Looking for Spookin' Light circuit
January 26, 2010 9:38 PM   Subscribe

I'm looking for an old (ca. 1965-1969) article for an electronics project. It was published in an electronics magazine like Radio & Electronics, Popular Electronics, etc.

The article was titled something like "Build a Spookin' Light." I've tried the reference section of the library and Readers Guide, but have drawn a blank (The entire period was not covered and the library has since withdrawn the Readers Guide in favor of on-line indices that don't cover the period or all the publications in question. The author may have been Don Lancaster (but I've drawn a blank there, too.)

If I had access to a better research library, I'd probably have it by now.

What it is: a unijunction transistor oscillator triggers an SCR. The beat frequency of the oscillator against the 60Hz line frequency produced interesting and complex strobing of a light controlled by the SCR. Having failed to find the original article, I've been experimenting with various circuits to try and reproduce the effects, so far without much success. I can reproduce a simple beat frequency effect, but the circuit I'm trying to reproduce had a much more complex result.

I had built one of these many years ago and would like do it again.
posted by warbaby to Technology (6 answers total)
Did it possibly use 3 UJTs?

I built one years ago - the same circuit, from an old 1970's '101 Circuit'-style magazine, modified for 240v. As an alleged flame effect it's pretty crap, but it's kinda interesting.

(BTW, I was amazed to learn that Don Lancaster is still alive & kicking!)
posted by Pinback at 12:07 AM on January 27, 2010

The three UJT design is similar, so that's a good starting point. I'm beginning to suspect that my attempts are not getting the right effect because the pulse width from the trigger circuit is too wide.

I found Don Landcaster's site when I started searching for this circuit. I probably misremember it as one of his. I spent a lot of time on his designs for color organs and active filters. the publication date is probably earlier than I remember it because it was very difficult to obtain the SCR and it was very expensive.

I'm going to try a monostable 555 oscillator as a pulse shaper and see if that cleans things up.
posted by warbaby at 7:18 AM on January 27, 2010

Well, the SCR is going to latch up & conduct until the next half-cycle anyway (unless it's a GTO type with pulldown?) - you could probably just stick a small cap in series with the gate to limit the pulse length, maybe with a pull-down resistor / clamping diode.

Depending on the exact effect you're after, the thermal mass of the filament will come into play. Sounds like you know what you're doing, so I'm just throwing this out there : you probably don't want the SCR conducting every other 1/2 cycle. I haven't bothered working it out, but I'd guess the freq of the oscillators in that circuit are at least down in the single digits; maybe even lower?

(Just did the math - I make the freq of the middle oscillator in that cct as ~0.05Hz. Hmmm, lower than I thought…)
posted by Pinback at 10:15 PM on January 27, 2010

Further thought: if you make the osc freq very close to, but not quite, mains freq, you'll get a slow semi-random-ish dimming & brightening of the lamp. In practice, if you calculate R & C for 60Hz, component tolerances & mains variations will do the rest. Is that the effect you're after?

(I used to design & look after thyristor-controlled power supplies in a former life. There, you have to use feedback to fire them at the right part of the cycle to maintain a stable output voltage.

The tricks of how to maintain an adequate deadband, and eliminate explosive destruction of the output stage, will be left for a later question ;-)
posted by Pinback at 10:34 PM on January 27, 2010

The oscillator has a total range of about 18 - 300 Hz. The interesting effects are when the frequency is very close to 60 Hz (or one or two octaves above or below) and the beat frequency is very slow. When tuned correctly, you can get a pulsing light that slowly fades in and out.

It works OK at the higher end, but the 555 is nearly a square wave, so the pulse width is too long at the low end. It shuts down when the line voltage hits zero unless the pulse is still high (which is the problem).

So I've got it working tolerably, but don't have as wide a range on the oscillator as I'd like. I tried using a small cap to AC couple it, but it didn't pass enough current to trigger the opto-isolator in the SSR.

I think the UJT circuit was a relaxation oscillator with a very short decaying pulse.

For the time being, it's close enough. Thanks for your suggestions.
posted by warbaby at 10:58 PM on February 7, 2010

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