I have a small TV box which needs a fan because it overheats causing the cpu to throttle. I would like to have the fan turn on when the box is on, and turn off when the box is off. Unfortunately, the USB ports on the box always have power even when the box is off. The box also has no physical power switch. The only power indicator is an LED that is red when the box is off, and blue when it is on. I would like to use this LED signal to turn a small fan on and off.
The fan has a very low draw, and I will be reducing the voltage to the fan with a buck to 1.85V at which point it should draw 0.15A (It draws 0.40A @ 5V).
I have been reading about using a transistor as a switch, but most articles talk about using 5V at the base to turn the transistor on, and 0V to turn it off, while the LED on my box has much less than that. To make it more difficult, the LED never has 0V at any time on any leg. The voltages on each leg of the LED are as follows:
Leg: One Two Three
Blue: 0.60 3.26 1.90
Red: 1.97 3.26 1.39
The voltages were read using the ground of the power supply since that's what the emitter leg of a transistor would be connected to in order to switch power to the fan.
I have an S9014 NPN transistor and the specifications seem to indicate that it will turn on and off at the voltages indicated on the first leg of the LED, however I'm not certain about that. Also, many diagrams using a transistor as a switch include a resistor on the base and sometimes also on the collector of the transistor, but those diagrams are also often using 5V and not 1.97V.
Can I use the S9014 transistor with the base connected to the first leg of the LED, the emitter connected to ground, and the collector connected to a fan, with the fan connected to a 1.85V source, to switch the fan on and off? Will I have any problems with the transistor pulling too much current from the existing LED circuit? I will not be using the LED circuit to power the fan, just to signal the the transistor to turn on and off. Or am I looking in the wrong direction entirely?
Note: I need to keep all of the components being used to a minimum so they can fit inside the small TV box, so I can't use anything like an arduino, or a thermistor relay.
Note: Measuring voltage from leg 2 to legs 1 and 3 respectively, I get -2.65 and -0.15 on blue and -1.29 and -1.87 on red.