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Post by william on Jul 1, 2015 5:44:31 GMT -8
What is the feasibility of modding the back-light to be powered independently of gameduino2 -- with a constant dc voltage controlled by a potentiometer? Assumptions: - The backlight is currently pulsed on and off These are drawbacks I imagine but don't know if they are valid: - Power consumption - Maybe the lcd takes time to be in a consistent state, and the backlight is only flashed when it is "good"? IE -- Tearing screen updates" - Heat? - There should be a minimum turn-on voltage for the leds -- so will it be able to become suitably dim? - Perhaps pulsing the backlight can achieve higher perceived brightness per a given level of wear of the led lifetime? - small circuit, not easy mod? A bit of background: For scalar-continuous things, like volume, brightness, etc, I prefer to just use a physical knob than doing it in software. I think that many features found on consumer stuff, like pwm brightness are done to keep things cheap, not for quality reasons. If I am going to build something for myself - 90% of the cost is my time, I don't mind adding a potentiometer. I am building my dream videogame console. I would like to play at night or day with an easy to adjust brightness, while preserving the dynamic range of the color space provided by the lcd. Here is what I have so far:
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Post by jamesbowman on Jul 1, 2015 17:57:05 GMT -8
Awesome stuff. Beautiful case. When I was prototyping GD2, I ran the backlight from a bench PSU, and it seemed to work fine. So as far as I know there are no drawbacks to using a constant voltage. You are correct, the backlight is currently pulsed on and off. Although its default state (maximum brightness) it is just continuously on. You could mod the circuit to provide a 0-19V supply on the backlight pin. That should work, although to be safe I would recommend removing the backlight driver IC (MIC228). There is also no easy solder point to replace the voltage. There is an alternative, which is to keep the MIC2289, but replace its output voltage resistor. Instead of using PWM to adjust brightness with a constant output voltage, this setup would use the variable voltage and keep the PWM line always high. The Micrel datasheet mentions a variable resistance as a control scheme, on p.7: www.micrel.com/_PDF/mic2289.pdfReplacing R1 with a couple of wires leading to a potentiometer would be fiddly but not impossible The range of the potentiometer should be 2.375 ohms to (about) 20 ohms. 2.375 ohms is the value of R1, so is maximum brightness. From their page 5 formula, at 2.375 ohms backlight current is 40 mA. At 20 ohms the backlight current would be 4.75 mA, which is quite dim.
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