Featured Product Archive
The inventions and products featured on these pages were chosen either for their
uniqueness in the RF engineering realm, or are simply awesome (or ridiculous) enough
to warrant an appearance.
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A couple days ago I posted an update on
the Watkins-Johnson databook page that had an unauthorized gag graph titled, "WJ-G1/SMG1 Phase vs. VCTL vs. Frequency
vs. Phase of the Moon." When RF Cafe visitor and sometimes contributor Dr. Marek
Klemes* read that, he sent me a note about
remembering this "Delayed Light Turn-Off" circuit from the
Timer Databook. It took a bit of creative Googling, but he managed to find the
datasheet (to the right). The text was a bit washed out from the original low resolution
scan, so I reproduced the labels (green).
Signetics NE555 timer schematic for a "Delayed Light Turn-Off"
circuit from their 1973 databook.
This Rube Goldberg-ish
contraption works thusly:
After a delay determined by the values selected for R1 and C1,
the output of the NE555 timer goes high and causes resistor RL to heat
up enough to ignite match M1. M1 subsequently lights the fuse
on firecracker FC1, which has tied to its body a string that wraps around
a pulley and holds a rock (which weighs precisely 2π
pounds) suspended in space atop a bellows. Once the firecracker explodes, the rock
falls on the bellows, causing a puff of air to snuff out the targeted candle.
Highly predictable timing is made difficult by the variability of the fuse burn
time, the time required for the rock to compress the bellows, and for the air blast
to reach the candle flame. The time of flight of the rock is calculable with the
first-semester physics free-fall equation of t = √(2h/g), but empirical
measurements are required to determine the fuse burn time, bellows compression time,
and subsequent air puff travel time to the candle, which compared to time constant
of the R1/C1 combination, is quite imprecise.
databook includes a half dozen or so comic type drawings to help readers grasp
the concepts of integrated circuit timer applications. In some aspects it is rather
insulting, although it was probably the ploy of the same engineers who dreamed up
the "Delayed Light Turn-Off" circuit. I remember being amazed while in the U.S.
Air Force seeing manuals in the vehicle maintenance shop that were made to look
like comic books, illustrating how to perform tasks ranging from changing oil on
a 2-ton truck to overhauling a diesel engine. That was done due to a directive that
the manuals be comprehendible by someone with a sixth-grade education. Now that
* See "Therapeutic Radio," "In the Days of Spark - A Rescue at Sea," and "Standards of Measurement."
Posted December 7, 2018