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


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 two loads.

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

Why should this post be closed?


2 answers


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".


@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 14 days ago

... 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 14 days ago


It's not clear what you are really asking, but here is something that might fit your requirements:


First, R1 and R2 can be considered separate loads. The transistor therefore switches two loads.

This circuit can also be used to produce two signals, one inverted from the other. This used to be more common in old tube amplifiers, and was called a "splitter". SIG+ and SIG- would then go off to control the drivers at different ends of the center tapped push-pull output stage, for example.

With bipolar transistors, the currents thru R1 and R2 aren't exactly equal, but can be close enough for many purposes. The discrepancy is 1/gain of the transistor. When this matters, use a Darlington pair, which acts like a single transistor with gain2. Or, nowadays, you'd usually just use a FET.

My idea was to make two non-linear loads (e.g., LEDs with slightly different VF) "help" the SPST switch when switching.

We do engineering here. That includes describing specifications properly so that others can understand them. I've mention before that your descriptions are vague. This needs to be fixed.

I can't even guess what "help the SPST switch when switching" is supposed to mean. SPST switches don't generally need help. If you're talking about a relay, then you either energize the coil or not. No "help", whatever that means, is otherwise required.

A single transistor can certainly drive multiple LEDs with different forward voltages. Just give each their own series resistor to get the current you want thru that LED:


In this example, R2 and R3 can be adjusted so that the current thru each LED is the same, even if they have different forward voltages. For this to work, the V+ voltage must be known and reasonably constant.

My idea was ... the LEDs to act themselves as switches ... consider them together with the transistor as a "composite SPDT switch"

Now it's even less clear what you are asking than before. However, one thing can't be changed, which is that a SPST switch has two output states. No matter how many LEDs you have, there will ultimately be only two different configurations of on/off for the LEDs that you get to select between.

Define what states you want each LED to have for the switch being on and off, and it will be straight forward to design a circuit to achieve that. Put another way, give us a truth table. This must have only two rows, one for switch off and the other for on. It can have as many columns as you like, one for each LED that is to be controlled.

Enough with the hand waving already!

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

It seems we finally have a somewhat coherent question. Yes, a single SPST switch can switch between loads. Here is an example where one or the other LED is always lit, depending on the state of a SPST switch:


In this example, the forward voltage of the LEDs is 2.1 V with 10 mA thru them.

When the switch is open, current flows thru R1 and D2. Since D2 drops 2.1 V, the voltage across R1 is (3.3 V - 2.1 V) = 1.2 V. The current thru R1 is therefore 10 mA. This current flows thru D2, so D2 is lit. The voltage across D1 and R2 is only 1.2 V, which is insufficient to get any meaningful current thru D1, so D1 is unlit.

When the switch is closed, D2 has 0 V across it, so is off. The bottom end of D1 will be at ground, so its top end will be at 2.1 V. Using the same math as above, that means 10 mA flows thru R2, which also flows thru D1, so D1 is lit.

Therefore, D2 is on and D1 off when the switch is open, and the opposite when the switch is closed.

However, there is a drawback to this scheme, which is the wasted current thru R1 when the switch is closed. Since the full supply voltage will be applied to R1 in that case, the current thru it will be (3.3 V)/(120 Ω) = 27.5 mA, which dissipates 91 mW as heat.


An interesting example... I like this circuit. As though, it is a voltage divider of three resistors in series. The middle "resistor" is variable and it controls the common current that, like an "electrical transmission", connects the voltage drops across R1 and R2 by Ohm's law - V1/R1 = V2/R2, so V1/V2 = R1/R2. It is like a "perfect voltage divider" with a simpler ratio. My idea was to make two non-linear loads (e.g., LEDs with slightly different VF) "help" the SPST switch when switching... ‭Circuit fantasist‭ 14 days ago

Thanks for the responce; it means a lot to me. My idea was to do the opposite - no equalizing resistors... and then the LEDs to act themselves as switches. Then the idea came to me to consider them together with the transistor as a "composite SPDT switch"... and I decided to ask this question... but I am still thinking about it... (I slightly corrected the question to make it more clear - "switch loads" -> "switch between loads"). ‭Circuit fantasist‭ 14 days ago

Thanks again for the efforts! I feel a little awkward about the time taken. Rather, I expected you to be a little affected by my naive question. My idea was even simpler. An LED is supplied by a current and is lit. The SPST switch connects another LED with lower VF in parallel. It diverts the current of the first LED, turns it off and lights up. As though, the first LED acts as a NC SPST switch that complements the NO SPST switch... and the combination of them acts as a composite SPDT switch. ‭Circuit fantasist‭ 14 days ago

@Circuit: If you already had a specific idea you weren't sure of, then why didn't you ask about that? If you already had an answer, then what was the point of asking the question in the first place? Either way, it seems you were just wasting other people's time. ‭Olin Lathrop‭ 14 days ago

@Olin Lathrop‭, I am sorry if it is taken this way. I have already told how the idea for this conceptual question arose from something specific from the distant past. My idea was to get confirmation of my current hypothesis, so I asked it. I did not want to direct the answers in a certain direction in advance. For me, this is not a waste of time and I keep thinking about summarizing the problem... Thank you and sorry again. ‭Circuit fantasist‭ 14 days ago

Show 3 more comments

Sign up to answer this question »