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Voltage controlled oscillator

Discussion in 'The Projects Forum' started by CraziestOzzy, Jul 4, 2012.

  1. CraziestOzzy

    Thread Starter New Member

    Jul 4, 2012
    Firstly a big g'day from Down Under.
    I require some help if you guys and gals don't mind assisting?
    I have done the hard yards this past week, searching the WWW and fiddling with circuit simulators to solve a problem.
    I am at the end of my tether and am now here as a newbie asking questions to get me back on my horse.
    Here are the nuts and bolts:

    I am wanting to control a motor through PWM.
    I wish to control a DC motor speed for a 6 volt brush motor through a linear pot and also have that speed oscillate (throb) with the oscillation frequency controlled by a linear pot to have a complete cycle peak to peak of around 1 to 5 seconds.
    I can create a PWM using either an OpAmp or a 555 timer.
    I can also create a crude sine wave with either IC and output a positive voltage output and manipulate that wave to the low frequency of around 1 to 30Hz.

    The challenge for me is combining the two together...I just don't know how to describe what I want in a Google search.
    I am at the moment using the 555 timer to create a sine wave in a brutal manner and using an OpAmp as a voltage controlled pulse oscillator.

    So here is the simulation results to roughly what I am after:


    you can see the sine wave controlling the pulse frequency. The pulse frequency can be controlled seperate to the sine wave bya pot and also be influenced by the sine wave ... hope that ain't confusing :p
    That's good so far, but what is this process called?
    I have an issue...the circuit I created to produce this frankenstein thingy, roughly works but shifts the width of the each pulse MARK relative to the volatge in the sine wave.
    I do need to know how I can keep the MARK of the pulse the same width. Any idea?

    ..and here is the completed circuit and do excuse my layout...I did my best to make it easy to read. I think there is some stuff that is not needed and also think there is stuff I need :p


    Basically, the 555 produces a sine wave as a result of cap C1 charging and discharging and the R3 pot controls the width of the sine wave.
    (I butchered that idea from a "police siren" with a 555 timer...saw a nice sine wave :p
    I could not for the life of me use another 555 timer to get the effect I want, that of a shifting pulse frequency relative to a sinewave.
    I have referenced U2C OpAmp to the output of the 555 timer (as opposed to using a typical voltage divider to the 6volt supply volatge).
    The U2B has a pot (R8) that controls the pulse output frequency.
    Ignore the discreets attached to the final output from U2A (2n3904). The final output here will be attached to a TIP31c that will drive the Dc motor.

    I hope that was as clear as mud.
    If you have an alternative super simple method to have a pulse frequency shift relative to a sine wave without having the mark of each pulse peak shift it's width relative to the voltage, I would love some thoughts.
    Any add-ons or exclusions to help improve my circuit would be greatly appreciated.

    To help you guys, if you think up another circuit...I have included my list of at-hand IC's:

    555 TIMER
    74HC14D HEX Inverting Schmitt Trigger
    MC34025P Double Ended PWM
    TL1431CD SMD Precision Programmable Voltage Reference
    UCC27322 Gate Driver for MOSFET/IGBT 9A
    MC33039 Closed-Loop Brushless Motor Adapter
    MC33035 Brushless DC Motor Controller
    LM324N Quad OP Amp
    CA3140E 4.5MHz BiMOS Op-Amp
    BQ2004/2002 NiCd charger
    UCC27322P Mosfet Driver
  2. Bernard


    Aug 7, 2008
    Possibility: Use two 555 triangle wave generators, one slow to modulate speed, one for PWM clock of about 2 k HZ. Q2, emitter follower or OP Amp buffer to present high Z for SLOW output. Seems we have two R 19's, pot R 19 is part of modulation control. R 15 & R 17 may need to be adjusted to keep PWN to 0-100%.
  3. wayneh


    Sep 9, 2010
    I don't quite follow all this, so sorry if this is not relevant, but I posted a similar concept here in a recent thread regarding a bicycle light flasher.