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Comments on Is it possible for one transistor to switch between two loads?

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Is it possible for one transistor to switch between two loads?

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The output collector-emitter part of a transistor can be thought of as a 2-terminal SPST switch controlled by the input base-emitter voltage or base current. So this transistor switch can control only a single collector load.

Is it possible to make the transistor act as a 3-terminal SPDT switch? If so, one transistor will be able to control (switch between) two loads.

In general, can a SPST switch act as a SPDT switch?

transistor-switch switching
posted about 4 years ago
4y ago
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Circuit fantasist‭
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I suspect this is another of your trick questions and you are waiting for the chance to show us your unrivaled knowledge, but I will play the straight man for you.

No. By definition, an SPST switch can not act as an SPDT switch in general. If that were possible then there would be no need to manufacture SPDT switches, would there?

However, if the trick question really is "Can you use an SPST switch to control two LEDs, so that one goes off and the other goes on when you move the switch, and one of them is always on?" then sure, you can envision a circuit that will do this. But that is not causing a SPST switch to "act as a SPDT switch".

posted about 4 years ago
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Elliot Alderson‭
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Circuit fantasist‭ wrote about 4 years ago
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@Elliot Alderson‭, As usual, you are in the right place at the right time:) In general, you are right… but not quite. Really, in 1984, I got a patent for an LED voltage indicator where two transistors controlled three LEDs. Now, inspired by the new EE network, I began to realize the idea in a more general way and to think about it. That is how the idea for this question was born… I do not think there is anything wrong with that...

Circuit fantasist‭ wrote about 4 years ago · edited about 4 years ago
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... I guess you will agree that coming up with a good question is no less difficult than answering it... Really, "an SPST switch can not act as an SPDT switch" but the latter can be assembled by two (NO and NC) SPST switches (transistors). And here is an idea (I have realized it just now) - one of the loads can replace the NC SPST switch. So, the NO SPST switch will initially turn on the one of the loads and the other load will turn off itself thus acting as an NC SPST switch.

Circuit fantasist‭ wrote almost 4 years ago · edited almost 4 years ago
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@Elliot Alderson‭, Are you happy with my answer? If this circuit solution seems too complicated to you, I will offer you an even simpler recipe for "converting" SPST into SPDT switch - only through a "pulling" resistor. It was dominant before the complementary stage was invented...

Elliot Alderson‭ wrote almost 4 years ago
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My problem with your writing is that you misuse language to the extent that it confuses inexperienced readers. The fact that you feel the need to "surround" misused "words" with quotes is "revealing".