HELP V. Indicator

Discussion in 'Homework Help' started by Sarat Rakngan, May 22, 2014.

  1. Sarat Rakngan

    Thread Starter New Member

    May 22, 2014
    Hi everyone,

    Recently I was assigned my first term project on voltage indicator via LEDs. I have uploaded the photos of the instruction and also the first draft of my plan below. The only part I am not so sure of is how I am using the comparator in association with the zener diode. The comparator will be used to decide the voltage level in which the supply voltage will start sourcing. Two comparators will be used since one is connected to 555 timer IC (for blinking mechanism mode 2 when voltage level is between 9V and 11V) and the other is going straight into the LED connection (for ON condition when voltage level exceed 11V). To keep this short, I am looking for some idea to modify the circuit to make it works efficiently and correctly. I would really appreciate any suggestion and comment from you guys.

    Thank you very much!
  2. MrChips


    Oct 2, 2009
    Moved thread to Homework Help.
  3. crutschow


    Mar 14, 2008
    Look at the data sheet for the LM311. It is an open-collector output and cannot supply any output voltage, only sink current to ground (from an external load resistor to the supply voltage for example). Your circuit requires that the comparator supply all the current to the 555 and the LEDs. That's not practical. Better to use the LM311 to hold the reset of the 555 low to inhibit its operation and power the 555 and LEDs directly from the supply.

    Also look at the data sheet for the 7404. It cannot provide enough output current to power the LEDs. You will need an added buffer circuit (transistor).
  4. Sarat Rakngan

    Thread Starter New Member

    May 22, 2014
    Thank you for your reply! I managed to work out the solution to the comparator section for battery saving mode (9-11V). However, I can't seem to come up with a way to switch to full power mode (>11V) via another comparator. Is there maybe a simple trick I am not thinking of? A lot of my friends are using arduino board for all the conditions, but I don't want mine to be the same. I really do appreciate your help crutschow! :)
  5. WBahn


    Mar 31, 2012
    If you look at your schematic you will see that you are shorting out the 7404 since you have the output of the bottom LM311 connected to all four LED strings.

    The instructions say that Mode 1 is optional but, apparently, Mode 2 is not. So, if you implement Mode 1, what determines whether Mode 1 or Mode 2 is used. Or do you just have to implement one or the other and it is up to you which?

    Focus on Mode 1 (which is what you seem to be doing anyway). If I asked you to design a circuit using 16 LEDs, some resistors, and a transistor such that a single logic level input could turn all of them on or all of them off, could you do that? If so, you now have a building block for your lights.

    Now, consider just the 555 timer circuit configured as an astable multivibrator. Can you figure out a way to force the output to go HI and stay HI? Can you figure out a way to force the output to go LO and stay LO? If so, you now have a building block for your modulator.

    If you can accomplish the two tasks above, then all that is left is to figure out how to use comparators to make a control block that is used to place the 555 block into the proper mode (LO, Running, HI) and use the output to control the LED block.