Circuit Component Overview:

• +5V Power Supply - This takes power from an 6V to 18V unregulated DC power source and converts it to the 5V DC regulated power that is needed by the electronics. The power supply includes a voltage regulator that reduces the voltage down to 5V and some capacitors that act as noise filters to smooth out the power spikes. One way to get your power source is to buy a 9V AC adapter, or you can build a transformer into your circuit. If you choose to use an AC adapter, a 300 mA adapter should do.

• IR LED - Emits the infrared light beam. Using infrared light has the advantage of not being affected by flash photography. This can be a bit dependent on which phototransistor that you use. If you find that your phototransistors are too sensitive, you can try ones from a different manufacturer. Be careful though, as some phototransistors have a very narrow wavelength range that they can detect, so make sure it will work with your IR LED, or also buy compatible IR LED's.

• IR Phototransistor - Detects the infrared light beam. This acts like a switch, so when the beam is blocked, the switch turns off, which results in a voltage change on the comparator's negative input. (IR LED's and Phototransistors can be purchased in sets from Radio Shack and others, I suggest buying them this way.)

• Quad Comparator - A group of 4 individual comparators placed on one IC chip. Each one compares the voltage of the input from the phototransistor to that of a reference voltage and provides an output of either 0V or 5V. When the light beam is on, the voltage of the comparator's negative input is 0V, but when the beam is blocked, the voltage increases to about 4.5V. When the voltage provided by the phototransistor becomes greater than the reference voltage, in this case 2.5V, the comparator's output changes from 5V to 0V (from on to off). This is the signal that the computer detects via the parallel port.

• Start Switch - You have the choice of a Normally Open (N.O.) switch, which is activated when pressed, or a Normally Closed (N.C.) switch, which is activated when released. Which one to use depends on how you wish to activate the switch. If you want the start gate to press the switch when released, use a N.O. switch. If you want the start gate to release the switch, use a N.C. switch. For my system, I used a N.O. switch that the start gate falls on closing the circuit. If you use a N.C. switch attach a small piece of wood, to the start gate, that will hold the switch down (opens the circuit) while the start gate is closed. When the start gate is released, the switch is also released and the circuit is closed. This is easier with a magnetic switch, but you need a N.C. switch that opens when the magnet is brought close to it, the opposite of a standard alarm contact.

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