Hi!
I just bought the Micro Maestro 24 channel ( still in the mail).
I have been going thru the documentation and would like some confirmation that my theory might work.
I intend to use the Micro Maestor to contol High power RGB leds up to 3w per colour. This means up to 1 amp per channel. This excludes the Shiftbar.
My plan is to use the LM 3406 http://www.national.com/pf//LM/LM3406.html#Overview as the led driver.
This is a 1.5A led driver with enable input, possibly used for PWM control of the led.
The control of this chip I intend to accomplish by increasing the rate to 333Hz and the Max position to 3000 uS (is this possible?).
In theory this should give me 24 0 - 100% PWM outputs channels. 333Hz should be enough for lighting applications.
Am I missing something?
best regards
Chris
You are missing something: Section 9 - Maestro Settings Limitations from the Maestro user’s guide.
How many LEDs do you want to control, and how do you want to control them?
Every Mini Maestro comes with one channel that can be used as a PWM output which is ideal for driving LEDs and allows you to get full 0-100% control of the duty cycle with high resolution.
But if the channels are configured as servo outputs, you won’t be able to get full 0-100% percent control of the duty cycle.
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The Maestro lets you turn off the pulses (achieving 0% duty cycle) but it can not produce pulses that are less than 64 microseconds long.
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The Maestro’s pulse scheduling algorithm imposes a limit on the maximum pulse width you can generate. This limit depends on your pulse frequency (e.g. 333 Hz), the number of pulses you have enabled, and the minimum pulse lengths of the other channels. Note that this limit is not strictly enforced by the Maestro, but if you exceed the limit then the Maestro will warn you with a performance error. You can see tables of those limitations here:
pololu.com/docs/0J40/9
According to that table, at 333 Hz you can control 6 servo channels with pulse lengths between 192 (6.4%) and 2648 (88%). Is that good enough for you?
Alternatively, instead of configuring the channels as servo outputs you could configure them as digital outputs and write a script inside the Maestro that turns them on and off. There are a few difficulties with doing it this way and I can elaborate on what they are if you are interested.
–David
Hi,
Thank you for the fast reply David. I Never got to chapter nine, sorry about that!
I am interested in scripting with Digital outputs, I would be wery pleased if you could elaborate.
I was hoping to use all 24 outputs. If using all at 250 Hz I would get 19.4 % to 75+%. The top can easilly be compensated by increasing the current in the led accordingly, but this will move the bottom equally high up.
First I thought I was going to be able to use the ShiftBar, just use the PWM outputs on this one to control the bigger LED Drivers, but the frequency is way to high on the ShiftBar. Using the ShiftBar would have been the better solution , to bad it wont work. I’ve also looked innto the octobrite, same frequency issue here.
Well I guess I have to go back to the drawingboard, even though it did not work as i hoped it is still a great product to a wery resonable price!
regards
Chris
Never mind about the scripting idea. I just did a test: the Maestro scripting language runs too slowly to handle the super fast toggling that you would need to get fine control over the brightness of the LEDs.
I looked in to the ShiftBar a little bit and I don’t think their frequency is too high. According to the A6281 datasheet, the PWM counter operates at 800 kHz, and you have 10 bits of resolution, so the PWM output frequency should be 800,000/1024 = 781 Hz. You can also configure it to divide by 2 or 4. Dividing by 4 gives you 195 Hz.
–David
Hi,
Guess i got a lot to learn! I thought the PWM frequemcy was the actual output PWM frequency not the counter frequency. Sorry about that, below 1 khz will work without problem on the LM3406.
Thank you so much for your time DavidEGrayson!
regards
Chris
