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Question about led flasher circuit

Discussion in 'Electronic Basics' started by [email protected], Apr 10, 2007.

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  1. Guest

    I want to build some circuits from

    http://ourworld.compuserve.com/homepages/Bill_Bowden/page10.htm

    About 1/3 down there are 4 1.5V led flasher circuits.

    The first one uses a 74hc04 hex inverters chip.

    I have tried to use a chip marked DL004D which is supposed
    to be an sn7404.

    I have looked in the datasheet and it is supposed to be powered
    by 5V. So how can it possibly work from a 1.5V battery?

    Thanks.
     
  2. The 7404 is a logic gate, the output will be either a "high" or "low"
    voltage, depending on the input state, and nothing in between.

    That circuit linearizes the gate with that 1megohm resistor, so it
    can oscillate.

    The 7404 that you are using won't work in the circuit. It is internally
    TTL (a forty year old logic family), and must run off a 5v supply, and has a
    low input impedance. The large value resistor won't work with it, and the
    IC will not work off a lower voltage supply.

    The 74HC04 came later, and uses CMOS internally instead of the bipolar
    transistors of the original. This has a very high input impedance, meaning
    that 1 megohm resistor will linearize the gate enough, and it will work
    as intended (at least, if the original does work, I've never tried it).

    CMOS logic is less restrictive about power supply voltage. TTL wouldn't
    work at a much different voltage than 5v, CMOS will. I can never remember
    the exact intend of the "HC" in the middle of the number, but the original
    CMOS variant of the 7404, the 74C04, allowed for operation up to about
    fifteen volts.

    A 1.5v supply would seem to be a bit low for the 74HC04, but in this
    sort of thing likely it can be done.

    If this was just using the IC as logic, ie that "high" or "low" state,
    likely you could get away with the 7404, if that was the only IC being
    used. (You can do some mixing of the logic families, but there are
    limits.) But this is an odd use of the IC, and in this case you really
    do need the 74HC04 (not just because it can be the straight 7404, but
    because when someone cooks up a use for an IC that is or may be out of
    spec, they are counting on some vagary of the specific IC)

    Michael
     
  3. The 74hcxx series is CMOS, so even though it's specified at 5V, it will
    work at lower voltages. This is not necessarily true of other families.
     
  4. Guest

    I have just tried the one with just one inverter with an 74hct00b1,
    tieing the inputs together gives us invertes. It doesn't work with
    1.5V. The only good one is the transistor
    based one, which works very very well.
     
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