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.

The New Hotness

  • Astable Multivibrator: SMT LED Ballast Resistor

    The original astable multivibrator ran from a dead CR123 primary lithium cell:

    CR123A Astable - front
    CR123A Astable – front

    With a terminal voltage falling from barely 3 V, the LED drew about 3 mA (1 mA/div), tops, without a ballast resistor:

    Astable - CR123A 2.8 V - 1 mA -green
    Astable – CR123A 2.8 V – 1 mA -green

    Hacking in a charged NP-BX1 secondary lithium cell boosted the supply to 4 V:

    NP-BX1 Holder - SMT pogo pins
    NP-BX1 Holder – SMT pogo pins

    Which, diodes being the way they are, raised the LED current to nearly 400 mA (100 mA/div):

    Astable - NP-BX1 4V - base V - 100mA-div
    Astable – NP-BX1 4V – base V – 100mA-div

    Somewhat to my surprise, a few weeks of abuse didn’t do any obvious damage to the LED, but I added a resistor while I was soldering up another holder:

    Astable - 51 ohm SMD ballast
    Astable – 51 ohm SMD ballast

    There’s not quite enough room for a 1/8 W axial resistor, so why not blob in a surface-mount resistor?

    Which cuts the current down to a mere 15 mA (10 mA/div) from a lithium battery at 4 V:

    Astable - NP-BX1 - 51 ohm ballast - 10ma-div
    Astable – NP-BX1 – 51 ohm ballast – 10ma-div

    It’s still blindingly bright, but now I don’t feel bad about it.