Instructables

Step 7: Solder the Serial Port Connector

Now attach the smaller gauge wires to the large wires and solder them to the serial port connector. You'll need to solder the wires to pins 4 and 6. Since we're just completing a circuit, it doesn't matter which wire goes to which pin just make sure you get 4 and 6. The connector you get will be labeled with the pin numbers and you can also see which pins you need to attach to below.

Pin 4 on a serial port is also known as "DTR" or Data Terminal Ready. Pin 6 is also known as "DSR" or Data Set Ready. When we write our program we'll be able to set DSR to +5V and check if DTR is +5V. As a result we will be able to determine whether or not the magnet is over the switch. And if we check very quickly then we'll be able to figure out how often and how quickly the wheel turns to determine both number of rotations and speed of each rotation.
 
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