# transistor as a switch circuit problems

#### bluemarshall

May 30, 2012
14
Hi,

I'm trying to build a circuit with two key switches, where both key switches need to be closed for the current to go through. I am trying to use a transistor so that one key switch feeds the collector, and the other feeds the base so that both need to be on to allow current to go through the emitter. I've tried this also with a MOSFET gate-drain-source and well as a general purpose transistor with no luck. I've seen similar tutorials on youtube where a guy is using a transistor to turn a bulb on and off by touching the base to the same source as the collector with the addition of a resistor. The image is my circuit. If there are things missing in the diagram it's because i don't know that they should be there. the resistor is brown, black, black, gold which according to the web means 100k-ohms +- 5%. Can someone please show me the correct circuit? Thanks.

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#### bluemarshall

May 30, 2012
14
something along the lines of this but with key switches on the collector and base::

#### davenn

Moderator
Sep 5, 2009
14,213
hi there

1) put the light ... is it a LED or a globe ? ... in the collector side NOT the emitter side
if its a LED then it will need a current limiting resistor ~ 1k Ohms would be an ok start for a 12V supply
if its a globe what is it's rating Volts and watts ?

2) what transistor were you using, did you correctly identify the B, E and C ?

3) the resistor on the base , going by your colour code is 10 Ohms, probably too low should more likely be ~ 1k Ohms bn bk rd

cheers
Dave

May 30, 2012
14

#### weird_dave

May 9, 2012
36
Why not just have the 2 switches in series and do away with the transistor?

As already said, your circuit isn't right, the Emitter should be at 0v and your lamp on the collector side.

#### bluemarshall

May 30, 2012
14
Hey weird_dave,

These key switches are being used as safety switches in a fireworks firing system, and the effect that I'm going for is the "you need both keys to turn on the system" sorta thing. But for each key switch i would like an indicator light (not pictured in the schematic) that operates based on the state of the switch it's tied to, not the throughput of the system as a whole. The "light" in this schematic represents the remainder of the system; another couple safety toggles, and all the NO mom push switches.

So anyways, I moved the "light" to the collector side of the transistor and added the 100k (brn\blk\red) resistor as davenn suggested and the system works as intended. Below is the schematic. Maybe there's still something wrong and it's working by fluke?

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#### davenn

Moderator
Sep 5, 2009
14,213
.....................
So anyways, I moved the "light" to the collector side of the transistor and added the 100k (brn\blk\red) resistor as davenn suggested and the system works as intended. Below is the schematic. Maybe there's still something wrong and it's working by fluke?

1k ..... not 100 k I gave the correct colours

Dave

#### weird_dave

May 9, 2012
36
OK, it's getting closer. I did attempt to edit the circuit in paint but failed miserably as I'm not familiar with the windows 7 version......
"The Rest of System" looks fine where it is now.
Those squiggly circled things below each switch, I'd have those as lamps but connect them to the right hand side of the switch to indicate they've been activated (or is the intention that they are lit when they keys are in the off position?).
I'd remove those 3 diodes, they're not doing anything as far as I can tell.

#### bluemarshall

May 30, 2012
14
@ davenn yes 1k sorry. I was tired when i finally got it working

@ weird_dave Yes those squiggly things are lamps haha. It was quick in photoshop. My bad for their positioning as the key switch doesn't have 3 posts. They are off the right side in the circuit. So yes they are only on when the corresponding key switch is thrown. As for the diodes; shouldn't they be necessary because of the common ground, and you don't want current going back up the circuit the wrong way? The whole water in pipes analogy i think only works for some things.

#### CocaCola

Apr 7, 2012
3,635
and you don't want current going back up the circuit the wrong way? The whole water in pipes analogy i think only works for some things.

GND is the finish line, it's not going to go back in reverse especially when there is resistance in it's way Some strange things can happen if the ground loops back upon itself but that is a whole separate issue...

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