Electrical – How to safely interface an NPN or PNP sensor output with the GPIO pins on the Raspberry Pi

Your LED strip runs at \$I_{LED} = 32W/12V = 2.7A\$. This is the current running through the transistor.

The MPF102 is an N-channel JFET that is not meant to drive a lot of current (datasheet specs 20mA at \$V_{GS} = 0V\$).

The MJE3055T and TIP31A are both bipolar power NPN transistors. You can approximate them as current controlled current sources; their current amplification, \$h_{FE}\$, is less than 100. This means that your base current \$I_B\$ will have to be more than \$I_{LED}/100 = 27mA\$. Your rPi GPIOs will probably not be able to run that amount of current (typical is 4mA for GPIOs).

The TIP102 and TIP120 are both NPN Darlington transistors. Their \$h_{FE}\$ are well over 1000, so you will not have problems driving the transistors. The high gain comes at a price, though: the collector-emitter voltage \$V_{CE}\$ is at least 0.7V; at your 2.7A it will be just under 1V (see Fig. 6 in the datasheet). This means that the power dissipated in the transistor will be \$1V \times 2.7A = 2.7W\$. Looking at the transistor's thermal resistance \$R_{thj-amb}\$, the junction temperature will be at \$2.7W * 62.5^{\circ}C/W = 170^{\circ}C\$ above ambient temperature. This exceeds the maximum allowed junction temperature of \$T_J = 150^{\circ}C\$. Without a heat sink, this transistor will not work either.

If you decide on trying either of the NPN transistors (Darlington or not), be sure to use a base current limiting resistor between your GPIO and the transistor base. To limit the current at 4mA, you would choose \$R_B = (3.3V-0.7)/4mA = 650\Omega\$. Of course you do not need exactly this resistance. A 1kΩ or 470Ω resistor would do the job as well.

From RPi 3.3V to 5V input, depends on the input IC's Input Logic High minimum. Most ICs are 0.7 * VCC, or 3.5V. So the 3.3V won't be enough to register as a Logic High. Some ICs are 0.6 * VCC, or 3V. It will be enough, maybe. Sometimes it will act in weird ways at such a low voltage. You need a level translator circuit. A simple NPN transistor can do it, but inverts the logic (3.3V Output = 0V Input, 0V Output = 5V Input). You could fix that in code, or you need a more complex circuit or dedicated IC for non-inverting.

From 5V output to RPi 3.3V input, you always want to convert it, as the RPi does not have 5V tolerant inputs. NEVER connect it directly, even with a serial current limiting resistor, unless you want to buy a new RPi. A simple way could be a resistor voltage divider. A better way would be a real logic level translator circuit.