The Do-It-Yourself Race Timer


This document contains supplemental information to help with the construction of the race timer circuit and preparation of the race track to work with the timer. Another document outlines procedures for race day setup.

Safe Handling

Some of the circuit components are sensitive to static electricity. Electrostatic discharge (ESD) can permanently damage these components. Use ESD protective grounding techniques for all personnel, tools, and work areas when handling static sensitive components. Avoid unnecessary handling.

Connections Not Shown

In this edition of the race timer plans, an effort was made to be more explicit about the connections to the various pins on the integrated circuits. Despite this effort, some connections are not shown in the schematic diagrams. This section details where to connect the pins that are not shown in the diagram.

On all integrated circuits, pin VCC should be connected to +5V, and pin GND should be connected to ground. Other pins should be connected as follows, unless shown otherwise in the schematic diagram:

Component Placement

The voltage regulator requires two ceramic capacitors to maintain the stability of its internal circuitry. See IC1, C2, and C3 on Page 1 of the two lane timer to help identify these components in your circuit. These capacitors must be located as close to the voltage regulator as possible in order to be effective.

The LCD display modules should be arranged so that their lane assignments are obvious. For example, the displays might be arranged in a vertical row with the display for Lane 1 on top, the display for Lane 2 just below it, and so on. Also, the yellow and green LED's should be mounted near the LCD display of the corresponding lane. In the two lane circuit, D6 and D7 would be mounted near IC10, while D8 and D9 would be mounted near IC11. See Page 3 and Page 4 of the two lane timer to help identify the corresponding components in your circuit.

The wires leading to the phototransistors should be kept as short as practical. Excessive length invites problems with electromagnetic interference and signal attenuation. A length of 10 feet (3 meters) is recommended in most cases. The wires leading to the infrared LED's and switches can be any length.

For most applications, S3 should be mounted on or near the main unit of the timer circuit. Its function is to prevent the elapsed times from the most recent race from being cleared until the race officials have confirmed and recorded them.

The clock source components (everything shown on Page 2 of the two lane timer circuit) should all be placed as close to each other as possible. Wires between these components should be kept short.

Mounting And Soldering

Most electronics hobby stores sell perforated circuit boards made of plastic, fiberglass, or similar materials. They are typically available in 3 varieties. The first variety has no metal on its surfaces. This type is the most difficult to work with, and is not recommended. The second variety has metal strips on one or both sides connecting nearby holes. This type is the easiest to work with, but causes circuit components to be spread out over a wide area. It is also more expensive than the other two varieties. The third variety has narrow metal rings around the holes on one or both sides. This third type is moderately easy to work with, and allows circuit components to be placed more closely than with the second type.

Place the components on the side of the board opposite the metal, with the leads poking through the holes. On the metal side of the board, solder one lead of each component to the metal ring or strip to hold the component in place. Use as little solder as practical to make these connections. Large balls of solder tend to get out of control and form bridges with nearby solder, causing short circuits.

If you use the second variety of board, then you should run your wires neatly on the component side of the board, poke the stripped ends through holes, and solder them on the opposite side to complete the connection. Before soldering, trim each wire to the length needed. A small amount of slack is good, but if you can twist a loop in it then the wire is probably too long.

If you use the first or third variety of board, then it will be easier to run all of your wires on the solder side of the board. Only strip 1/16 inch (2 mm) at each end of the wire. Hold the exposed wire against the component lead and use just enough solder to join the wire, component lead, and metal ring together. If you prefer to run the wires on the component side, then allow extra length in each wire and strip more of each end. After poking the stripped end through a nearby hole, bend the end into an 'L' shape so that it touches the desired lead. Trim any excess before soldering.

Starting Gate

It is assumed that the starting line has some type of mechanical gate which quickly opens to release the racers at the start of the race. Locate a place to mount the starting gate switch (see S2 on Page 1 of the two lane timer) so that the gate, when fully closed, will press on the switch. Solder the wires to the switch and run them along the track, fastening them periodically to keep them tidy and out of the way. The wires should reach to the finish line, plus 10 feet (3 meters).

Finish Line

At the finish line, mount the infrared LED's in the floor of the track such that one LED is centered in each lane. The lens of each LED should face upward. Make sure the track surface surrounding each LED is non-reflective. An 8 inch (20 cm) wide stripe of flat black paint across the finish line helps to keep unwanted reflections to a minimum.

Above each infrared LED, vertically mount one of the metal tubes that you have prepared. Ensure that the distance between the LED and the upper end of the metal tube is between 12 and 18 inches (between 30 and 46 cm), unless your racers require additional clearance. (Be sure to tune the circuit sensitivity if your distance is outside this range.) Use mounting hardware that allows you to adjust the position and angle of each tube for best alignment with the LED below.

If you can not find mounting hardware that allows the tubes sufficient freedom of motion, you can use stiff wire to attach the tubes to a sturdy wooden frame. 12 AWG solid copper wire works very well, and it can be found in most hardware stores. Tightly wrap about 6 to 8 turns of wire around each tube. The insulation on the wire helps it to grip the tube. Leave approximately 6 inches (15 cm) at each end of the wire not wrapped. Poke about 2 inches (5 cm) of each loose end into a hole drilled in the wooden frame. The angle and position of each tube can be adjusted by bending the wires between the tube and the frame.

Solder each phototransistor onto its own small piece of perforated circuit board. Solder a twisted pair of wires to each phototransistor. Insert the phototransistors, with lenses facing downward, into the upper ends of the tubes. Use two wraps of tape to secure the wires to the metal tubes. This will provide some strain relief in case somebody accidentally trips over or yanks on the wires.

Copyright © 2001 Kristin Hammond

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