The Do-It-Yourself Race Timer

Parts For The 2-Lane Race Timer

Capacitors
C1 = 1000 uF, electrolytic, 16 V minimum rating (see note)
C2 = .0047 uF, ceramic
C3 = .0047 uF, ceramic
C4 = 47 uF, electrolytic, 10 V minimum rating
C5 = .001 uF, ceramic
C6 = .001 uF, ceramic
C7 = .0047 uF, ceramic
C8 = .001 uF, ceramic
C9 = .0047 uF, ceramic
C10 = .001 uF, ceramic

Diodes
D1 = general purpose diode, Silicon or Schottky (see note)
D2 = general purpose diode, Silicon or Schottky (see note)
D3 = general purpose diode (see note)
D4 = general purpose diode, Silicon or Schottky (see note)
D5 = general purpose diode, Silicon or Schottky (see note)
D6 = indicator LED, 2 V nominal, 20 mA, green
D7 = indicator LED, 2 V nominal, 20 mA, yellow
D8 = indicator LED, 2 V nominal, 20 mA, green
D9 = indicator LED, 2 V nominal, 20 mA, yellow
D10 = GE model LED55C
D11 = GE model LED55C

Integrated Circuits
IC1 = 7805, voltage regulator, +5 V fixed
IC2 = 74HC00, quad NAND gate
IC3 = ECS model OECS-163.8-3-C3X1A, 16.384 MHz crystal oscillator
IC4 = 74HC393, dual binary counter
IC5 = 74HC123, dual monostable, retriggerable
IC6 = LM324A, quad low voltage op-amp
IC7 = 74HC123, dual monostable, retriggerable
IC8 = 74HC175, quad D flipflop
IC9 = 74HC32, quad OR gate
IC10 = Red Lion Controls model MDMU
IC11 = Red Lion Controls model MDMU
IC12 = 74HC32, quad OR gate
IC13 = 74HC08, quad AND gate

Transistors
Q1 = Motorola model MRD300 (see note)
Q2 = Motorola model MRD300 (see note)
Q3 = general purpose transistor, silicon, NPN, hfe>100
Q4 = general purpose transistor, silicon, NPN, hfe>100

Resistors
R1 = 100 K ohm, 1/4 W, 10%
R2 = 33 K ohm, 1/4 W, 10%
R3 = 10 K ohm, 1/4 W, 10%
R4 = 100 K ohm, 1/4 W, 10%
R5 = 10 K ohm, 1/4 W, 10%
R6 = 10 K ohm, 1/4 W, 10%
R7 = 100 K ohm, 1/4 W, 10%
R8 = 100 K ohm, 1/4 W, 10%
R9 = 220 K ohm, 1/4 W, 10%
R10 = 150 K ohm, 1/4 W, 10%
R11 = 22 K ohm, 1/4 W, 10%
R12 = 100 K ohm, 1/4 W, 10%
R13 = 100 K ohm, 1/4 W, 10%
R14 = 220 ohm, 1/4 W, 10%
R15 = 220 ohm, 1/4 W, 10%
R16 = 220 K ohm, 1/4 W, 10%
R17 = 22 K ohm, 1/4 W, 10%
R18 = 100 K ohm, 1/4 W, 10%
R19 = 100 K ohm, 1/4 W, 10%
R20 = 220 ohm, 1/4 W, 10%
R21 = 220 ohm, 1/4 W, 10%
R22 = 220 K ohm, 1/4 W, 10%
R23 = 10 K ohm, 1/4 W, 10%
R24 = 10 ohm, 1/4 W, 10%

Switches
S1 = single pole, single throw, rocker or slide, 1 A minimum rating
S2 = single pole, single throw, low pressure pushbutton, normal open
S3 = single pole, single throw, pushbutton, normal open

Other Parts
transformer power supply, 9 VDC, 150 mA minimum rating (optional)
connector for 9 V battery (optional)
metal tube, in 2 lengths of 8 inches (20 cm), inside diameter matches the outside diameter of Q1
wire, unshielded twisted pair, 22 or 24 AWG (see note)
various mated connectors (see note)

Notes

The value of C1 may be varied somewhat, according to the available current vs. output voltage of your power source. 1000 uF is sufficient for the 2 lane version of the circuit to run reliably on a fresh 9 volt alkaline battery. Higher values may be needed for other battery types or batteries that are not quite fresh. Lower values may be used if a transformer-adapter is to be used exclusively.
Diodes D1, D2, D4, and D5 are included to harden against electrostatic discharge when remote components are being connected or handled. Although a fast-acting Schottky barrier diode (1N5818 for example) may offer slightly better protection, a general purpose silicon diode (1N4148 for example) will suffice.
Diode D3 serves as a low-speed logic gate. A general purpose silicon diode (1N4148 for example) will work well, or other diode types may be substituted.
The Motorola MRD300 phototransistor is apparently no longer in production. You may need to consult a replacement guide to find a substitute part number. One such guide suggested the following part numbers as approximate replacements:
MTD6040
L14G2
BPW14
OP804
BPX43-3
If the phototransistor that you select is not an exact match for the MRD300, then you will need to retune the circuit to work with your selection. You will also need to retune the circuit if the separation distance between the IR LED and the phototransistor will be much more or less than 12 to 18 inches (30 to 46 cm). Consult the section on tuning the sensitivity for details of the procedure.
Standard telephone crossconnect wire is acceptable for connecting the remote components, such as switches and phototransistors, to the main circuit, but you may find it more convenient to use multipair cable such as Category 3 or Category 5 network cable. The twisting of wires in a pair helps to protect the circuit against electromagnetic interference.
When selecting the connectors for use with remote components, consider the potential benefits and risks of the various connector types. Exposed conductors can be short circuited to each other or to ground, potentially damaging the circuit. Using the same type of connector for different types of remote components may lead to circuit damage if a component is accidentally connected to the wrong place in the circuit. You may even decide not to use connectors at all, but to hard-wire all remote devices permanently to the main circuit.

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