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Transistor switch design problem

Discussion in 'Electronic Basics' started by todd fahrner, Jul 18, 2004.

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  1. todd fahrner

    todd fahrner Guest

    First: I'm a complete electronics novice, in over my head. Apologies
    in advance for poor descriptions and follow-up questions.

    The overall problem: I have a Hall effect sensor (in a throttle).
    There's a main, a ground, and a feedback wire. In the rest state, the
    feedback wire returns ~5V. As the throttle is opened, the feedback
    wire voltage drops progressively. I wish to close a switch powering a
    ~9V component when the throttle feedback voltage drops below 4V, and
    open it as the voltage rises again above it.

    All of the discussions and diagrams of transistor switches I've found
    work by saturating a transistor to close a switch; i.e.; rising V
    would do it. I need to have falling V do it. Seems like a simple
    inversion should be possible, but I can't figure out how. Hints?
    Alternative suggestions? Thanks.
     
  2. todd fahrner said
    Try a PNP transistor:


    5V 9V
    / |
    |< |
    Sig---R--< PNP LOAD
    |- |
    \ |
    | |
    R /
    | |-
    ---R--< NPN
    | |<
    R \
    | |
    | |
    gnd gnd


    Hope the load isn't a Nitrous bottle. ;-)
    This is very crude.

    Sorry for the poor ASCII. This is my first!
     
  3. John Fields

    John Fields Guest



    +12>-----+----------------+-------+------------+----------+
    | | | | |
    | | [10K] | |
    [330R] | | | |
    | +-----[1M]--+ | |
    | | | | | |
    VIN>--------[10K]-----+--|+\ | E |K
    | | >-----+--[3K3]----B PNP [1N5359]
    +------+ +--|-/ C |
    | | |1/2 LM393 | |
    |K | | +----------+ |
    [Z5.1V] [10K]<--+ |K | |
    | | |+ [1N4001] [RELAY COIL] |
    | | [1µF] | | |
    | | | | | |
    GND>-----+------+-----+-------------+----------+----------+


    Adjust the pot for 4V at the - input of the comparator, then when VIN
    falls below 4V the output of the comparator will go low, turning on
    the transistor and the relay.
     
  4. View in courier font (using notepad perhaps?)

    5V --------------o-------------o---------.
    | | |
    .-. .-. |
    | | | | 8.2k |
    | | 100 | | |
    '-' '-' |
    | | |
    ,-o-. | |
    4.7k | | | |
    ___ |< >| | |
    Input -|___|--| |---------o |
    |\ /| | |
    | | | |
    | | | |<
    | o-------------------|
    | | | |\
    | | | |
    | | | |
    | | | o--------.
    | | | | |
    | .-. .-. | C| Relay
    | | | 3.3k | | 33k - C|
    | | | | | ^ C|
    | '-' '-' | |
    | | | | |
    GND ------------o---o-----------o---------o--------o



    Diode = 1N4148
    PNPs = 2N3906
    Relay = 5V switching

    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    This design assumes a regulated 5V supply.

    When the input gets down to 4V, the differential amplifier will turn
    on the rightmost transistor, causing the relay to be closed.

    You could replace the differential amp with a comparator, but that
    would be cheating...

    Regards,
    Bob Monsen
     
  5. The specific component designed to make such a one bit analog to
    digital decision is called a comparator. It has two inputs and one
    output. one input is labeled + and one is labeled -. When the +
    input is more positive than the - input, the output goes positive,
    which in most cases means that an output transistor connected to the
    negative supply rail, turns off. The inputs can compare any voltages
    that are in the common mode range, usually somewhere within the supply
    rails. So, to use a comparator to make this decision, you need some
    source that provides 4 volts to be compared to your hall effect
    signal. The comparator will need a supply that surrounds the 4 volt
    reference. Comparator outputs typically can carry between a couple
    milliamps to up to about 30 milliamps. If you need more output
    current than that, you will have to add a transistor to the output to
    provide a higher current output.

    Here is a data sheet for a common dual comparator and a single:
    http://cache.national.com/ds/LM/LM193.pdf
    http://cache.national.com/ds/LM/LM111.pdf

    You can also use an opamp as a comparator in any application where the
    output does not have to switch off or on in sub microsecond time.
    Opamps are essentially comparators with push-pull output stages and
    are slowed enough that the inputs cap hold the output in any
    intermediate stage between full pull down and pull up.

    Here is the data sheet for the opamp version based on the same front
    end as the above dual comparator.
    http://cache.national.com/ds/LM/LM158.pdf

    These are very useful components to be familiar with.
     
  6. todd fahrner

    todd fahrner Guest

    Three replies, three approaches... Your design seems simplest, but
    still a little confusing: my circuit design reading skills are below
    fundamental. Concerning supply, I have been assuming that I could tap
    the throttle feed wire without interfering with its function.

    I'd say I'm experiencing a fight or flight reaction to the challenge
    of learning this stuff well enough to solve my problem in a reasonable
    timeframe.
     
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