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Discussion in 'Electronic Basics' started by Frank, Nov 26, 2004.

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  1. Frank

    Frank Guest

    I have a simple circuit for a time delay. Small problem is that it does not
    switch entirely as I want.
    With a transistor I fill an elco of 100uF and then switch off that
    transistor. The voltage on the elco is used to switch on a second
    transistor. As long as the voltage is above about 1V the transistor stays
    open. But, between 1 and 1,5V (around that, dunno) the switched current goes
    down and the LED on the second transistor fades out. I want the led to
    switch off quickly. How could I do that? It should be something like: if
    voltage on elco goes below 1,5V switch immediatly to 0V. Any website a could
    Hope this is clear enough, drawing schemas in text is very tiresome.
    Thanks for your ideas.
  2. John Fields

    John Fields Guest

    It's also tiresome trying to figure out what you mean. For example,
    you don't say whether the output of the electrolytic is supposed to
    supply the operating current for the LED or whether you're only using
    it for timing. Then you say that the LED stays on with 1V on the cap,
    but that it starts to dim when the voltage on the cap is between 1 and
    1.5V, which doesn't seem to make much sense. Since you're the one
    asking questions it seems to me that you're the one who should be
    putting forth the effort to at least make yourself understood for the
    convenience of those whom you are asking for help.
  3. CBarn24050

    CBarn24050 Guest

    Subject: timer
    Learn to post your circuits in spice format, it's a whole lot quicker and can
    be read on any text program.
  4. Jamie

    Jamie Guest

    Look at Schmit triggers and voltage comparators.
    you need a circuit that can switch on at a higher
    point than what it takes to switch off.
    you can also acheive this by putting a bias
    feed back transistor to help drive the base
    as soon as current starts to conduct.
    there are lots of options but i would look
    into the use of a simply Op-amp.
    with that you can simply use a loop back
    resistor to the + input that would counter
    act the fixed voltage point.
    look up voltage comparator's
  5. Frank

    Frank Guest

    Thanks, that sounds like what I need. I'll look into it.
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