Hello, i've recently done a modification to my xenarc 700IDT to dim the backlight with a 12V external signal in the RCA audio IN. I've also added attenuation control with volume digital potentiometer, i've used the internal potentiometer used for audio, the xenarc 700idt indash has no speaker, but amplifier and potentiometer are in the board!!!.
The backlight intensity is modulated by a signal from 0 to 3.3V (being 0V the brightest) located at the inverter conector. Note that in my screen 3.3V is backlight at 50%, if backlight is lowered more, backlight turns off!!!!!. I think it is a design problem.
This signal is produced by an LM358, a signal amplifier, configured with a gain of 2.
The input to this amplifier (4) is an analog signal provided by a filter (RF & CF , 10K and 39n), from a PWM signal around 800Hz from the main controller, PW113.
To make the screen dimm we need to mix this signal with the input to raise the voltage when input is 12V.
The circuit has two parts, first part is an input signal conditioning and level control, and second is a mixer and filter enhacement.
Gray parts are present in the board.
R1, DZ1 and R2 limit the point (3) voltage 3.11V.
R3 and CF2 are the mixer an filter enhacement (we need to raise the filter because we reduce the filter resistor with R3)
With screen set to full backlight and input set to 12V, the mixer point (4) raises from 0 to 1.244, multiplied by 2 (lm358), this makes 2.488, so backlight is at 60% (with pot at max value).
Location of LM358 and digital potentiometer
0 ohm resistor must be eliminated (this is the input from the audio RCA)
LM358 and filter
Final aspect:
My original post about this in solocarputer forum in spanish:
http://www.solocarputer.com/phpbb2/viewtopic.php?t=5122