/diy/ - Do It Yourself

>>2741472
Pic related. As the lower impedance is on the collector, this circuit will act like a common-collector circuit, which is unusual (and kinda shitty) for turning on an LED. When the button isn't pressed, you should find 11-12V across the switch and across the collector-base junction, and 0V across R1, R2, R3, and D1. When the button is pressed, the voltage across R1 should be about 2V, and the voltage across R3 should be about 7V. Importantly the voltage from base to emitter should be around 0.7V, and the voltage across the LED should be 2-3.5V, depending on colour.

If there is a voltage at the emitter of 4-5V, it means either the transistor is properly turned on, or it's broken and passing current anyhow. If the LED isn't lit up, then it may well be backwards or broken. The short leg should be on the negative rail.

IIRC this is a reason we decided to remove this book from the OP.