by Paul Finwall
If you have time on your hands, want a low cost differential timer, and know a little about electronic circuitry you may want to give this a try. It provides differential times to 0.001 seconds, displays the lane that trips the timer first and comparators circuit to allow setting the trip voltage. The comparator is required to override sunlight effects that can cause a low voltage output from the photo-cell. A few notes are provided to explain timer construction and operation:
1) Don't package the timer in too small a box: a box about 8"x l0"x 3", deep will give you no problems.
2) The test switch disconnects the photo-cell and allows you to trip lane 1 and lane 2 manually for checking operation and for you initial setting of the clock.
3) The 10 K pot. on the 555 timer will allow you to calibrate the clock to an exact pulse rate (for 0.001 second accuracy this would be 1000 pulses/sec)
4) The 1 K pot. on the comparator allows you to set the trip voltage of the photo-cells (1-5 volts). Normally sunlight can cause a photo-cell output up to 1 volt depending on conditions. For most race day conditions a mid-position (2.5 volts) works very well.
5) For the photo-cell timer system the pho-cells are located in the middle of the race track with a 75 watt or 150 watt spotlight directed at the photo-cell from each side of the track. A 6 inch long plastic lawn sprinkler riser with 3/4" pipe threads acts as a light shield. The photo-cells are packaged in a single box approx. 3"x3"x4" with a 3/4" pipe flange centered over each photo cell. The 6" risers are the attached to the pipe flanges. LED's 1B and 2B are indicator lights on the photo-cell box that will glow only when sufficient light is received for operation. They are only used to align the spot lights at the side of the track.
6) A six conductor wire and connectors are used between the timer and photo-cell package. This requires about 501 of length.
7) You may not find a DL727 (2 digit, Ill numeric LED). That isn't important, any numeric LED operated by +5 V DC can be incorporated. You can choose.
8) I found construction to very easy using the wirewrap technique. Don't attempt soldering the IC circuit. You can buy a punchboard, wirewrap IC sockets, and all the rest of the components at most electronic supply houses.
Timer Schematic - (On-line editors note: for best results we suggest saving this as a file and printing this out with a graphics editor that can manipulate GIF images such as Paint Shop Pro)
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