Optimizing astable multivibrator for minimum power consumption

Discussion in 'The Projects Forum' started by aeftimia, Sep 16, 2011.

  1. aeftimia

    Thread Starter New Member

    Sep 16, 2011
  2. Wendy


    Mar 24, 2008
    First of all, I would loose the transistors. If you must have them raise the resistors as high as possible, with the collector resistors being 1/10 the value of the base resistors. The base formula's for frequency still apply, you adjust the values of the capacitors to match.

    Having said that, have you considered CMOS gates? A CMOS inverting Schmitt Trigger would be very low current, and have good drive characteristics.

    If you haven't studies Hysteric Oscillators before here is a good example.

    555 Hysteretic Oscillator

    You could even use a CMOS 555 oscillator for good results.
  3. aeftimia

    Thread Starter New Member

    Sep 16, 2011
    Thank you for your reply. I suppose getting rid of the transistors would mean using an integrated circuit--which is perfectly logical (pun!)--however I am trying to make due with just transistors, capacitors and resistors right now. I am trying to get a feel for how these things work without buying a black box with 6 pins--at least until I understand the circuitry well enough that these ICs are no longer black boxes to me.

    PS: Why should the ratio between the collector resistors and the base resistors be 1/10? That seems rather arbitrary. Could that be optimized?
  4. SgtWookie


    Jul 17, 2007
    The 1/10 rule involves transistor saturation characteristics. In this circuit, the transistors are being used as saturated switches. For most transistors, the base current necessary to saturate the transistor is 1/10 the desired collector current.

    By the way, "saturated" means that a further increase of the base current will not result in a corresponding further reduction in Vce (voltage on the collector with respect to the emitter).

    There are some high-gain transistors that will saturate pretty well with less than the 1/10 rule. However, they are the exception and not the rule. Those transistors are also usually quite a bit more expensive than standard versions.
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    Bear in mind that if your supply voltage is higher than the transistor base-emitter breakdown voltage, the oscillation frequency will be higher than the calculated frequency, and the transistors may eventually be damaaged.