Nine Transistor Switching Problem

I'm new to electronic circuits, so if this is a simple problem, I hope you'd be willing to help me out.
View attachment SCAN0001.PDF
Above is a (rough) schematic of the circuit I'm trying to build. Here's what I'm trying to do: I have three toggle switches that make it possible to activate a load down the line. I want it so that if one toggle switch is on, the signal goes through, but if two or more are active, the circuit will shut down. I thought I could achieve this by using three NPN transistors and six PNP transistors as shown above. I'm trying to set it up so that the NPN transistors function as normally-closed electronic switches, and the PNP transistors function as normally-open electronic switches. I've managed to build and test each component of the circuit. But when I link two PNP transistors together as shown, there isn't enough current (I think) to activate the LED. Is there something wrong with my set up? What do I need to do differently? Or is there an easier method to do what I'm asking?

It'll take a while to try and figure that out, but in the meantime I see that transistors B and C in the top row have their bases connected to battery minus, so they will always be off.

btw, What you're trying to make is an XOR gate. It may be easier just to buy one.

---------- Post added at 19:33 ---------- Previous post was at 18:40 ----------

OK, I gave up trying to understand your circuit, but this one should work.

Firstly:
Q7 (on the far right) switches on and lights up the LED whenever Q1 or Q2 or Q3 are on.

If only one switch is on at a time:

• If switch 1 is on, current flows through R1 and switches on Q1.
• Same story for switch 2, R2 and Q2 as well as switch 3, R3 and Q3.

If more than one switch is on at the same time:

• If switch 2 (or 3) is on, current flows through R7 (or R4) to switch on Q4. Q4 switches off Q1 even if switch 1 is on and current is flowing through R1.
• Similarly, if switch 1 (or 3) is on, current flows through R8 (or R5) to switch on Q5 and make sure Q2 is switched off.
• Also, if switch 1 (or 2) is on, current flows through R9 (or R6) to switch on Q6 and make sure Q3 is switched off.

So you managed to do it with one PNP and 6 NPNs then? Well, that does seem more efficient than my model. Thanks, I'll give this circuit a try. Also, yes, I was trying to make an XOR Gate, but I wasn't aware they had XOR Gates with three lead ins or that there were just individual gates for sale somewhere.

thelaughingfool said:

So you managed to do it with one PNP and 6 NPNs then?

Click to expand...

I realized later it can be made a bit simpler. If you leave out the last transistor and turn the whole circuit upside-down, you can drive the LED directly from Q1, Q2 and Q3, as in the picture below. That way, you just need 6 PNP transistors.

Also, yes, I was trying to make an XOR Gate, but I wasn't aware they had XOR Gates with three lead ins or that there were just individual gates for sale somewhere.

Click to expand...

Logic gates are still available, but I expect they're not as common as they once were. e.g. Here's what's available at RS: https://za.rs-online.com/web/c/semiconductors/standard-speciality-logic/gates/

It seems like you're right about 3-input XORs though, so you'd probably have to make one out of several other gates. There's some interesting info here: https://en.wikipedia.org/wiki/Xor_gate#More_than_two_inputs