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PIC :: Resistor Varied Output - Why Ground?

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superflux

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Sorry for the confusing title.

I'm creating a simple Tri-State situation like "ronsimpson" suggested using a resistor ladder to give an LED multiple brightness characteristics.

I'm curious to know if the ground (circled in yellow) is necessary and why is it necessary - what is it's function?

Thanks!

8983-PIC_LED_OUTPUT.gif
 
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kpatz

New Member
It makes the ladder act as a voltage divider.

When RB6 is high, you get 5 * (1000/1680) = 2.97 volts. When RB7 is high, you get (1000/1010) * 5 volts = 4.95 volts. Without the 1k resistor to ground, you'd always see 5 volts, though the current going into the LED would be different. Thus that resistor isn't strictly needed in this particular application, since you're going to get a voltage drop across the load anyway.
 
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superflux

New Member
Well said. I figured since the LED is simply a diode going to ground that it would complete the voltage dividing circuit anyway.
 
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