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Thread: Robotics with Dan's PIC SBC
face BY : Dan Michaels email (remove spam text)

At 11:54 AM 12/15/00 -0700, Dwayne Reid wrote:
>>I designed a really cool ckt today which I "think" can pulse
>>an LED at up to 40Khz and "also" control its intensity by varying
>>the width of the 40Khz pulses. 5 components, including 2 R's,
>>1 Cap, NPN, and LED. Requires only 1 PIC pin.
>>Think you can duplicate it? [just try :) - hint - the tricky part
>>is the cap - you have to be able to get a graded current in the
>>LED, but still interrupt the current at a 40khz rate - tricky
>>time constants]. I estimate I can get at least 6-8 intensity levels
>>running off a 4 mhz xtal - 1 usec cycle time.
>Probably similar to what I use to drive triacs: capacitive discharge.  A
>resistor charges the cap, another resistor limits the peak current when the
>transistor connects the cap to the LED.  The longer you keep the transistor
>turned on, the less charge on the cap and the lower the peak current.

Dwayne, sorry for the long delay in reply, I'm just catching up on
missed emails.

Close? yes, a little different arrangement.

         D1         LED
    +---|<|---+      |
    |         |      C
PWM--+----Rb---+----B    NPN
20Khz    200   |      E
         .1uF C      |
              |      Re  50
             gnd     |

Original ckt didn't work as well as I would have liked --> so
added more components. Still pretty simple for controllable
intensity at variable frequencies using 1 pin.

Time constant Rb*C sets the cap charge time so that increasing
PWM pulsewidths give higher Vb within the time PWM is high, for
--> intensity control. Once Rb & C are chosen, you are somewhat
limited in the selection of PWM frequency.

D1 discharges the cap fast, so get fast turnoff. Re more or
less independently sets the max LED current. Did have a little
problem with stability/oscillations [emitter followers tend to
do that], so also put a 1000pF across Re. Also tried the LED
in the emitter leg, but didn't work as well.

best regards,
- Dan Michaels
Oricom Technologies

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