I am building a gauge to replace another in my instrument cluster. I am adding some LEDs and an OLED screen and I would like them to have adjustable dim with the old incandescent backlight bulbs. I plan on using a voltage divider and low pass filter, as well as averaging the ADC value to compensate for voltage swings as the alternator, alternates? (programmed to vary voltage pretty widely in operation)
My car has a low side switch for controlling the dimming, that is all bulbs are directly connected to the +12V headlight/indicator relay/fuse, all then go to the dimmer switch which has an oscillator and some sort of switch, also powered by the +12V from indicator fuse.
If I am not mistaken, if I measure the lead before the PWM switch and pass it through a low pass filter I should get a voltage, right? If the dimmer is at minimum pulse width, minimum brightness, I would expect to read nearly 12V, and if the dimmer was at full brightness I would expect to read much lower voltage, pretty much the resistance of the PWM switch times total circuit current? (was also hoping to use this ADC to detect when the headlights are on!)
I have attached a schematic of my dimmer circuit. The wire I think I should measure is GY/R
Thank you in advance for any advice!
Edit: I went ahead and measured the frequency and duty cycle of the dimmer, it varies from 60Hz 1%DC at minimum dim, and maxes out at 100hz and 80%DC.
I am trying to cram this all in place of a factory gauge, I can't really fit any easily available opamps. Instead, I plan on using a rail to rail opamp.
Here is my proposed schematic, the response time is slow, but not so slow to be unmanageable for this application. The high impedance of the divider circuit and opamp input should keep any ground leakage to a minimum when the lights are fully dimmed. The divider is set for a maximum of 14.7, and I plan on letting the opamp manage any extra overvoltage from there. The divider circuit and RC should limit current to a level the internal clamp diodes should handle.
Thank you for the advise!