Signetics NE555 timer schematic for a "Delayed Light Turn-Off" circuit
from their 1973 databook.
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
Signetics 555/556 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).
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.
The same Signetics 555/556 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 was insulting.
* See "Therapeutic
the Days of Spark - A Rescue at Sea," and "Standards
Posted December 7, 2018
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