Operational Amplifiers or Op-Amps are usually high performance linear amplifiers with an amazing variety of uses and applications. There are so many applications for an op-amp, but I will only cover the basic circuits I mostly use for my projects, I will add more when I build other circuits based on Op-Amps... Most commonly used Op-Amps are the LM339 (Quad op-amp),1458 (Dual Op-Amp), the 741 General Purpose Op-amp, and also the LM386 Audio Amplifier....
Most Op-Amps and op-amp circuits require a dual polarity power supply. Here is a simple dual polarity supply made from 2 - 9 volt batteries. The leads from the supply to the circuit should be short and direct, if laeds are longer than 6", add bypass capacitors on the op-amps power supply pins. make sure both batteries have the same voltage output and both should be new... and never apply an input signal to the op-amp when the power supply is off!
Piezo Electric Buzzers/ speakers can also be used for robotic sensors, these could be used to detect pressure, touch or even vibrations, these type of sensors are used in automobile security systems and for industrial sensing applications.      Piezo Crystals used in these devices are unique in a way to where when voltage is applied, the crystals resonate and produce sound, on the other hand, if the piezo disc is bent, tapped or pressure is applied, you will see it produce a voltage. I have seen some piezo's put out quite a bit of votage when tapped hard, so in order to interface to a TTL circuit we have to make sure the voltage output of the sensor stays within the 5 volts range of our TTL circuit. Here is a simple Piezo Disc touch sensor interface circuit I made.... Simple Piezo disc touch sensor interface circuit. D1 - D2 prevent the voltage output of the piezo element from going above 5 volts.
Here is a circuit for interfacing to a Photocell/Phototransistor, with variable gain adjustment..   Typical circuit for amplifying an output from an IR phototransistor	Typical CDS/Photocell  comparator coupling circuit...
Sometimes you will need to have some audio output from your robot, and sometimes you need to amplify the audio output a little... this is the typical circuit I use when making an audio amplifier for my robotics projects... this is a really basic circuit designed around the LM386 Audio Amplifier... Typical LM386 Audio Amplifier circuit
Typical circuit for a power output meter suitable for testing Laser and LED output power on a multi-meter or scope....
This circuit was used to start robots in a sumo competition by a whistle blow, it is set to detect a certain pitch sound usually from a piezo buzzer...