pulse motor circuit help

Discussion in 'General Electronics Chat' started by jasonspinmaster, Mar 2, 2008.

  1. jasonspinmaster

    Thread Starter Active Member

    Mar 1, 2008
    57
    0
    i am making a pulse motor with 6 magnets.
    i am using an optic sensor (OPB 876) as the trigger, and not a reed switch. i can fire where i need on the magnet all of the time.
    my circuit is a ne 555 timer as a schmitt switch.
    i am getting a very small pulse but the faster it goes the smaller the pulse gets.
    power supply is a 12 volt motor bike battery
    col resistance is 10 ohms
    i am really new at this, which i hope you under stand
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Jason,

    If you want a constant-width output pulse, regardless of the input pulse frequency, you need to change the NE555 circuit to a "monostable".

    http://home.cogeco.ca/~rpaisley4/LM555.html#2

    This contains a calculator, so you can select the component values for the timing you need. :)

    Ken
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    I would help if you could explain what you perceive is the problem, and how you would like it to be resolved.
     
  4. jasonspinmaster

    Thread Starter Active Member

    Mar 1, 2008
    57
    0
    i see the problem as the transistor does not saturate fully. to resolve it i think placing a 1k resistor from base to postive, to place more current in the base. or place the resistor to ground to source the base. ??????????
    not to sure but i am really new at this
     
  5. jasonspinmaster

    Thread Starter Active Member

    Mar 1, 2008
    57
    0
    hi ken ,
    i have tryed a monostable circuit, but i need to fire the coil when the optic sensor is bloked, not with a resistor and cap.
    i need 12 volts to pass through the coil for the time the sensor is bloked

    jason
     
  6. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    With your detector schematic, the length of time the sensor is blocked gets shorter with the increase in speed/pulse-rate. Is the output of your 555 not the same width as the input pulse width?

    ken
     
  7. jasonspinmaster

    Thread Starter Active Member

    Mar 1, 2008
    57
    0
    hi ken
    i am really new at this.
    i understand that the fast it gets the shorter the pulse, but i am not putting the full 12 volts in to the coils from the start.
    i am using a scmitt switch so the pulse width, in, should be the same, as going out !!!!!!!!!!!!!:confused:
    i have placed a LED in a test circuit and it lights up ONLY when it is BLOKED.
    pin 2 on the 555 timer is 1/3 the supply 4.5 volts (round about).
    jason
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    The optocoupler circuit you posted should fire the monostable when the light is blocked. If it will light an LED (with a resistor in series) when the light is blocked, it should be able to trigger the 555 in monostable mode.

    I think you are saying that the motor is not getting full voltage. How are you measuring the voltage?
     
  9. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Do you have an o-scope to see what's happening? What's the resistance of the coil? What's the voltage at pin 2 when the detector is lighted and when blocked?

    Ken
     
  10. jasonspinmaster

    Thread Starter Active Member

    Mar 1, 2008
    57
    0
    i have just recieved my new o-scope. i am getting out off pin 3 (0.7volts). i need to increase this to about 2 volts to trigger the TIP 120 transistor
     
  11. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    The output at pin 3 should be switching between about zero to near the supply voltage. The TIP120 only requires about 12mA for a 3A collector current (datasheet) so a 1KΩ base resistor should be enough for a 10Ω coil. The 555 is capable of supplying 200mA to drive the transistor. Have you tried testing the transistor output by not using the 555, and just connecting the 1KΩ base resistor to the +12v? Does it work? Remember that Vsat on that transistor is ~2v, so you should see the collector switch between +2v and +12v.

    Ken
     
  12. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    jasonspinmaster,

    The TIP120 has a spec'd gain (Hfe) of 1000. So to drive a 12v/10Ω relay, a 3.3KΩ resistor between pin 3 of the 555 and the base should provide adequate drive to guaranty saturation of the transistor, and allow cutoff too.

    A 2N3055 has a gain of only 20 to 70, so would require a lot more drive current to saturate it...about 50mA. Maybe 270Ω to the base.

    Ken
     
  13. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Hi Ken,
    hFE, current gain and beta are used for a linear transistor that has lots of volts between its collector and emitter. The saturation voltage loss is spec'd with a base current of 1/20th, 1/10th and 1/3rd the collector current.

    The TIP120 darlington is spec'd with a saturation voltage loss when its base current is 1/250th its collector current. Not 1/1000th. Its current gain is a minimum of 1000 when it has at least 3V between its collector and emitter so it is not saturated.
     
  14. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Audioguru,

    You're right about the Hfe and switching. I suggested a base resistor for the TIP120 that would probably be adequate for a real gain of 300. For the 2N3055 I was thinking maybe a gain of 20. Not bench tested so just a guess as a starting point for them.

    Ken
     
    Last edited by a moderator: Jul 12, 2008
  15. guruji

    Member

    Nov 11, 2005
    29
    0
Loading...