Multiple inputs - Need a single 1 second output.

Discussion in 'The Projects Forum' started by Mike4Chevy34, Nov 3, 2008.

  1. Mike4Chevy34

    Thread Starter New Member

    Nov 2, 2008
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    I am a newbe. Wired some projects With a Radio shack project kit.

    Here is my dilemma: I just installed a remote car starter in my Chevy.
    My goal is to have the car start by itself while I am by my machine at work. (I cannot leave my machine to start my car when it's -10*F + the keychain remote will not reach through a concrete/steel building)

    The module has an "activation trigger" wire. (this will start the car W/O pushing the remote)
    The trigger requires a neg. (-) pulse of 1 second or less. I will use a N.O. relay wired to chaises ground and the coil powered by this project.

    I purchased a $12 countdown timer/alarm from a local hardware store that runs on a single AAA battery. I would like to use the timer/alarm to activate this Neg. (-) trigger (relay) but the alarm will output a Pos. (+) .75 Volt pulse (4 pulses a second for 30 seconds to a piezo buzzer)
    I need to build a circuit that will give a Pos. (+) output of one sec. or less to a relay even though the circuits trigger will be Pulsing. ie. after the alarm has reached Zero.
    I have up to 12V DC to work with.

    Any ideas on how to make this work?
    Thank you in advance for any help on this mater.
    I understand that the LCD on this cheap timer may freeze up.
    Is this a huge deal?
    How picky are those displays? I haven't trashed one yet.
    Trying to keep warm in Wisconsin.
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Hey neighbor. I was in Minnesota for 17 years. It's hard to say live, as I was frozen at least half the time.

    I think you are looking for something called a "one shot" or monostable. Look up the LM555 or other version (e.g, NE555) and check the section in the datasheet on monostable operation.

    Basically, you start a timing sequence by using a negative going trigger, and it outputs a pulse a certain duration. By negative going, I mean the trigger input is held high and you momentarily ground it.

    Before getting into details, though, you refer to negative pulse as well. Do you me a negative going pulse from a high state (i.e., from high to ground) or do you mean something that goes negative with respect to ground?

    As for LCD's, they are temperature sensitve. I have been out of the tundra for 15 years now. Maybe the new ones are not as temperature sensitive.

    John
     
  3. Wendy

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    Mar 24, 2008
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  4. Mike4Chevy34

    Thread Starter New Member

    Nov 2, 2008
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    Hello from the tundra of WI.
    I have done some research on the 555 timer and from what I can tell, the chip wants a negative input pulse from high to low and then back high. I was hoping to trigger the chip to activate via. many +.75V pulses on the input. Could I tie a transistor in with my input to get high to low inputs?
    On the output, I need ONE SINGLE SHOT of +12V for one second or less (To operate a relay - pull it down for one second)

    Should I go with a 74121 chip instead? That seems to take a + input pulse. one positive edge Schmitt-triggering input. What is that?
    Sorry for any confusion, the fact that I am so new to this and thanks for any help.
    To sum this up, I have a cheap countdown timer that outputs 120 .75V pulses in 30 seconds to a piezo buzzer. I will remove the wires to the buzzer and I need to convert the first .75 pulse into a single +12V output signal and ignore the rest of the 119 pulses. The chip will be powered by the 12 V car battery.
    I thought this would be easy!
     
  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Unfortunately, I am tied up moving my shop this week. I am not experienced with simulations and generally build to test things, but just don't have the time right now to do that. So, here are just some initial impressions.

    1) The 74121 chip runs on 5V, has weak current output, and requires a 1.4V trigger according to its datasheet. You might find a relay that could be driven by that (e.g., a 5V reed relay), but you still would have need for a 5V supply and a way to get adequate trigger voltage. How much current will your relay need to handle?

    2) The NE555 and its generic equivalents run well on 12V and supply adequate current to run most relays.

    3) How did you determine the magnitude of the 0.75V pulse? If it was done with a voltmeter, it is possible the actual voltage is higher, as the meter does not respond instantly to a voltage change. Assuming the pulse is something less than 1V, you will need a voltage amplifier to get to a usable trigger for most monostables. A single transistor (e.g., 2N3904) in a common emitter configuration with a collector resistor of 12K or so would give you the inversion you need for the 555 input. That is, a positive pulse on the transistor base will give a negative going pulse on the transistor collector, which would be your output to the 555.

    4) In order to get the single, 1-second pulse with inhibition for the next 119 pulses consider using 2 one-shots. The first would initiate the relay pulse and trigger the second one-shot after and an RC delay of 1 second. The second one-shot would give a much longer negative pulse to inhibit any further pulses from the first one shot (e.g., by using the reset pin) regardless of its input.

    5) Once this thing runs its cycle, how often will it need to be reset and repeated?

    Bill Marsden is a 555 guru and hopefully will chime in.

    John
     
  6. Mike4Chevy34

    Thread Starter New Member

    Nov 2, 2008
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    Mike

    I take it I shouldn't have put my answers in with Johns' quote? Oops.
     
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