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Trying to make a transistor cancel another transistor

David Lacroix

Sep 15, 2016
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Hi,

I tried making a circuit with 2 transistors as switches with an LED in which the first one would be cancelled if the second one is switched one. I thought about giving positive current to the negative of the one I want to cancel to make no difference in potential. Hasn't work yet, can't wrap my head around this little issue. Any help would be appreciated.

Thx
 

Arouse1973

Adam
Dec 18, 2013
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Is It just one LED? which transistor is the LED connected to?
Thanks
Adam
 
Last edited:

dorke

Jun 20, 2015
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"cancel",like matter and anti-matter ;)
What are you trying to achieve?
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Please post a schematic with part designations (R1, C2, etc.) and part specifications. Show electrolytic capacitor polarity if these are used. A well-lighted, high-contrast, hand-drawn schematic is acceptable. Use a dark felt-tip pen on plain white paper. Preview the image for clarity before uploading it.
 

Alec_t

Jul 7, 2015
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If I've understood you correctly you could arrange the second transistor so that when it turns on it shorts the base-emitter junction of the first transistor.
 

David Lacroix

Sep 15, 2016
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2016-09-25 10.28.11.jpg

I manage to make something work. Basically, transistor 1 will only work if transistor 2 is activated, which then can send current to the base of the third transistor which this one permits the number 1 to complete it's circuit. PrettyI know this must be pretty basic stuff but trust me I scratched my head several minutes to figure it out haha...
 

Alec_t

Jul 7, 2015
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Since you have no current-limiting resistors it's surprising something didn't fry!
 

David Lacroix

Sep 15, 2016
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Since you have no current-limiting resistors it's surprising something didn't fry!

They are resistors before LED and on transistor bases, they're just not on the drawing, which I made for myself to have a complete view of my layout.
 

AnalogKid

Jun 10, 2015
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They are resistors before LED and on transistor bases, they're just not on the drawing, which I made for myself to have a complete view of my layout.

But *we* don't know that. And it's not actually a complete view, is it? Are there any other circuit components or connections that are not on the drawing you posted?

ak
 

davenn

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Sep 5, 2009
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They are resistors before LED and on transistor bases, they're just not on the drawing, which I made for myself to have a complete view of my layout.

As you were asked near the start
supply a FULL circuit diagram of your planned construction
 

Bluejets

Oct 5, 2014
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Perhaps the Op is referring to a transistor flip flop where one transistor section "steals" bias from the other .
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Yes, if we had a good idea of what the OP was trying to achieve we could be much more helpful.

Something like: "I have 2 LEDs. I want one on and the other off. When I close a switch I want the one that was off to go on, and the one that was on to go off. When I open the switch I want them to go back to how they were."
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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Perhaps the Op is referring to a transistor flip flop where one transistor section "steals" bias from the other .
Yup, sure sounds like it.

Chris
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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There is a lot of criticism that could be made about the "schematic" that @David Lacroix made, photographed, and uploaded, but lack of contrast is certainly not one of them! Excellent job of drawing with high visibility of everything drawn in black ink on white paper. Good lighting for the photograph too.

Now if only he would identify the emitter and collector of each transistor, add some resistors that apparently are in there somewhere but not included in the "schematic" and follow certain drawing conventions such as crossing lines NEVER connect (so don't need a semicircle to show that), lines that the DO connect do so ONLY at "T" junctions, and (unnecessary, but desirable for easy reading) there is largish dot placed to show there really IS a connection at the "T". Oh, and some component numbering like Q1, Q1, etc. for transistors, R1, R2, etc. for resistors, and some values associated with those components like 2N2222 or 2N3906 for transistors, 330 Ω or 10 kΩ for resistors, red LED or blue LED for Light Emitting Diodes... that sort of thing makes it easy to discuss the circuit in a forum such as this one.

I have no idea what David is trying to accomplish, but perhaps a DPDT switch would do the job... or (as @Bluejets mentioned) a flip-flop configuration. Looks like there are a lot of switches on that "schematic" to just turn a couple of LEDs on and off. Or maybe this is just a learning experience.
 

dorke

Jun 20, 2015
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David,
Please explain in plain words what is it you are trying to do and for what purpose.
That is the only way to give you productive and meaningful help .

BTW,
I think stopping this time-wasting "game of guessing" is a worthy goal as well ... o_O
 

Herschel Peeler

Feb 21, 2016
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dESIGN 879 A ORIGINAL.PNG
View attachment 29340

I manage to make something work. Basically, transistor 1 will only work if transistor 2 is activated, which then can send current to the base of the third transistor which this one permits the number 1 to complete it's circuit. PrettyI know this must be pretty basic stuff but trust me I scratched my head several minutes to figure it out haha...

One step at a time. This is what your original drawing looks like. Switches 1 and 3 must be on before LED 2 comes on. Switch 2 must be on before switch 3 can come on.
 

Herschel Peeler

Feb 21, 2016
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So many simpler possibilities if you don't want to use so many transistors, but I assumed there was a learning experience in this.
 
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