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Electronics Puzzler by Mr. Forrest M. Mims III himself ! Answered

Below is a link to Jameco's latest Forrest M. Mims III electronic's puzzler: can anyone figure it out without cheating? The link provides more detail. plus some hints....

JAMECO PUZZLER 2 - What's the Missing Component?


Would you believe it is JUST an LED ? A clear cased RED LED. That's all folks ! LOL

Yes, that's what I thought see below


The red LED is being used as a sensor in this case. The clearer the case it is in, the more sensitive it is. I had always known that LED's and other clear cased diodes reacted somewhat to light, but I never did much experimenting with it.....I am going to do some now though...

Light Detector thing

Ok, if you look at the answer (at the site ) you'll be surprised :-)

That would be seen as a resistor and probably would not work ;-p


9 years ago

o, oh my....well, it has a graph i assume thats what it is that has volts to light....so my guess is an LED or light bulb circuit?

the missing component, is to detect certain wavelengths of light....

one of the hints states: The missing component is clearly some kind of photodiode or photoresistor. But what kind?

Did you go to the site and read the article? It says it will pick up certain reds, but not the near IR of a remote control.

It isn't what most would consider a normal detector but they do react to light in their wavelengths....

one doesn't think of one of these as a detector normally ;-)

Really? It's what we used in our school projects, with a pretty much identical circuit.


9 years ago

I know I'm going to just kick myself once I look up the answer.

*Kicks Himself... hard*

I can't believe I didn't figure that out. Haha, I must have been overthinking it.

I had seen something similar to this YEARS ago (maybe even submitted by Mr Mims), and I had forgotten about this particular "ability" of this component :-)

. Woot! . If it had been asked as a "I found this PCB and can't figure out what '?' is" question, I never would have figured it out. Since it was a puzzle, it started thinking "backwards." It's a common part - if it had been more "esoteric" I wouldn't have gotten it. . But in the end, it was just a lucky guess. ;)

The right hand arrow isn't labeled, but at a guess the missing piece creates the LIGHT output represented in the I-doubt-it's-actually-that-linear-graph?
(e.g. a bulb / LED)


/facepalm It was so obvious once I looked up the answer...