It is a current limiting resistor. Without it the transistor base would use too much current. This is similar to the current limiting resistor used with an LED.Hi all,
I need your advice on something. May I know why do we need a 1kOhm resistor between a transistor and the output pin from a PIC? can it be any value?
I did a charlieplexing project with 20 leds. They switched fast, nanoseconds. So the current is on for only a fraction of the time. Think that will cause problems ? I left it on for a few hours, and everything appeared to be ok. Though the intended purpose is for those charlieplexed leds is to be on no more then 5mins, after that, they turn off.The max allowed output current from a PIC is 25mA.
The PIC is made with High Speed Cmos (74HCxxx) so it will try to drive 60mA and blow up with out current-limiting.
There is a Mood Light project where the author did not use current-limiting resistors from the PIC to LEDs. The PIC got so hot that it failed or stopped.