i have recently built a speedometer (version 2) with an LCD display driven by a LCD driver chip and a microcontroller (atmega168). All the electronics are small and compact to all fit behind the LCD.
The Speedometer will only have two functions
1. show speed
To change between either showing speed or distance i had an idea of using a capacitive touch pad behind the LCD so that you could just effectively touch the screen and the function switches from one to the next. I would like to hear your opinions on this idea? will it work? will the electronic signals behind the pad cause false triggers? Do you guys thing the capacitance of the copper pad behind the screen will change enough to detect it if someone puts their finger on the screen?
I am shying away from using a mechanical button because i want a sleek looking LCD and to have a "touch screen" speedometer will be awesome. Here are two pictures of the speedometer. the one is before the LCD was mounted and the other one with the LCD mounted. There is enough space for the capacitive pad pcb to be sandwiched between the LCD and the electronics.
As for the coding part i was planning on using an RC network and measuring the time constant to charge or discharge the touch capacitor and toggling to switch between functions when the capacitance is above a certain limit above what the idle capacitance would be. i have just been reading about this and it seams possible but i will need to approach this is small steps.