Auto-switch and auto power on mod for generic touchscreen
Thought I'd share the method I used to modify my DWW-700H-like (non pop-out) touchscreen for auto-switch to rearview cam, and auto-on. The same method should work on most any LCD, with minor modifications.
I threw together a basic PIC microcontroller circuit, which is connected to the touchscreen's control board, and to the rear backup-light circuit. The touchscreen I have uses a resistor-ladder type of switch board, with a ADC on the main controller converting a voltage into a button function (i.e. there are 5 buttons on a two-wire interface). I first attempted to use a simple transistor output circuit from the PIC to simulate button presses. Unfortunately, however small I made the base current, it resulted in different voltages than the controller was expecting, and therefore wouldn't function.
I ended up measuring the voltages as each button I was concerned with was pressed (Power and input), and then using a voltage divider driven by the PIC to reproduce the voltage. The output is protected by a diode, allowing the regular hardware buttons to still function.
Input from the 12V reverse-light circuit is clamped down to 5V with a zener diode.
Power for the board comes from the 5V regulator on the LCD's main PCB.
I don't have a schematic drawn up, but I can if people are interested. However, it's quite basic - I just threw this together from parts I had laying on my desk! A couple pics are attached. I'll post a video of the screen switching if I get a chance later this week.
The code is simple - just a main loop checking the status of the inputs, and tracking the state of monitor. It low-passes the inputs to deal with any noise, and always ensures the screen input mode is correctly synced. The current screen mode is saved in non-volatile EEPROM, such that if the car is shut-off while in reverse, the controller is still synced when it is powered back up.