No LEDs Required For This Servo-Controlled Larson Scanner

Servo Larson scanner

All things considered, it’s pretty easy to get one LED is a strip to light up sequentially, and have it bounce back and forth. Turning that simple animation into a real Larson scanner, with smooth transitions and controlled fade-out, is another thing entirely. And forgetting the LEDs altogether and making a servo-operated Larson scanner is — well, let’s just call it an interesting lesson in hardware abstraction.

The Larson scanner, named after famed TV producer Glen A. Larson for his penchant for incorporating it into shows like Battlestar Galactica and Knight Rider, is actually hard to execute in hardware thanks to the fading tail that follows the lead pixel as it dances back and forth across the display. [Eric Gunnerson] decided to make this and other animation effects easier to achieve with Fade, a custom framework for LED animations that runs on an ESP32.

LED animations are fine, but what about servos? Could Fade be modified to support them? This turned out to be a fairly easy mod thanks to Fade’s architecture and [Eric]’s existing support for non-addressable LEDs via PWM signals. And it was even possible to support more than the 16 PWM channels on an ESP32by adding a UDP connection that puts multiple ESP32s under the control of a central microcontroller.

The video below shows [Eric]’s demo of servo support, with an eight-channel electromechanical Larson scanner. Each “pixel” is a painted ping pong ball swinging back and forth on a hobby servo, and the whole thing sounds just about as awful as you’d expect it to. If you squint just right, the effect looks pretty convincing, but that’s hardly the point. The real story here is [Eric]’s thoughtful architecture, which made the mods easier than starting from scratch.

31 thoughts on “No LEDs Required For This Servo-Controlled Larson Scanner

      1. For arbitrary rotations from either side, it would need continuous rotation, but for this purpose, you could get away with just gearing the servo output to provide 360 degree rotation, since the motion reverses with each pass.

        If you go with continuous rotation, steppers motors (with a zero sensor) would be better and quieter than common servos.

  1. Servos were not a good choice for the simple fact that they are noisy and complicated. Using electromagnetism directly would ensure it’s perfectly silent though it could still be quiet (but not silent) as a purely mechanical device.

    1. Probably harder to get a smooth fade then, which was the main goal of the project.

      If we’re putting forward completely different ways to build a mechanical Larson scanner, those linear PCB motors would do it. Or a stepper motor with a red card on a timing belt :)

    1. I don’t think that actually works. You may not be able to tell in the video, but in a larson scanner you get a lot of red at the leading part with it fading out in the trailing part. That means the pattern looks different going left than it does going right.

  2. 1 DC gear motor, 2 limit switches, and a current reversing relay. Each ball offset by 45 degrees to its neighbor, and all balls geared to turn together.
    Won’t be any quieter, but the noise might be more pleasant, and the effect will be much better.
    Doesn’t really serve his purpose, but what the heck.

    1. Don’t think that works, as the pattern is not symmetrical. The leading spot is always the reddest with those trailing becoming less red. When you switch directions the relationship flips around.

      And you might have cases where the redness is 25% 100% 75%

      1. It doesn’t work perfectly but it’s close enough – I’m sure with some clever linkages you could make the whole thing work perfectly from a single DC motor in a continuous rotation.

  3. “actually hard to execute in hardware”?

    Over 30 years ago I made one out of hardware (not servos though) and it was super simple.

    A 4011 quad 2-input NAND as a low frequency square wave oscillator with a potentiometer to adjust the speed.
    This fed the clock of a 4017 decade counter where the outputs each fed a transistor to turn a LED on. A simple capacitor made the LED fade when the transistor turned off.
    There were 6 LED’s and output 7 fed back into LED 5, 8 into 4 etc. to make the trail snake from left to right to left…..

  4. The Data General Eclipse computers had old-school front-panel LEDs for address and data as well as switches for front panel hacking. There was a ‘Cylon patch’ available for the OS that would show this. It was very cool! IIIRC, it also indicated system load by the scanning speed.

    1. The scanning panel lights in the address register display was a thing on Digital PDPs running the RSX operating system. I don’t think the RT-11 operating system ever inherently supported the feature. I believe the older PDPs had cartridge type 5 Volt lamps.

      1. The PDP 11/34 I used at work had boot ROMs, hardware initial jump, and a seven segment LED display. There was no toggle switch gymnastics to enter the jump address and data, or nifty lights to watch while pondering FORTRAN statements at the ASR-33 TTY.

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