This page contains links to general schematics that can be used with a variety of sensors, as building blocks. Schematics for each specific sensor can be found in the sensor reports themselves.
Voltage Divider - The voltage divider is the most basic analog input circuit.
RC Circuit - a resistor-capacitor circuit can be used to measure the change in a variable resistor or capacitor. It's a useful way to get analog in when you have no available analog-to-digital converters on your microcontroller.
Wheatstone bridge - Wheatstone bridges are used to measure the voltage difference across two leads of a sensor.
LM358 Op Amp - LM358 op amp used as a summing amplifier and a DC voltage gain amp to raise a piezo's voltage to 0.2 - 3.0V DC.
simple amplifier circuit