Use bipolar transistor to power LED from a certain power on?
There are two simple approaches you could take. The first would be to add a second transistor for additional gain, which will make the off-on transition happen over a narrower range of voltages. Something like this:







simulate this circuit &ndash; Schematic created using CircuitLab

The second approach would be to use the second transistor to create positive feedback for the circuit, causing it to have a "snap action" (hysteresis). However, this would also mean that the on-off transition happens at a lower voltage than the off-on transition. Something like this:







simulate this circuit