Practical Arduino Cool Projects for Open Source Hardware- P42: A schematic or circuit diagram is a diagram that describes the interconnections in an electrical or electronic device. In the projects presented in Practical Arduino, we’ve taken the approach of providing both a photograph and/or line drawing of the completed device along with a schematic. While learning to read schematics takes a modest investment of your time, it will prove useful time and time again as you develop your projects. With that in mind, we present a quick how-to in this section | CHAPTER 16 RESOURCES prudent for high-current applications even at lower voltages simply to avoid the risk of damaging the relatively sensitive circuitry in the Arduino and or connected devices. Semiconductor and Mechanical Switches For practical applications there are two broad types of switching devices available semiconductor and mechanical. Semiconductor devices include a few different types of transistors optocouplers and solid state relays SSRs mechanical devices come down to different types of relays. Solid state devices are often physically smaller consume less power and are able to switch more rapidly than mechanical devices because there are no moving parts. The lack of moving parts nominally makes semiconductors more reliable but well-designed relay circuits will often last just as long. Only certain classes of semiconductor devices provide electrical isolation between input and output. Some solid state devices are a little trickier to use and less forgiving if used incorrectly. Mechanical switching devices have the advantages of providing electrical isolation being pretty bulletproof to design with and are easily understood. On the downside they tend to need higher drive currents and so can themselves need additional circuitry to be driven from the Arduino. They are also limited in switching frequency. This latter limitation can manifest as either being unable to switch fast enough to support higher frequencies or wearing out too quickly limited number of cycles. As a rule of thumb switching frequencies of 1Hz or faster for any continually operated circuit with a mechanical relay is marginal at best. Having discussed some of the general issues in selecting output circuits we now look at some practical examples. Reed Relays Reed relays consist of a switching element the reed within a coil of wire. When current flows through the energized coil the resulting magnetic field closes or in some cases opens the contacts. If the switch is open when the coil is .
