The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

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  • Astable Multivibrator: 2N7000 MOSFET First Light

    Taping a cardboard support under the soldering fixture helped hold all the parts in place:

    Astable - 2N7000 soldering
    Astable – 2N7000 soldering

    The struts fit neatly into an NP-BX1 battery holder and the circuit began blinking merrily:

    Astable - 2N7000 assembled
    Astable – 2N7000 assembled

    My photography hand is weak …

    The circuit schematic / layout resembles this:

    LED Schematic - MOSFET transistors
    LED Schematic – MOSFET transistors

    The missing 1 MΩ resistor at the LED would serve as a physical support to tether the loose end of the 100 (-ish) Ω resistor, which desperately needed some stabilization under the LED spider.

    The simulation says it should blink about every 4s:

    Astable - 2N7000 buffered - true model
    Astable – 2N7000 buffered – true model

    The 2N7000 MOSFETs use a SPICE model from the Motorola ON Semi downloads, although they behaved about the same way using the LTSpice 2N7002 model.

    It really does blink every 4s:

    Astable - 2N7000 overview
    Astable – 2N7000 overview

    The LED pulse width should be about 50 ms:

    Astable - 2N7000 buffered - LED current - true model
    Astable – 2N7000 buffered – LED current – true model

    The voltage at the bottom of the ballast resistor is directly proportional to the LED current:

    Astable - 2N7000 pulse detail
    Astable – 2N7000 pulse detail

    So the pulse is actually 80-ish ms, which is Close Enough™ for my purposes.

    The key advantage here is making both the astable’s period and the blink’s duration (roughly) proportional to the component values, so I can tweak them with some confidence the results will come out more-or-less right.

    I love it when a plan comes together!