Practical Arduino Cool Projects for Open Source Hardware- P31: 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 14 RFID ACCESS CONTROL SYSTEM But before installing the rest of the parts the relay itself needs to be fitted and that can be more tricky than it sounds because many relays don t fit neatly on a grid. We used a SPST singlepole single-throw relay. The four outer pins could have been bent enough to fit through normal holes on the prototyping shield but the center common pin for the outputs fell directly between four holes. We had to drill a new hole through the prototyping shield keeping in mind that something to be very careful of is that many prototyping shields have a ground plane that covers the entire shield surface between all the pins. If you drill a small hole through the shield just big enough for the pin to fit through you ll probably find that the pin will short-circuit to ground when you solder it in place. We had to drill a large hole that cleared the pin by a good margin and then solder a jumper wire to the pin while keeping the joint clear of the edges of the hole. A short length of heat-shrink tubing is a good idea to prevent short-circuits if the wire is bumped see Figure 14-9 . Figure 14-9. Connection to common connection on relay through a hole drilled in the shield Once the relay is in place use a short length of hookup wire to join one of the coil connections to the NO normally open connection and then to the 12V terminal on the power connection. Then fit the second 1N4001 power diode across the coil connections so that the end with the band the cathode is joined to the coil connection that is linked to 12V. The purpose of the diode is to short out any reverse-voltage charge that is induced across the relay coil when it turns off. Without this protection diode in place a high-voltage reverse spike could damage other parts in the circuit. Even if you don t see a failure immediately it can still cause degradation of parts over time. Insert the transistor so that the collector pin connects to the relay coil connection at the .
