voltage prescaler question.

Quick question,
I’m going to build a switchable voltage prescaler on the front of my arduino based oscilloscope.
between groind and the line to the analog pin will be 10k ohm as per instructions in Practical Arduino article project derived from.
between analog pin and the scope’s probe will be the switched resisters.
I know how all that should behave

<<<gnd ----10kohm------arduino Analog pin----- switched prescaler resister-----probe …circuit being tested

but what would happen if the switched prescaler resister was set to 0.0 ohms?
What effect will the grounded 10kohm have on the signal if the prescaler is set to 0

Thanks.

but what would happen if the switched prescaler resister was set to 0.0 ohms

The you would apply the full input voltage to the arduino. If this is over 5V then you could damage it.

What effect will the grounded 10kohm have on the signal if the prescaler is set to 0

Nothing much except to present a 10K load to the input voltage.

What you need is some protection on the input. I would suggest a small series resistor, say 47R and catcher diodes to each rail. See here:- http://www.thebox.myzen.co.uk/Tutorial/Protection.html or here:- http://www.digikey.com/us/en/techzone/microcontroller/resources/articles/protecting-inputs-in-digital-electronics.html

Thanks, in the end I decided to use a twin pole switch so I am switching both the upper and lower resisters in the prescaler, so on the lowest scale I have no resiister on either side of the prescaler thus eliminating the 10kohm acting as a pull-down resister on the analog pin. The project it's for is an oscilloscope and will probably be used mainly with audio - not power and when using meters I tend to start on high scales and then switch down if I don't know roughly what value I'm looking at.

OK make sure your switch is a "break before make" type and not a "make before break".