Using USB Servo Controller to dim an LED.... need to smooth strobing effect
Bfarn
Posts: 1
Hi all,
I know that the Parallax USB Servo controller isn't strictly designed for dimming LEDs, but it ALMOST does it perfectly. I've wired the LED to the - and S pins of one of the servo outputs, and it's using PWM to brighten and dim the LED without any further modification.
The problem is that it's visibly strobing. It's slight, but enough to be unacceptable. I mean, I know that's how PWM works, but a dedicated pwm LED dimmer just has a faster strobe speed so that it's not visible to the eye.
What I'm wondering is how (or even IF) I could use a capacitor to smooth out the signal. It's my understanding that, in theory, the right capacitor would slowly ramp up and down between the pulses, effectively smoothing things out and making steady light. But when I add small capacitors (500pF or so) they don't do anything, and when I add larger capacitors (500uF or so) they only dim the LED, but leave it strobing.
I know that the proper solution is to not to try to get this board to do something it's not meant for.... but if I could use a single controller board for this project it would make my life a LOT easier.
If anyone's dealt with this kind of issue before, or even just has some general circuitry advice, I'd really appreciate the help.
Thanks!
--David
Post Edited (Bfarn) : 11/1/2006 6:51:01 AM GMT
I know that the Parallax USB Servo controller isn't strictly designed for dimming LEDs, but it ALMOST does it perfectly. I've wired the LED to the - and S pins of one of the servo outputs, and it's using PWM to brighten and dim the LED without any further modification.
The problem is that it's visibly strobing. It's slight, but enough to be unacceptable. I mean, I know that's how PWM works, but a dedicated pwm LED dimmer just has a faster strobe speed so that it's not visible to the eye.
What I'm wondering is how (or even IF) I could use a capacitor to smooth out the signal. It's my understanding that, in theory, the right capacitor would slowly ramp up and down between the pulses, effectively smoothing things out and making steady light. But when I add small capacitors (500pF or so) they don't do anything, and when I add larger capacitors (500uF or so) they only dim the LED, but leave it strobing.
I know that the proper solution is to not to try to get this board to do something it's not meant for.... but if I could use a single controller board for this project it would make my life a LOT easier.
If anyone's dealt with this kind of issue before, or even just has some general circuitry advice, I'd really appreciate the help.
Thanks!
--David
Post Edited (Bfarn) : 11/1/2006 6:51:01 AM GMT
Comments
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·· The strobing effect is no doubt caused by the fact that the PSC is sending out the varying pulse widths (1 mS to 2 mS) but there is a 20 mS delay between each pulse.· One thing I would be concerned about is the more LEDs you add the more current you will draw and eventually it will be too much for the controller on the PSC.
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Chris Savage
Parallax Tech Support