If you are working on a project that responds to sounds, your circuit will require some type of microphone to transduce the sound into a modulated voltage and, most likely, some type of amplification of this modulated voltage. I like to use electret microphones to transduce sound because they require relatively simple circuits. For example,

The following schematic demonstrates a simple infrared (IR) photodiode circuit using a analog-to-digital converter (ADC) to monitor the photodiode. An analog pin on an Arduino used with the analogRead() function will do the job. Photodiodes are two-pin analog devices. This makes them relatively easy to use and makes their output graded. Some of my students

Let’s say you are working on a project that requires you to turn on an appliance plugged into a household main (115-ish V AC) using an Arduino (3.3-5 V DC). What would you use as a switch. For some applications you can use a bipolar junction transistor as a switch (see this post). However, such