555 Timer Help

Discussion in 'The Projects Forum' started by Zaraphrax, Sep 21, 2009.

  1. Zaraphrax

    Thread Starter Active Member

    Mar 21, 2009
    47
    3
    Hi guys,

    I'm designing a circuit for a project and need some guidance.

    I'm designing a circuit which is basically a window comparator, connected to a buzzer and an LED. I'm setting up the window comparator, such that the output will be high when the input voltage is above 14V and less than 10V. I'm feeding the input from the two comparators in the window comparator circuit into a NAND gate, which is to drive the buzzer. The buzzer will be on when the voltage is above 14 and below 10 (because only one of the comparators will be high, thus the output from the NAND gate will be high. If it is between 10 and 14, both comparators will be high, thus the buzzer will be off).

    Now, comes the tricky part. I need to use a timer circuit of some fashion to make the LED flash when the voltage goes below 10V, but remain solidly on when it's above 14V (and remain off when it's between 10 and 14). I've read up on 555 timers in monostable and astable configurations, but can't quite work out what I need to do to integrate this into my circuit.

    Any help is appreciated.

    Thanks.
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Interesting.

    But first, show us your circuit please.

    I assume you want to use one 555?
     
  3. Zaraphrax

    Thread Starter Active Member

    Mar 21, 2009
    47
    3
    Well it doesn't really matter, but I suppose if I could do it with one, then one will be fine. Or, if there's an easier way to do it than I suppose that would work also.

    Unfortunately I don't have a scanner handy to scan my circuit in....I'll try drawing it roughly in Microcap (sorry, it's not very good - we'll just have to use our imaginations).

    [​IMG]

    The DC source there is a 14V source. Imagine that the comparators are configured with reference voltages such that when and input voltage voltage is higher than 14V, the top-most comparator produces a HIGH output, and the bottom one is HIGH when it's below 10V.

    So, when the top comparator is HIGH, the buzzer will be on, but I also need the LED (not shown) to be on constantly.

    When the bottom comparator is LOW, the buzzer will be on, but the LED will flash at about 1 Hz.

    (So basically it's a circuit one would use in a car for example, to tell you if your battery is flat, or the regulator is bad and is overcharging that battery).

    I was thinking about configuring a 555 as an astable to give the 1 Hz flash, using the output from the bottom most comparator to drive the trigger. I would then need to make this retriggerable somehow, so when the top comparator goes high, it will pull the trigger high but also allow the LED to be on constantly.

    I hope that makes some sense.

    Thanks.
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    555 timers have a reset line (pin 4) that can also be thought of as an enable.

    Instead of thinking of controlling a 555, think digital logic where the pulse train from a 555 is routed. When the second comparitor is high (14V) it feeds a signal to the gate that controls the LED, similar to an OR gate. When both comparitors are low a second gate kills the signal, similar to an AND gate.
     
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