# Fun little brain teaser

Discussion in 'General Electronics Chat' started by KL7AJ, Sep 22, 2015.

1. ### KL7AJ Thread Starter AAC Fanatic!

Nov 4, 2008
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I ran across this ages ago, and I present it to each new electronics class as an exercise.

You have a black box with four leads coming out of it. You apply a voltage (ac or dc) to any two adjacent leads. The two remaining leads output precisely 1/4 of the applied voltage. What is in the box?

Eric

blocco a spirale and AnalogKid like this.
2. ### strantor AAC Fanatic!

Oct 3, 2010
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a wheatstone bridge with an inverted side? Like this:

the 12V source represents your applied voltage, and points C & D represent your output leads. Potential difference between C & D is 4V (1/3 applied voltage in this case).

Apr 30, 2011
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I agree in principal but don't the resistors have to hold the ratio X:3X to satisfy the 1/4 voltage requirement?

4. ### strantor AAC Fanatic!

Oct 3, 2010
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The values can be changed to make it hold a 1/4 voltage when voltage is applied from opposite corners, but when it is applied on the same side, it no longer works. So I proved myself wrong. I'll think about more tomorrow. Good night.

5. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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Here's what I think it is:

Last edited: Sep 22, 2015
cmartinez and strantor like this.
6. ### strantor AAC Fanatic!

Oct 3, 2010
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Nice! And you even made it look like a box!

7. ### KL7AJ Thread Starter AAC Fanatic!

Nov 4, 2008
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Free hint: It can be done with just 5 resistors

8. ### AnalogKid AAC Fanatic!

Aug 1, 2013
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If the four leads are in a row, does the outside pair constitute an adjacent pair as if the four leads were in a box?
Or does four-in-a-row intentionally have one fewer possible connection?

ak

9. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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Version 02:

Last edited: Sep 22, 2015
10. ### GopherT AAC Fanatic!

Nov 23, 2012
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So, I assume "Blocco a Spirale" translates to, "Master of Voltage Dividers"

PS: I know it is a "spiral notepad" but I like my translation better for today.

11. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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I'll settle for "Master of guesswork that just happened to lead to a quick solution but may not be so lucky next time"

GopherT likes this.

Nov 4, 2008
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Here ya go.

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13. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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Well, that makes sense