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Thread: Need help designing latching circuit

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    Default Need help designing latching circuit

    Hi Guys,

    I want to design a circuit that does the following:

    • "arms" (output high or low) when the input frequency reaches a certain point
    • "disarms" (output low or high) when Vs is supplied to an input



    My input "arming" frequency is 5vdc, and the trigger point will be high for around 15ms and low for 15ms.

    My input voltage for the "disarm" will be around 12-14vdc, but I can drop that to 5vdc if required.


    I know how to build a "one shot" latching circuit, but that doesnt deal with an input of a frequency.

    Hope that makes sense
    Thanks in advance.



Look Here ->
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    Member GavinSV's Avatar
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    Is this another "Help me do my school assignment" request?

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    no. Its a "real life" situation.

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    what frequency.
    you can use a pic chip for this.

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    Whats the application?

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    Sounds like related to a variable frequency motor drive.
    It's hard to make a come back when I havn't been anywhere
    I finally got my head together, now my bodies falling apart.
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    Quote Originally Posted by eeprommemory View Post
    what frequency.
    you can use a pic chip for this.
    The signal will start from 0Hz and build up at an unknown rate of increase. As the increase rate is unknown, I want to trigger at about 33Hz.

    The trigger point would then ideally drive a relay to close the contacts.

    Once triggered, it would hold the relay closed, until a second input occurs.

    The second input will come in the form of DC voltage of 12-14V.

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    Member GavinSV's Avatar
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    QUOTE=eeprommemory;76347]what frequency.
    you can use a pic chip for this.[/QUOTE]

    I'm with eeprommemory, much easier to use a pic. You could do the whole thing with a single chip. If microcontrollers aren't your thing though, how about something like this. Use the VCO and phase comparator from a 4046 PLL and use the output of the phase comp to drive a latch


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    ^^

    Awesome.. I love it..

    Thanks very much for that.

    I should get into microcontrollers. Maybe I might look into it now that I have a real application.

    Thanks very much for the quick response.

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    555 Timer and a transistor is all that's needed.

    555 timer set to respond to the frequency or period which then activates the relay. The transistor is then bias to disable the relay when 12V is present.

    Err, yep, A 4046 can also do it.

    Could also do it with a PIC chip. That's a little overkill and not much fun for this task.

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    Member GavinSV's Avatar
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    Quote Originally Posted by trash View Post
    Could also do it with a PIC chip. That's a little overkill and not much fun
    OVERKILL!....NOT FUN!.....Wash your mouth out with soap. PICs have a built in pulse width modulated Fun generator

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    Ha, movwf ARSE ! (code for blow it out ya arse)
    I have no problems programing for any CPU, especially PICs.

    I can think of about 20 different ways of doing this project.
    The 555 timer and a transistor just wins hands down for simplicity.
    Slightly simpler is a high pass filter and op amp used as a slope detector but the 555 offers a little more flexibility and precission than the slope detector.

    I could show off a little and do the same (slope detect) with just two transistors.

    Precission could also be retained using just two transistors with a resetable monostable circuit where the 12V is used to inhibit the circuit.

    Two transistors beats one PIC chip for simplicity any day.

    Anybody want to try it with just one transistor ?
    I think it can be done.


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    TRASH...

    Could you help me with the 555 and transistor method.

    I cant work it out...

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    I'll try and draw it now.
    Ok... this is the basic config. I have left a design error in it just in case this is a high school project to ensure failure. (I'm evil)
    However if you build it and test it, you will quickly understand to error and be able to fix it.

    Last edited by trash; 12-08-08 at 09:25 PM.

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