I would like to use two TIP120 transistors in series so that both pushbutton switches have to be pushed at the same time for the lightbulb to light. I tested one transistor and it worked as intended. I just cloned the transistor and connected the collector of one transistor to the emitter of the second transistor. I think i am making this an and gate with transistors. Will this work?

 

You don't need two transistors. Just two diodes would do it.
Both inputs have to be HIGH for the transistor to turn on.

 

Why don't you just use one transistor and put the two switches in series?

 

MrChips, thanks for your reply. My original question stands. Will the circuit that I show work? I don't see where the 12 volts I need to light the light bulb comes from in your circuit.


[/QUOTE]

 

Wbahn, thank you for your answer. The circuit I provided is a test for what I want my circuit to do. Eventually I will be connecting an Arduino to the circuit and will need two separate inputs. Will my circuit as shown still work?

 

Wbahn, thank you for your answer. The circuit I provided is a test for what I want my circuit to do. Eventually I will be connecting an Arduino to the circuit and will need two separate inputs. Will my circuit as shown still work?

It will have a problem. If both push buttons are open, the emitter of the upper transistor and the collector of the lower transistor will be at an undefined voltage level. This will make it difficult to establish the required Vbe of about 0.7 V to turn on the upper transistor. If you always turn on the lower transistor first, then it may work for you.

 

If you are going to use an Arduino, it can do the and operation. Two inputs, one output that is high when both inputs ate high.

 

If you are going to use an Arduino you don’t need two inputs.
Do the AND logic in code and activate a single transistor.

 

It will have a problem. If both push buttons are open, the emitter of the upper transistor and the collector of the lower transistor will be at an undefined voltage level. This will make it difficult to establish the required Vbe of about 0.7 V to turn on the upper transistor. If you always turn on the lower transistor first, then it may work for you.

If the lower transistor is off, it doesn't matter whether or not Vbe the upper transistor can be turned on, since the goal is for the light to be off in this case.

If the switch to the upper transistor is closed when the switch to the lower transistor is closed, the lower transistor will go into saturation which will pull the emitter of the upper transistor down and establish the necessary Vbe to turn it on.

I'm more concerned about there being a sneak path from the collector to the base of the top transistor when it is nominally on and the bottom one is off, but I'm not sure one way or the other. Even if there is, the 2.2 kΩ base resistor would limit the current to just a few milliamps even if it were a dead short (which it wouldn't be).

 

You don't need two transistors. Just two diodes would do it.

You need to add a diode in series with the transistor base to insure that the transistor is fully off when when one diode input is low (unless the diodes are Schottky).

 

Wbahn, thank you for your answer. The circuit I provided is a test for what I want my circuit to do. Eventually I will be connecting an Arduino to the circuit and will need two separate inputs. Will my circuit as shown still work?

It should. As others have pointed out, you should be able to save yourself a pin and perform the AND logic in code. Are either of those signals going to be used for something other than determining whether to turn on the light or not?

 

Will this work?

Yes it will, but .....
Having two TIP120s in series is going to drop more voltage and waste more power than necessary.
Base-emitter pull-down resistors could be added to reduce the chance of spurious lamp switch-on.

 

It will have a problem. If both push buttons are open, the emitter of the upper transistor and the collector of the lower transistor will be at an undefined voltage level. This will make it difficult to establish the required Vbe of about 0.7 V to turn on the upper transistor. If you always turn on the lower transistor first, then it may work for you.

The upper transistor might not turn on until the lower one is on. But the logic still works.

 

The upper transistor might not turn on until the lower one is on. But the logic still works.

Agree. The only valid state is both switches closed. Otherwise, if the transistor pins are at some unknown voltages because on is on and one is off, that is a don't care condition as long as there is no collector current.

ak