The switchmode boost regulator used to create the backlight voltage for the chumby was intended to be operated in an "on or off" only mode; it's actually a boost voltage-mode regulator by design that we've modified to be a current-mode regulator. Current mode regulation is important because the LED light output is better correlated to the current through the LED, and not the voltage. Lot to lot variation and aging of the devices means that a voltage mode regulator will lead to non-uniformity in backlight brightness. By utilizing this regulator instead of including a separate current mode regulator, we save significant $$$, which is a constant challenge when building a device as feature-rich as the chumby selling at such a low price point.

Since the regulator wasn't designed to be operated in a PWM mode, it was decided not to drive it as such. It works with a slow PWM chop but the waveform is pretty unstable and you get some rippling/shimmering effect on the screen when this is happening. So, you could apply a PWM signal to it and it will work on some units but the light uniformity output was not deemed adequate for a product that we ship to customers.

On the other hand, if the dim mode is not dim enough for you, a simple fix is to just increase the value of R525 and R526. The reason it wasn't trimmed any dimmer is that the current is already so low that we run the risk of not having adequate current to start the regulator up. It will work on some units, but again parametric variation is your enemy and we want a high-yielding product. It was reasoned that most people would be okay with how dim the night mode is on the chumby--there's always someone who wants it dimmer, but perhaps for those individuals its best to just shut the screen full off, which is possible in software.

I think it's the case that this is a hardware feature but the current control panel that you see does not expose it. So you can cause this behavior to happen but only if you are willing to ssh into the device and cause it to happen.

I'll point duane at this thread to see what he thinks.

In my opinion it should be a Night-Mode-option to turn off the LCD, the touchscreen and the wifi. I use a  script that does exactly that and it works great and very intuitive: after some seconds of nightmode the screen turns to black and when I hit the top-button the display goes on again and I'm in the control panel as every user would expect.

The brightness is controlled by the procfs file /proc/sys/sense1/dimlevel:

0 = full bright
1 = dim
2 = off

For instance:

# echo 2 > /proc/sys/sense1/dimlevel

...will turn off the screen.  Replace the '2' with '0' to turn it back on, and '1' for dim.

That's correct, the proper procedure for going from off to dim is to quickly transition through the on state. The scripts that run behind the scenes on the chumby do this for you. If you just do

echo 0 > /proc/sys/sense1/dimlevel; echo 1 > /proc/sys/sense1/dimlevel

the transition is so brief you cannot see it.

The reason for this is that the integrated LCD bias generator uses the backlight voltage to prime a set of charge pumps that generate the bias voltages for the LCD. Once the pumps are primed, the voltage can drop to the dim state. If you go from off to dim, the voltage going to the LCD backlight is so low the charge pumps never prime and the LCD never get biased correctly. 

Just a tiny pulse of voltage is needed to prime the pumps, and executing the two commands in rapid sequence does the trick for you. This could also have been integrated into the kernel driver, but it was decided that instead of creating a timer process or dead-waiting to do this, it was more flexible and efficient to do the call at the user level.

From Flash, you'd use the appropriate ASnative calls.

These calls are disabled for widgets, but are accessible to movies run locally.

I get no response when echoing 0, 1 or 2 to /proc/sys/sense1/dimlevel... any thoughts on fixing this?

at the moment my day brightness is about 50% and the night is just a touch above completely dark... and i do get accurate responses when i use the brightness sliders in the control panel, just nothing from the command line.

Thanks Duane,

looking in /psp, there's a daymode_brightness and nightmode_brightness... are those just multipliers to output a number to /proc/sys/sense1/brightness?

edit: nevermind, after playing in the CP, i realized that it's the brightness percentage 0-100... so yes, it's a multiplier for the 0-65535 brightness range.

Chumby One:

The Chumby One doesn't appear to have a /proc/sys/sense1.  Any tips on dimming the display?

Do you know why some people can write 10 to it, but with my chumby one the lowest I can send is 15, anything below 15 the screen goes completely dark?

Duane thanks for the clarification.