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Low Frequency Oscillator

Discussion in 'Electronic Design' started by Ban, Sep 6, 2003.

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  1. Ban

    Ban Guest

    Hi Ban,
    I find them in my mind, they just pop up there. Seems I have the older type
    of mind then. Well boy, we talk opamps not tubes here!

    I got this by private mail, but since it might be of interest, pasted it
    here into the group.

    Hi Peter,
    1. Win Hill recommends in a similar situation a CA3060 opamp, generally most
    CMOS opamps can be taken as they swing rail to rail at the output. In this
    case a dual opamp in a 8-pin casing can be chosen.
    The transistors can be BC548C and BC558C, jelly bean parts.
    I modified the circuit a bit to better meet your needs like low power
    consumption and operability from a 9V battery.

    +-----------------+-------+----+-----------------o
    | .-. .-. .-. 9V VCC
    | 10k| | 10k| | 100k 100k
    | | | | | | | +-|___|-+
    | '-' '-' '-' | |
    | | | | | |\ | VCC
    | >| |< +---+--|+\ |CA3060 +
    | |-+-| | | >-+ op1 |
    | /| | |\ +----)--|-/ | |
    | +---+ | | | |/ | +---+
    | | | | | | | _|_
    | | +---)---)----)-------+ - |_/_|
    | frequency | | | | | ___ ^ |
    .-. | | +-+ | +|___|-GND | |
    100k| |<-------+ | | V V | 100k +---+
    | | .-. | +-+ | | |\ |
    '-' | | | | +---+----+ +---|+\ CA3060 |
    | 100k| | | V V |.47u| | | >--+ |
    +---+ '-' | +-+ --- +--)- -|-/ .-. |
    | | | | 1N4148| --- VCC | |/ | | |
    | |/ +---+ | | | | | | | |
    +-| | | | | === .-. | 10k'-' |
    |> \| | |/ |/ GND | | | | |/
    | |-+---------| 100k | +--|2N3904
    GND <| |> |> '-' | |>
    | | | | | |
    .-. .-. .-. .-. | ===
    10k| | 10k| | 10k| | | |<-+ GND
    | | | | | | 100k duty cycle
    '-' '-' '-' '-'
    | | | | ___
    GND GND GND +-|___|-GND
    100k
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


    2. Let me explain the circuit:
    The frequency pot adjusts the current flowing into the bottom current mirror
    from 0.09mA max to nearly zero. This mirror has to equal outputs, one goes
    to another mirror on the positive rail, the other to the diode switch. This
    way the diode switch is fed with 2 equal currents of adjustable polarity.
    Lets say op1 output be high(+9V), so is then the left side of the diode
    bridge.
    The current from the positive current source flows now into the capacitor
    and charges it
    delta_Vc = I*t/C
    with I=0.09mA; C=0.47uF; delta_Vc=3V
    we get t=15.67ms
    The current from the negative voltage source flows through the lower left
    diode into the output of op1.
    The voltage across the cap is sensed by op1 with the -in, whereas the +in
    sits on a voltage of 2/3*Vcc= 6V. Op1 works as a comparator and when the
    capacitor voltage has reached 6V the opamp toggles and its output swings to
    gnd.
    Now the current from the upper source flows through the upper left diode
    into the op1 output, whereas the negative current source discharges the
    capacitor until it reaches the voltage on the +in, which is now +3V.
    This way we have made a triangle generator with a swing from 3 to 6V.

    The second opamp compares again this triangle wave with an adjustable
    potential in the same range as the triangle wave (3-6V). When we adjust it
    to the lower side, the cap voltage is almost always higher, and so the
    solenoid gets activated only a fraction of the period, in the middle the
    duty cycle is 50% and rises to the right. This way the period stays the same
    and both adjustments are completely independent.

    It is better to drive the solenoid with another transistor, best would be a
    small NMOSFET here, as it doesn't consume any power at these switching
    rates.

    ciao Ban
     
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