Transistor Affecting Timings?

[RossWaddell]

I need to set precise on/off timings for a blinking LED and I was surprised to see that when I switched to a 2N2222 transistor (with a 10k resistor between the GPIO.0 output and Base) in order to drive 4 LEDs that the overall on/off rate changed (it appears longer). I'm using a 20Mhz oscillator to get the best resolution possible, so am I doing something wrong? If I want a precise duty cycle is the only way by trial and error?

DEFINE OSC 20
 
@ DEVICE pic12F629, HS_OSC
@ DEVICE pic12F629, WDT_ON
@ DEVICE pic12F629, PWRT_ON
@ DEVICE pic12F629, MCLR_OFF
@ DEVICE pic12F629, BOD_ON
@ DEVICE pic12F629, PROTECT_OFF
@ DEVICE pic12F629, CPD_OFF
 
CMCON    = 7         'Turn off comparators
TRISIO.0 = 0         'Make GPIO.0 pin output
 
LED_0 VAR GPIO.0       ' Alias GPIO.0 to LED
DUTY VAR BYTE
CYCLE VAR BYTE
STEP_CNTR var byte
 
LGHTS_ON_MS CON 1410 '36 frames given the oscilattor accuracy
LGHTS_OFF_MS CON 550 '12 frames given the oscilattor accuracy
 
Cycle = 1
STEP_CNTR = 2
 
lblLoop:
   ' Fade in
   For Duty = 0 TO 255 step STEP_CNTR
         PWM LED_0, Duty, Cycle
   Next
 
    ' Stay on LGHTS_ON_MS
    High LED_0
   Pause LGHTS_ON_MS
 
    ' Fade out
   For Duty = 255 TO 0 STEP -STEP_CNTR
         PWM LED_0, Duty, Cycle
   Next
  
   ' Stay off for LGHTS_OFF_MS
    Pause LGHTS_OFF_MS
 
   GoTo lblLoop
 
End

Also, I'm using PWM to fade in/out the LED to give the appearance of incandescents - does that even work with a 2N2222 transistor?

[Ramius]

I can only assist with the transistor itself which is good to 250 MHZ so at 20 MHZ is well within its frequency limits. If timimg is a concern you may wish to go with the faster 2N2222A. You might also wish to go with a 4.7K, and then consider creating a darlington configuration (collectorof transistor 1 to collector of transistor 2, emitter of transistor 1 to the base of transistor 2, base of transistor 1 to the resistor to the PIC and the emitter of transistor 2 to your LED's). My first guess is that your are not turning the transistor on hard enough. Best, Ed

[RossWaddell]

I think I'm already using a 2N2222A - the writing on the casing says, "MPS2222A". I'll try with a 4.7k first and then look into the Darlington configuration (I assume you're talking about two (2) 2N2222A's, right?)

[RossWaddell]

Neither replacing the 10k resistor with a 4.7k nor a darlington pair (your connections didn't work for me; this did, though) helped - after about 1.5 minutes they are out of sync.

[Dave]

Out of SYNC?.....

[Ramius]

Not sure what "out of sync" means either. I doubt the transistors are causing much of a problem and the original idea/suggestion was to ensure that the transistors were driven hard enough to light the LED's. A schmatic/drawing would help if possible. Just an idea, maybe using "HPWM" to set your frequency might help? Perhaps even a combination of "HPWM" and "pause"? Yes I was talking about using 2 transistors. My guess is that you are placing your LED's on the emitter side of the transistors and not the collector?