Practical Arduino Cool Projects for Open Source Hardware- P12: 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 6 SECURITY AUTOMATION SENSORS Figure 6-7. Connections for _12V and 0V for sensor terminals Because the 12V supply for the sensors is totally separate to the Arduino supply we also added a power LED across the and - inputs to provide visual feedback that it s on. Insert a green or blue LED through the shield just behind the pair of 12V terminals so that the anode long lead is nearer the input and the cathode short lead is near the - input. Solder the legs in place then use a wire link and the 1K5 resistor to join them to the appropriate power connections. In Figure 6-8 the left-hand link to the connection looks like a black component but it s actually a short length of heat-shrink tubing threaded over the LED leg before it was bent over and soldered directly onto the connection. The 1K5 resistor then joins the cathode lead to the - connection. Figure 6-8. Common ground connections for sensor outputs 89 CHAPTER 6 SECURITY AUTOMATION SENSORS 1K5 is a fairly high value to use as a current limiting resistor on a 12V supply but because the LED is just a status indicator there s no need to run it at full brightness. Using a high resistance value is also a bit of extra protection that may be helpful because plugpack power supplies are notorious for not providing a clean and consistent output and their voltages can vary significantly depending on how much load they are under. Next use short lengths of jumper wire to connect one of the sensor connections from each set to the Arduino s GND on the shield. Because this particular prototyping shield provides GND and 5V rails down the sides it s a simple matter of bridging across between the screw terminals and then to the GND rail. The SparkFun prototyping shield has a handy spot for a reset button to make it easy to reset the Arduino when the shield is in place so insert and solder the reset button. The other sensor connection needs to be pulled to 5V using 4K7 pull-up resistors so insert one resistor for each channel .
