using an NPN, the transistor is the wrong way round

How can the pros be "the led is off at power up" and the cons be "the led is off at power up" ?



The 270 Ohm resistor is responsible for the brightness of the LED in this circuit.
WHAT 270 R resistor ?


The third method does NOT need to feed the second transistor with the 5V supply, it can be unregulated.

Last circuit LED IS ON at power up, and you are sourcing base current from the 8052 pin, which, as you set more current - the zener also steals current from the base which is all sourced from 8052 pin.

This circuit is what I alluded to in my last post.

To a reasonable approximation, the voltage across the 100R resistor is fixed by the zener, in actual fact, it is Zener volts - 0.6 volts, so current = (zener-0.6)/(100 r)resistor

The 100 Ohm resistor and the zener diode are responsible for the power through the LED(s)

NO, the 100 Ohm etc is responsible for setting the CURRENT, the power is fixed by the LED volt drop AND the current.


I think you need to seriously re-read this because I am getting very confused too.

Steve