If we use 1 transistor, then we would need to have a current limiting resistor for each emitter, thus reducing the amount of current through each emitter.

Also, the transistor drivers can pump more current through the emitters if each emitter has its own driver. The current through the emitter is limited by the Vce-sat voltage of the transistor (we want this low). And the Vce-sat voltage goes up as the current through the transistor goes up. If we use one transistor per emitter, then there is less current through the transistor, so the Vce-sat goes down (thus bringing the emitter's cathode closer to ground, so more voltage is across it, and the current through it is limited by the batteries' internal resistance).

Mitch.

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Please be positive and constructive with your questions and comments.