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555 Timer with switch and pump

Discussion in 'Electronic Basics' started by Scott K, Mar 30, 2005.

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  1. Scott K

    Scott K Guest

    Greetings all! I am very new to electronics but heres what I want to do...

    I want a push button (spst) to start a liquid pump for x seconds (enough to
    fill 1.5oz of fluid)
    Here is the pump specs: http://www.smartproducts.com/sp2003.html

    I have a protoboard circuit that works with a trigger switch and LED that I
    copied from this site:
    http://www.doctronics.co.uk/555.htm#monostable

    I had no problems getting this monstable circuit to work...But since I want
    to power a pump that is 12-24vdc < 1amp (pump is on order, wont get it for a
    few weeks ) I am thinking I will need leg 3 of the 555 chip to go to a
    relay, I will probably need a full 12v to go to the pump to get it to work.
    I don't know how to integrate the relay and pump in place of the the LED...I
    picked up a 12VDC relay SPDT rated at 1a part # 275-241 radioShack.. It has
    5 contacts on it...

    Anyone have any suggestions?

    Thanks,

    Scott
     
  2. Rich Grise

    Rich Grise Guest

    You would put the relay coil in place of the LED+resistor, but only if the
    output current spec of the 555 is greater than the required current for
    the relay coil. So you'd need a transistor to drive the relay, but since
    you need a transistor anyway, the relay is redundant unless you need to
    isolate from the mains. So all you really need is a transistor that's good
    for an amp, and maybe a 2.2K resistor from pin 3 of the 555 to the base
    of your transistor. The emitter of the transistor should go to the
    negative pole of the 12V supply, and its collector goes to the negative
    terminal of the pump motor, and the positive of the pump motor goes to the
    positive of the 12V supply.

    Picking a transistor that can switch an amp is left as an exercise for
    the reader. (GAW! I _love_ saying that!!)

    http://www.google.com is your friend. :)

    Good Luck!
    Rich
     
  3. Scott K

    Scott K Guest

    Thank you for your reply Rich. Since the motor will draw .8 amps (I found
    this out today)
    Can I still use your method of boosting the output voltage through a PNP amp
    transistor? I was looking at the max current the 555 can output, it was
    listed at 200 or 300 ma... can't remember off the top of my head.

    Does the transistor amp, boost the current as well?

    Thanks in advance!

    Scott
     
  4. The issue is only that 0.8A will require something like 20mA of base
    drive. Thus, you need to make sure the resistor between the output of
    the 555 will let it pass that much. At 12V, you probably want to use a
    470 ohm resistor rather than the 2.2k resistor Rich suggested.

    You can also use an N-channel mosfet, which won't require any base
    current. If you do this, you probably want to do it like this:

    VCC
    ------------------------o--------.
    | |
    | |
    | .-o--.
    - | | Motor
    1N4001 ^ | |
    | '--o-'
    | |
    o---------'
    |
    ||-+
    ||<- N-MOSFET
    From 555 out----||-+
    |
    |
    |
    -------------------------'
    GND
    (created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

    If you omit the diode, the inductive spike from shutting down the motor
    can clobber the mosfet.

    Also, use a capacitor between the Vcc and Vdd pins of the 555 to prevent
    any false triggering.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  5. Rich Grise

    Rich Grise Guest

    And here, I just want to say Thanks to Robert for expanding on my reply.
    I neglected the diode, and was way too lazy to look up the current spec,
    beta, and all that.

    Cheers!
    Rich
     
  6. Scott K

    Scott K Guest

    Will a IRF511 N mosfet work for this application? I am going to the
    electronics store to get one now :)

    Thanks a bunch!

    Scott
     
  7. Yes, it'll work.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  8. Scott K

    Scott K Guest

    Thank you Robert! I got your design working great!
    I've been using a 12 V LED in place of the Liquid pump... I can't wait to
    give it a try when the pump arrives!

    I found this schematic just like you suggested:
    http://casemods.pointofnoreturn.org/pwm/circuit1.html

    One more question though... in the schematic they have, they have the Diode
    across D S of the mosfet, and not across the + - inputs of the motor... does
    this infact do the same thing by preventing feedback across the D S?

    Thanks again!

    Scott
     
  9. Their diode does nothing. There is already a diode there, inside the
    mosfet! If you look at the mosfet picture, there is a little triangle
    inside, pointing to the black vertical dashed line. That is a diode,
    which all power mosfets have, due to their construction.

    You should put the diode across the motor, pointing up (meaning the
    terminal with the line is connected to 12V).

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
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