10 years, 1 month ago.

To add to Wim's answer...

For voltages over 3.3V a simple resistor network is all you need. e.g.

Vin - 1k Resistor - CPU AnalogIn pin - 1k Resistor - GND

Will reduce the voltage by half allowing you to measure from 0V to 6.6V. Just remember to double the output you get from the ADC before displaying it.

Or in a more generic form if you have: Vin - R1 - AnalogIn - R2 - GND The voltage at the analog input will be Vin*R2/(R1+R2)

It's possible to do something similar for voltages under 0V by connecting the resistors to 3.3V rather than ground. However the analog input will be referenced to ground which means any noise on the 3.3V power supply (and there will be some) will show up in your measurement. Not recommended but you can do it as a hack if you need to.

Also using resistors like this makes two assumptions: That your voltage source is completely independent of the current it is supplying and that the analog inputs don't pull any current. For crude measurements these two are reasonable assumptions, higher values of resistors minimize the first effect, lower values minimize the second so it's best to pick somewhere in the middle. If you want a more accurate measurement then you'd need to look at a more complex circuit like an op-amp to buffer and scale the signal.

One further detail, if you have no idea what the input voltage will be then connect two diodes to the Analog input,

3.3V supply -|<=- Analog In -|<=- GND. (sorry about the poor ascii artwork)

Normally the diodes will do nothing but if the voltage on the analog input goes over 3.9V or under -0.6V then the diodes kick in and stop the pin from going any further out of specification protecting the CPU from damage. If you aren't using a resistor network to scale the voltage then put a 100 ohm to 1k resistor in series with the input to limit the current that flows if you do go outside the range.