Re: vibration sensor
You can ffed this signal directly to "comparator" (+) input and put a potentiometer between supply lines and feed its output to the comparator's (-); this will not reduce the range from accelerometer-buffer circuit.
As far as supply is concerned you can create an arfificial ground by deviding 12V into +/-6V, (and then reducuing this voltage to +/-5 if necessary) and I am pretty sure that these circuits can easily handle +/-6V as well as +/-5V..
This can be done by another 358 opamp, where the (+) input is connected to 2 identical resistors deviding the supply voltage by 2, and the (-) input is connected to the opamps output - voltage follower.
Can you visualize these circuits?
Regards,
IanP