adder help

Discussion in 'The Projects Forum' started by Gadersd, Dec 13, 2012.

  1. Gadersd

    Thread Starter Member

    Dec 8, 2012
    98
    1
    I have made a simple adder. It adds two single bit numbers and outputs a result. My adder works like expected, but the problem is that I want to light a led as the output. When I get a current of 20 ma at the led with a 150 ohm resister the current the voltage source outputs increases from about 20ma to 40 ma. I want to keep the current at around 20ma to increase the battery life. How can I achieve this?
     
  2. Gadersd

    Thread Starter Member

    Dec 8, 2012
    98
    1
    Maybe a better way to ask my question would be how would I add a led to the adder?
    I have seen so many adders, but none of them showed how to add a led to the end of the adder as the output.
     
  3. WBahn

    Moderator

    Mar 31, 2012
    17,748
    4,796
    So, if I understand you correctly, you are saying that, when the LED is off, your circuit is drawing about 20mA from the supply. Then, when the LED turns on and is, by itself, drawing 20mA that your total circuit draws about 40mA from the supply. Correct?

    Well, think of KCL. Why wouldn't your total current draw increase by 20mA if, all of a sudden, you start feeding an LED that you weren't feeding before.

    So you need to do two things - decrease the current that your circuit draws when the LED is NOT on, and decrease the current drawn by the LED. There are many LEDs out there today that are quite visible with less than 20mA of current.

    Is there a reason that you are using transistors instead of off-the-shelf digital logic components, which can be chosen to draw virtually no power by comparison to 20mA?
     
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