I'm trying to design a circuit to power some LEDs with a diode as a temperature sensor. I want the circuit to turn off the LEDs when it gets too hot. The diode forward voltage will rise as the temperature rises which I can use with a transistor to turn something on, but I want to turn it off. I figured out how to set it up with a transistor and a relay but I'd like to stick to only transistors. Is there a way to use a transistor as an off switch instead of an on switch?