I am trying to produce an LED dimmer circuit using a 555 timer circuit and a BC639 NPN transistor. I know there are better transistors that would suit the task better, but the BC639 is what I have to work with.
I've got the 555 timer configured so that it is outputting a ~12V 1.4kHz signal.
I am using a BC639 NPN transistor to drive a white LED, with a maximum forward current of 50mA. I have put a 330Ω resistor in series with the LED so that the current is 26.36mA.
I'm trying to figure out how to make it so that the LED turns all the way off when my 555 timer is outputting the lowest average DC voltage.
I have been told that this is down to the value of R3. I have calculated a value for R3 of 51.4kΩ:
This resistor value does not allow the LED to be turned off fully. Through trial and error I have found that the closest E12 resistor value that does allow the LED to turn off fully is is a 680kΩ resistor.