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Motor comtrol

Discussion in 'Electronic Basics' started by Robert Tracey, Oct 9, 2005.

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  1. Hi

    I am a supply teacher with a limited knowledge of electronics and have been
    given a class at short notice. I have a student who wants to control a
    simple 9v dc motor to make it rotate clockwise for a certain time then
    anticlock for another period of time, maybe just a few seconds in each
    direction. anyone have any ideas that is as simple as possible please.


  2. A double pole, double throw relay or switch is the simplest way to do
  3. Bob Monsen

    Bob Monsen Guest

    It is probably just a matter of switching the leads. JP suggested a "DPDT"
    switch or relay. You may not know what they are, so here is a little
    tutorial on switches:

    A switch has 'throws' and 'poles'. Think of those big switches you see in
    the movies that they use to fire up the electric chair. That is a single
    pole single throw switch. When it is up, there is no connection through
    the switch. When it is down, there is a connection.

    The 'pole' is the part that is always connected to the incoming line. The
    'throw' is the part that is either connected or not depending on if the
    switch is up or down.

    If there were a connection at the top as well, so you could use it to
    electrocute two people, but at different times, that would be a single
    pole double throw switch, or SPDT switch.

    If you were living in Texas, and needed to execute lots of people, you
    could install a DPDT, or double pole double throw switch. This means there
    are basically two electrically separate SPDT switches which are closed by
    the same mechanical action. This actually goes on, and you can get 3pdt,
    4pdt, etc, switches. You can also get what are called rotary switches,
    which have more than two throws. They can also have multiple poles, and
    again meaning they are stacked on top of one another and activated by the
    same rotary motion, so the general naming is NpMt, where N is the number
    of poles, and M is the number of throws. I guess they could use one of
    these for executions in China.

    Now, back to your question. You can usually reverse the direction of a DC
    motor by switching the polarity. Lets call the terminals of the motor A
    and B. So, with a battery, simply swapping both of the terminals makes the
    motor run the other way. Now, if you hook up the terminals of the battery
    to the poles of a DPDT switch, and hook up the throws to either sides of
    the DC motor, so when the switch is one way, A is connected to + and B is
    connected to -, and when the switch is the other way, A is connected to -
    and B is connected to +, then your student will be able to run the motor
    forwards and backwards by flipping the switch. Yu can get toggle switches
    that are DPDT at the hardware store or electronics store.

    You can also get relays that are DPDT, so when you energize the coil by
    putting current through it, the throws go one way, but when you stop
    putting current through the coil, the throws go the other way. However,
    there are a few gotchas on using relays, so post again if you intend to do
    this, and somebody can help out.

  4. There are no doubt more elegant methods, but this should do it:

    It will give about 3 seconds in each direction with the component
    values shown. Make R2 a variable pot or preset if you want to control
    the cycle time. And alter the relative values of R1 and R2 to make
    forward and reverse times different. Changing the timing capacitor
    will also change the cycle time proportionally.

    Also posted in alt.binaries.schematics.electronic with image attached,
    under 'Motor control' (not comtrol >g>).
  5. Another idea, to further simplify the sound section. Assuming you
    *can* find a suitable 'sound effects greeting card', you could use
    *that* as the contact separator. Paint it matt black inside and out.
    Protect from rain appropriately. Insert a small piece of sponge or
    other springy stuff inside. Carefully attach the card to the nylon
    line. So when the snagged line pulls the card from between the metal
    contacts, that now not only closes the NO switch in the circuit but
    also opens the card sufficiently for its built-in sound to be
    triggered. Its resting position would ideally be close to the 'eyes'
    and whatever mask you build around them, but if it's dark and spooky
    enough I doubt if anyone will notice a small discrepancy <g>.

    Note that the 555 can source (or sink) 200 mA, so no transistor driver
    should be necessary for typical relays.

    Have fun!
  6. steamer

    steamer Guest

    --Get on over to; i.e. Basic Stamp 101. Some may
    say it's primitive, but it's the best documented intro to control of
    electronics I've found..
  7. Jamie

    Jamie Guest

    find your self a LM555 timer pulse generator print.
    use this timer output to drive a transistor like a
    2n2222 type that will drive a simple double pole ,
    double throw relay..
    simply have the relay reverse the polarity when
    it gets switched on from the 555 pulser..
    look there for the specs' and a simple Astable
    operation or the 50% duty cycle osc..
    just use the math below in the spec's to give the
    time window needed.
    remember to use a transistor on Pin 5 from the timer
    to drive the relay, most relays
  8. Thanks for the circuit Terry. I tried to recreate it in Crocodile Clips and
    the 555 keeps blowing. I am sure the circuit is put together correctly and
    I cannot see what I am doing wrong. Has this circuit been proven to work?


  9. Bob Monsen

    Bob Monsen Guest

    The circuit should work, but there are a couple of issues.

    First, make sure the diode points UP, from ground to the node joining the
    555 output pin and the relay. It is drawn properly in the current
    schematic, but it is an easy thing to screw up. The line should be on the
    side of the GND. Also, make sure the power isn't reversed or something
    like that. Also very easy to do, and it'll kill a 555 fairly quickly.
    Ensure the connections are just like what are drawn.

    Secondly, if your relay has low coil resistance, the 555 may hurt itself
    trying to power it. You can help it out by using an NPN or N-MOSFET
    transistor... Just use a 1k resistor from 555 output to the base of an NPN
    like a 2N2222, attach its emitter to ground, and its collector to one
    terminal of the coil. Attach the other terminal of the coil to 9V. Also,
    keep that diode across the coil. Now, when the 555 pulls up the output,
    it'll turn on the NPN, which will power the coil. When it drops output,
    it'll turn off the NPN, turning off the coil.

    Now, one odd thing about this circuit is that if the motor is running one
    way at full steam, and you suddenly reverse the terminals, the inductance
    and possibly momentum of the motor will try to force current from ground
    to the 9V terminal. If the voltage source is fairly high impedance, what
    will happen is that a bit of current may flow backwards through whatever
    else is attached between the 9V and gnd; in this case, probably the 555.
    However, if you put yet another diode pointing from ground to 9V across
    the supply on the contact side (basically across the poles of the relay),
    then that diode may now be the preferable path if this happens, thus
    saving the 555. Another way to avoid this is to simply use a separate
    battery for the motor, as Terry suggests in his schematic.

    Bob Monsen

    Mathematics takes us still further from what is human, into the region of
    absolute necessity, to which not only the actual world, but every possible
    world, must conform.
    - Bertrand Russell
  10. Bob Monsen

    Bob Monsen Guest

    On Wed, 12 Oct 2005 12:33:36 -0700, Bob Monsen wrote:

    No, No, No. The line is on the cathode, just like the schematic diagram.
    Current flows towards the lead nearest the line.

    The line lead should be connected to the 'output' side, pointing UP in
    terry's schematic, the anode (without the line) should be connected to

    (painfully pulls head out posterior with a minor 'popping' sound)

    Bob Monsen

    Even the greatest of creations start from small seeds.
    - Unknown
  11. Yep. Most likely explanation is as Bob suggested: diode the wrong way
    round. And/or wrong relay connections used (maybe a NC pair!). Try
    with no diode and no relay, just a DMM on the 555 output, or an LED
    with 1k resistor in series, to see that astable is OK.

    If the astable itself (a bog standard circuit) is not working, post a
    picture as actually built - if possible a clear photo.

    For the motor circuit, I'd definitely use a separate supply until it's
    working OK. Then see what the effect is of sharing a single supply.
  12. ehsjr

    ehsjr Guest

    The diagram below shows the wiring to a DPDT switch.

    +-------> To
    | +-----> Motor
    | |
    +12 -------+ | | +-----+
    | | | | |
    --|---|-|--|-- |
    | O O | O | |
    | | | | |
    | +--------+ | |
    | | | | |
    | O O-+ O | |
    --|----------- |
    | |

    To "automate" the forward/reverse of the motor
    (I'm assuming the student just wants to demonstrate
    the operation), he can drive a relatively large
    circle - say 10 inch diameter. A bolt ( B ) screwed into
    the circle near the edge can operate the toggle
    switch, which will reverese the direction each time
    it is operated:

    * * ||
    * * < Circle > || -------
    * * ||- Motor |
    * o * || -------
    * * ||
    * /B Nut>[]==[]==|=[]<Bolt
    * */ || |
    -------- ---
    | Switch | | | < switch
    -------- ---
    Front View Edge View

    The challenge for the student with this method is more
    mechanical than electrical. As shown, the circle is
    directly driven by the motor, but he may want to use
    belt or worm drive - I dunno what his goals are.
    The "certain time" mentioned in your post would require
    converting rpm to time and would depend on dimensions
    and ratios if speed conversion via gears or drums
    is used.

    With Terry's (very nice) circuit, the learning is more
    electrical than mechanical.
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