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relaly driver latching

Discussion in 'Electronic Basics' started by Starbase, May 17, 2004.

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  1. Starbase

    Starbase Guest

    Hi all,

    My electronics is not good but I am trying to build a circuit which will
    monitor battery voltage (the battery is charged from a solar panel) and
    switch a relay on when it reaches a set value (say 13.5 volts) and off again
    when it falls below another value (say 11.5 volts). I built a voltage
    monitor with LEDs based on the LM3914N and that works OK. Also to conserve
    power I power it in short bursts through a circuit driven by a 7555 timer
    and that works OK. Now I am working on the relay driver which will be
    switched on from a positive pulse from the LM3914N on a selected pin feeding
    one of the high voltage indicating LEDs and switched off again on a positive
    pulse from one of the low voltage indicating LEDs. My theory was that I
    build a simple relay driver which switches the relay on and is then self
    sustaining from a lead with resistors from the emitter feeding back into the
    base, the coil load being connected between ground and the emitter of the
    driving transistor. Hopefully this would keep the relay latched and to
    unlatch it a second transistor would be used to sink the current feeding the
    base of the first transistor to ground. I am still working on this part of
    the circuit (using unknown transistors from an old TV with markings "JC 501Q
    230", I have established that the transistors are NPN with a hFE of about
    260 and though I can get one to switch a load (I am currently using an LED
    as the load) I still have some experimenting to do to get it to latch, i.e.
    it does not work yet.

    I just wondered if this was a standard way of doing things, what you think
    of it, or are there any other (SIMPLE) approaches that I should consider
    which will work.


    Regards Chris


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  2. <snipped description of very power hungry circuit
    MOSFET's and PICs will be your best bet IMO. You would then be able to
    cut the average required power to well below 1mA.
     
  3. You need a flipflop, either use a chip or implement one with your discrete
    parts... which is more punishment than you deserve.
     
  4. Starbase

    Starbase Guest

    I think I will experiment with this idea first (above), though I do much
    like the idea of using and programing a PIC, a leap I am almost ready to
    make.

    Many thanks for the ideas chaps.
     
  5. "Starbase" wrote
    It's one leap in life that you will never regret.
     
  6. OTOH just a simple comparator chip directly driving a power mosfet is the
    'conventional' approach and has very little current overhead ( 5mA tops).
     
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