Need some advice on how to use 4066 CMOS bilateral switch

Discussion in 'The Projects Forum' started by innominata, Jul 26, 2009.

  1. innominata

    Thread Starter Member

    Jul 25, 2009
    12
    0
    Hi everyone,
    I'm building a (rather complex) digital switching system for my 3 humbucker electric guitar using 4066 chips to control the signal.
    As the input to the guitar is grounded (and thus the signal from the pickups) I need to use a positive and negative rail for Vdd and Vss in order to keep the signal from the pickup within the range of the switch right?
    Assuming this, I have built a +/- 5v with reference to ground power supply to use on them (I have 10 4066 chips). The signal from the pickups goes +- 1.5v max.

    Next (and most important) question: On the 4066, to switch (the switches) on, I raise the control pin to +5v, to turn them off do I need to take the control pin to -5v or will ground work. I say this because the control signal is coming from a cmos 4017 decade counter, which I was hoping to run without using the -5v rail, as the available current on -5v is limited.
    OR do I use a pulldown resistor to -5v on the control pins on the 4066 board?

    I'm using transistors switched by the decade counter outputs to drive up to 4 switch controls + 1 led at a time.

    Any advice would be most appreciated, I can elaborate further if need be :)
    Regards,
    Tony
     
  2. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    Not necessarily. Generally, the old FET analog switches work fine for lower voltages. So, the negative voltages are not necessarily a problem. However, using Vdd-Vss as large as possible helps reduce the ON-resistance.

    This is fine and should work. However, 10V for Vdd and ground for Vss should work with a signal going +/- 1.5 V relative to ground.

    The control voltage should go to the negative rail Vss to turn them off. There is actually a spec that tells you just how much higher will still work, but driving in the center of the range is ambiguous. When using CMOS logic, you should keep all logic chips/signals at the same Vdd and Vss potentials.

    However, given your low current rating on the -5VDC supply, you could use 0-5V power for logic, but then your CD4066 chip will not operate with the best possible specifications.

    As a side note, remember that the switch has considerable resistance in the on-state (~100 ohms) with maximum voltage supply, and this gets worse as the voltage supply is reduced. This needs to be considered in the analog portion of the circuit, as it could affect volume and tone.

    There are more modern (pin compatible) versions of this device, with lower on-resistance. For example, Maxim has them. You can also get variants with inverted logic to simplify your design in some cases.
     
  3. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The CD4051, CD4052 and CD4053 are analog switches that can select signals or select where signals go to. They have the same low on-resistance as a CD4066.
    Their control voltages can be chosen for a single polarity supply or for a dual polarity supply.
     
  4. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    Here is an app-note that may be useful.

    This starts off by saying that these types off switches were available 25 years ago. Even noting the publication date (in 2000), this seems a little off. I thought they were available a few years before this - maybe even late 60's.

    There is discussion on make-before-break switching, to prevent click-noise in audio applications. Maybe Audioguru and some other audio people can comment on this further.
     
  5. innominata

    Thread Starter Member

    Jul 25, 2009
    12
    0
    Thanks for answering everyone :)
    I have a CD4051 and a CD4053 lying around, but I don't think they will work in this instance...allow me to elaborate with a crudely sketched drawing I took a photo of. :)

    [​IMG]

    This is a (hopefully recognisable) diagram of the two coils and 9 switches
    Each labeled box is an on/off switch, two lines on the right are gnd and signal. J is not being used due to lack of IO avaliability

    Will this even work due to the resistance of the switches...
     
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