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Weekend one-shot timer design

Discussion in 'Electronic Design' started by Tim Williams, Apr 19, 2006.

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  1. Tim Williams

    Tim Williams Guest

    While idly contemplating oscillators, I thought I might want to try a
    controlled delay oscillator. That is to say, one thing runs this much time,
    then another, then another, then another, then back to the first. My
    thought was to have two long "ON" delays spaced with short delays for MOSFET
    switching/"OFF" time. One way to do this would be a chain of one-shot
    timers.

    I like discrete, and I like short circuits. I figured, hmm, this is going
    to need a flip-flop, because it is a state machine after all, and I'll need
    a capacitor charger, discharger, and a comparator of some sort. The charge
    can come from a resistor or CCS, simple enough. The discharge could be
    turning off the CCS, and, uh, well hell let's just clamp CCS and cap
    together, that'll save a pair of resistors.

    For comparator I could wire up a seven transistor op-amp, and more than
    double my parts count, but hell, there's gotta be an easier way. Ya know,
    isn't it nice that a transistor only conducts when the base is forward
    biased? Fair enough! Now I just need some way to make that work, hmm a
    zener on the emitter will set trigger voltage solidly, I could use a
    collector resistor to drive the next transistor but saturation is slow,
    especially for low drive currents, ah let's throw in two base diodes and an
    emitter resistor. Now it switches over 0.8V or so, much slower, but output
    current is far more consistent. Now I can drive a base directly.

    Since this NPN is already a few volts above ground, and it doesn't work
    terribly fast in absolute terms, I'll toss on a PNP to drive the flip-flop.
    Ah, that crisps things up nicely.

    http://webpages.charter.net/dawill/Images/One-shot Timer.gif

    Overall, seven transistors. I could probably make it work with five, but I
    tried and the flip-flop was being very finicky about reverse bias from the
    trigger input (yeah, DTL would probably fix that). So I went with the pair
    of RTL NOR gates. And, I like RTL. :p

    Specs:
    Ct = 0, Rt = 1mA CCS: pulse width 0.8µs, maximum repettition rate 700kHz.
    Pulse width is maintained down to individual cycles, as near as I can tell,
    at least as long as the rising edge is fast enough to pass the 220pF (my sig
    gen has no shortage of speed for that).
    Ct = "1000µF", Rt = 60µA CCS: pulse width approx. 80 seconds (suggesting
    capacitor is actually 1240µF ;-). Decision made in roughly 0.1s, and fully
    discharged in 0.1-0.2s. (It's fun to watch the voltage cross 4.20V, pause
    for a moment, then quickly scan down to zero. -If that voltage is odd, it's
    because I'm using a pair of red LEDs, since I don't have a 3.3V zener on
    hand.)

    Because the circuit is edge-triggered, it ignores cycles during charging.
    Thus, the circuit is useless as a missing pulse detector. However, this has
    an interesting consequence, as the dead time inbetween output pulses
    directly corresponds to the input frequency, since for rates higher than the
    timing rate, one or more cycles are skipped. I'll have to watch this some
    more to figure out what the implications are, but I would suppose harmonic
    frequency division or something would be the idea. In this manner, with Ct
    = 0 and Rt = 1mA, the maximum input frequency is around 2MHz, above which
    the output rolls off because the drunken input 2N4401 slurs the pulses
    together.)

    Haven't yet built another (geez, I'm out of 1k resistors!), but two
    together, linked by the Q' output, should be a straightforward multivibrator
    (though with the cap and diode input network, it will need a starting
    pulse), and three or more should make a polyphasic squarewave output.
    Multiple modes may even be possible, as varying numbers of pulses cascade
    through what is effectively a looping shift register. Oooh, a megabyte of
    these would make a skookum digital delay line...

    Tim
     
  2. Deefoo

    Deefoo Guest

    I like discrete too, but I would have used a 4538 or something anyway. Two
    one-shots with only 5 components instead of 30 for one. But it is a nice
    exercise.

    --DF
     
  3. Tim Williams

    Tim Williams Guest

    Or a 74LS123, given a 5V regulated supply. But what fun is playing with
    something that's already been designed? :)

    Tim
     
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