Are there any sensors that would allow me to measure voltage/current/resistance similar to using a multimeter but would allow me to interface with them through a library? (for example, measuring village from a solar cell and plot it on a graph through python) I've heard about ADC expanders, do they work on voltages and if so what range? (I need up to about 10-20V) What kind of precision can you get? And are there similar sensors for current and resistance? Thanks in advance.
It depends on exactly what you want to do. It is relatively easy to measure small (~1V) +ve voltages RELATIVE TO THE PI GROUND. Any ADC can do this, depending on the precision required. Higher voltages can be measured with appropriate voltage dividers (which is what multimeters do internally).
I have done this with
MCP3002 ADC and also using the analog inputs of an
ATMEGA. If you go the latter either an
Gertboard may be an easier solution.
Measuring -ve voltages requires additional circuitry, and a -ve supply voltage.
Floating measurements (voltage or resistance) are harder, and you would be better to go for an external data logger module, with its own supply, which can float WRT the Pi, and use a serial protocol to log the data.