CaptainBlood
New Member
DIY DAC logic to manage a backlight led network light level with a MCU
I have differents panels which needs variable backlight. Light level will be driven by an MCU. Number of led for each panel may be different, as well as the number of available output pins. To quantify the problem a little bit more let say that per panel no more than 32 leds should be wired together, probably less. So I have to find out a generic solution to my problem that I would adapt to each panel case.
Here a scheme of my first solution to the problem :
**broken link removed**
This is a ktechlab study I did for 13 bits DAC which probably is a higher resolution to any of my real needs. Please note that the LED network is represented by a single LED, which is inaccurate. Each of the MCU Output is represented by a switch connected to Vdd. The 0V reference bottom left is probably useless. A pull-up or pull down resistor may be wired at each MCU output before the diode, they are not represented there.
The problem with this solution is that the transistor state of the transistor. Most of the time It be will in intermediate state. AFAIK (very little, I must say), the problem will be waste of energy converted into heat by the transistor. Maybe it's good start for a good solution ? Please feel free to comment !
So I've been thinking of an alternate solution with a transistor wired to each of the MCU output and that would be only open or saturated, which may (AFAIK) avoid energy loss and heat.
**broken link removed**
Again pull-up/pull down resistors are not implemented at MCU Output level. Resistor values are mostly irrelevant. Its only the general conception I'm willing to talk about.
So I expect this second scheme to loose less energy than the first one.
I have two questions there :
Do I have to limit current with a resistor at each transistor collector ?
Should I scale my DAC function through MCU Ouput resistor ? through transistor Beta ? Both ?
Generally speaking, and regarding energy loss, is there any benefit having lo drop diodes on both scheme ?
Both solutions are using classical transistor, but maybe other transistor are more interesting ?
A subsidiary problem could be the need to adress some of these leds for a short blink, but I gess this is another story.
Please excuse the poor representation of these two pictures as I have very generic tools and little practice to generate them.
Thks for your attention.
I have differents panels which needs variable backlight. Light level will be driven by an MCU. Number of led for each panel may be different, as well as the number of available output pins. To quantify the problem a little bit more let say that per panel no more than 32 leds should be wired together, probably less. So I have to find out a generic solution to my problem that I would adapt to each panel case.
Here a scheme of my first solution to the problem :
**broken link removed**
This is a ktechlab study I did for 13 bits DAC which probably is a higher resolution to any of my real needs. Please note that the LED network is represented by a single LED, which is inaccurate. Each of the MCU Output is represented by a switch connected to Vdd. The 0V reference bottom left is probably useless. A pull-up or pull down resistor may be wired at each MCU output before the diode, they are not represented there.
The problem with this solution is that the transistor state of the transistor. Most of the time It be will in intermediate state. AFAIK (very little, I must say), the problem will be waste of energy converted into heat by the transistor. Maybe it's good start for a good solution ? Please feel free to comment !
So I've been thinking of an alternate solution with a transistor wired to each of the MCU output and that would be only open or saturated, which may (AFAIK) avoid energy loss and heat.
**broken link removed**
Again pull-up/pull down resistors are not implemented at MCU Output level. Resistor values are mostly irrelevant. Its only the general conception I'm willing to talk about.
So I expect this second scheme to loose less energy than the first one.
I have two questions there :
Do I have to limit current with a resistor at each transistor collector ?
Should I scale my DAC function through MCU Ouput resistor ? through transistor Beta ? Both ?
Generally speaking, and regarding energy loss, is there any benefit having lo drop diodes on both scheme ?
Both solutions are using classical transistor, but maybe other transistor are more interesting ?
A subsidiary problem could be the need to adress some of these leds for a short blink, but I gess this is another story.
Please excuse the poor representation of these two pictures as I have very generic tools and little practice to generate them.
Thks for your attention.
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