Control large voltage supply using smaller voltage

Question

I have a 6 volt supply that I need enable/disable using another 3 volt supply.

If I use a PNP Darlington set-up like this: Then I get just under 6 volts at the Output node.

My problem is that I don't have any PNP transistors but I do have several NPN transistors and I was hoping that setting up a NPN Darlington would give me similar results, but building something like this yields about 2 volts at the Output node:

My question is; is there a circuit I can make using NPN transistors (2N2222 to be exact) what will give me a voltage close to V2 voltage when V1 is enabled?

Answer 1

Try this circuit:

^{simulate this circuit – Schematic created using CircuitLab}

The reason your circuit doesn't work is because you're using an "emitter follower" circuit. Basically what it does is the transistor maintains a voltage at the emitter that is the same as that at the base, minus a diode drop. Using a darlington in this case doesn't help because the transistor will not conduct if the base is lower than the emitter.

Note that the voltage at the collector of the transistor will not be 6 volts when it is on (you don't want that anyway), however the load will have very close to 6 volts across it when the transistor is switched on.

Edit: as mentioned by @jonk, your micro's output pin does not source enough current to drive this transistor enough to switch your pump. You have a couple of options here, you could buffer the output of your microcontroller through a buffer like a 74HC34, or you could use a darlington configuration with 2 2n2222 transistors as shown here.