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Help with circuit

Discussion in 'Beginner Electronics' started by Gene Hudson, May 3, 2005.

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  1. Gene Hudson

    Gene Hudson Guest

    I can build from a schematic, but do not have enough knowledge to design
    more than a simple circuit.

    I need a circuit for an automotive application.
    I have two relays that are switched on and off to achieve four states of
    They are always powered with 12vdc and sequence by grounding them in the
    following pattern:
    A B
    ON ON
    OFF ON
    ON OFF

    I am looking for a two button control where one button steps up through the
    sequence and one steps down. The time between button press will be at least
    2 seconds, but I guess I need something to make sure each press only
    registers once (ie no contact bounce).
    A plus would be a numerical (segment?) display numbering 1-4 for each

    Any help in setting this up or directing me to information on a circuit to
    do this would be appeciated.

    Thank you,
  2. Gene,

    You probably won't find a circuit for that, but why not take this as an
    opportunity to learn how to use digital ICs. You can make it out of 4000
    series CMOS, which will work directly from the 12V supply.

    For the inputs, connect one side of each pushbutton to ground and the other
    to a pull up resistor to the 12V. Add a capacitor to ground. That should
    debounce the switch sufficiently.

    For the outputs, the CMOS can't drive the relays directly so put your
    digital logic outputs through a MOSFET such as a 2N7000 or whatever is
    needed to handle the relay current. Add a catch diode for the inductive
    spike off the relay coil.

    You should play around to figure out how to do the logic between the inputs
    and the outputs. If you get stuck, ask for help and we'll get you pointed
    in the right direction.

    John Musselman
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